---
aliases:
  - nec-240
tags:
  - authorship/other
  - destiny/uncertain
  - exclude-from-word-count
  - status/draft
  - topic/construction/electrical
  - type/media/reference
title: Article 240 Overcurrent Protection
---
# Article 240 Overcurrent Protection

## Part I. General

### 240.1 Scope.

Parts I through VII of this article
provide the general requirements for overcurrent protection
and overcurrent protective devices
not more than 1000 volts, nominal.
[[#Part VIII. Supervised Industrial Installations|Part VIII]]
covers overcurrent protection
for those portions of supervised industrial installations
operating at voltages of not more than 1000 volts, nominal.
[[#Part IX. Overcurrent Protection over 1000 Volts, Nominal|Part IX]]
covers overcurrent protection over 1000 volts, nominal.

> [!info] Informational Note:
> Overcurrent protection for conductors and equipment
> is provided to open the circuit if the current reaches a value
> that will cause an excessive or dangerous temperature
> in conductors or conductor insulation.
> See also [[nfpa-70_110_requirements#110.9 Interrupting Rating.|110.9]]
> for requirements for interrupting ratings
> and [[nfpa-70_110_requirements#110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics.|110.10]]
> for requirements for protection against fault currents.

### 240.2 Definitions.

The definitions in this section shall apply only within this article.

#### Current-Limiting Overcurrent Protective Device.

A device that, when interrupting currents in its current-limiting range,
reduces the current flowing in the faulted circuit
to a magnitude substantially less than that obtainable in the same circuit
if the device were replaced with a solid conductor
having comparable impedance.

#### Supervised Industrial Installation.

For the purposes of Part VIII,
the industrial portions of a facility where all of the following conditions are met:

* (1) Conditions of maintenance and engineering supervision
  ensure that only qualified persons monitor and service the system.

* (2) The premises wiring system has 2500 kVA or greater
  of load used in industrial process(es), manufacturing activities, or both,
  as calculated in accordance with [[nfpa-70_220_load-calculations|Article 220]].

* (3) The premises has at least one service or feeder
  that is more than 150 volts to ground
  and more than 300 volts phase-to-phase.

This definition excludes installations in buildings used by the industrial facility
for offices, warehouses, garages, machine shops, and recreational facilities
that are not an integral part of the industrial plant, substation, or control center.

#### Tap Conductor.

A conductor, other than a service conductor,
that has overcurrent protection ahead of its point of supply
that exceeds the value permitted for similar conductors
that are protected as described elsewhere in
[[#240.4 Protection of Conductors.|240.4]].

### 240.3 Other Articles.

Equipment shall be protected against overcurrent in accordance with the article in this Code that covers the type of equipment specified in Table 240.3.

#### Table 240.3 Other Articles

| Equipment                                                                                   | Article |
| ------------------------------------------------------------------------------------------- | ------- |
| Air-conditioning and refrigerating equipment                                                | 440     |
| Appliances                                                                                  | 422     |
| Assembly occupancies                                                                        | 518     |
| Audio signal processing, amplification, and reproduction equipment                          | 640     |
| Branch circuits                                                                             | 210     |
| Busways                                                                                     | 368     |
| Capacitors                                                                                  | 460     |
| Class 1, Class 2, and Class 3 remote-control, signaling, and power-limited circuits         | 725     |
| Cranes and hoists                                                                           | 610     |
| Electric signs and outline lighting                                                         | 600     |
| Electric welders                                                                            | 630     |
| Electrolytic cells                                                                          | 668     |
| Elevators, dumbwaiters, escalators, moving walks, wheelchair lifts, and stairway chairlifts | 620     |
| Emergency systems                                                                           | 700     |
| Fire alarm systems                                                                          | 760     |
| Fire pumps                                                                                  | 695     |
| Fixed electric heating equipment for pipelines and vessels                                  | 427     |
| Fixed electric space-heating equipment                                                      | 424     |
| Fixed outdoor electric deicing and snow-melting equipment                                   | 426     |
| Generators                                                                                  | 445     |
| Health care facilities                                                                      | 517     |
| Induction and dielectric heating equipment                                                  | 665     |
| Industrial machinery                                                                        | 670     |
| Luminaires, lampholders, and lamps                                                          | 410     |
| Motion picture and television studios and similar locations                                 | 530     |
| Motors, motor circuits, and controllers                                                     | 430     |
| Phase converters                                                                            | 455     |
| Pipe organs                                                                                 | 650     |
| Receptacles                                                                                 | 406     |
| Services                                                                                    | 230     |
| Solar photovoltaic systems                                                                  | 690     |
| Switchboards and panelboards                                                                | 408     |
| Theaters, audience areas of motion picture and television studios, and similar locations    | 520     |
| Transformers and transformer vaults                                                         | 450     |
| X-ray equipment                                                                             | 660     |

### 240.4 Protection of Conductors.

Conductors,
other than flexible cords, flexible cables, and fixture wires,
shall be protected against overcurrent in accordance with their ampacities specified in 310.14,
unless otherwise permitted or required in 240.4(A) through (G).

> [!info] Informational Note:
> See ICEA P-32-382-2007 (R2013), Short Circuit Characteristics of Insulated Cables,
> for information on allowable short-circuit currents
> for insulated copper and aluminum conductors.

#### 240.4(A) Power Loss Hazard.

Conductor overload protection shall not be required
where the interruption of the circuit would create a hazard,
such as in a material-handling magnet circuit or fire pump circuit.
Short-circuit protection shall be provided.

> [!info] Informational Note:
> See NFPA 20-2013, Standard for the Installation of Stationary Pumps for Fire Protection.

#### 240.4(B) Overcurrent Devices Rated 800 Amperes or Less.

The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

* (1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.

* (2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

* (3) The next higher standard rating selected does not exceed 800 amperes.

#### 240.4(C) Overcurrent Devices Rated over 800 Amperes.

Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6.

#### 240.4(D) Small Conductors.

Unless specifically permitted in [[#240.4(E) Tap Conductors.|240.4(E)]]
or [[#240.4(G) Overcurrent Protection for Specific Conductor Applications.|240.4(G)]],
the overcurrent protection shall not exceed that required by (D)(1) through (D)(7)
after any correction factors for ambient temperature
and number of conductors have been applied.

##### 240.4(D)(1) 18 AWG Copper.

7 amperes, provided all the following conditions are met:

* (1) Continuous loads do not exceed 5.6 amperes.

* (2) Overcurrent protection is provided by one of the following:
  * a. Branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire
  * b. Branch-circuit-rated fuses listed and marked for use with 18 AWG copper wire
  * c. Class CC, Class J, or Class T fuses

##### 240.4(D)(2) 16 AWG Copper.

10 amperes, provided all the following conditions are met:

* (1) Continuous loads do not exceed 8 amperes.

* (2) Overcurrent protection is provided by one of the following:
  * a. Branch-circuit-rated circuit breakers listed and marked for use with 16 AWG copper wire
  * b. Branch-circuit-rated fuses listed and marked for use with 16 AWG copper wire
  * c. Class CC, Class J, or Class T fuses

##### 240.4(D)(3) 14 AWG Copper.

15 amperes

##### 240.4(D)(4) 12 AWG Aluminum and Copper-Clad Aluminum.

15 amperes

##### 240.4(D)(5) 12 AWG Copper.

20 amperes

##### 240.4(D)(6) 10 AWG Aluminum and Copper-Clad Aluminum.

25 amperes

##### 240.4(D)(7) 10 AWG Copper.

30 amperes

#### 240.4(E) Tap Conductors.

Tap conductors shall be permitted to be protected against overcurrent in accordance with the following:

* (1) [[nfpa-70_210_branch-circuits#210.19(A)(3) Household Ranges and Cooking Appliances.|210.19(A)(3)]]
    and [[nfpa-70_210_branch-circuits#210.19(A)(4) Other Loads.|210.19(A)(4)]],
    Household Ranges and Cooking Appliances and Other Loads

* (2) [[#240.5(B)(2) Fixture Wire.|240.5(B)(2)]],
    Fixture Wire

* (3) [[#240.21 Location in Circuit.|240.21]],
    Location in Circuit

* (4) 368.17(B),
    Reduction in Ampacity Size of Busway

* (5) 368.17(C),
    Feeder or Branch Circuits (busway taps)

* (6) 430.53(D),[[nfpa-70_430_motors#430.5]]
    Single Motor Taps

#### 240.4(F) Transformer Secondary Conductors.

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors
shall not be considered to be protected by the primary overcurrent protective device.
Conductors supplied by the secondary side
of a single-phase transformer having a 2-wire (single-voltage) secondary,
or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary,
shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer,
provided this protection is in accordance with 450.3
and does not exceed the value determined by multiplying the secondary conductor ampacity
by the secondary-to-primary transformer voltage ratio.

#### 240.4(G) Overcurrent Protection for Specific Conductor Applications.

Overcurrent protection for the specific conductors
shall be permitted to be provided as referenced in Table 240.4(G).

Table 240.4(G) Specific Conductor Applications

| Conductor                                                       | Article                            | Section                                                        |
| --------------------------------------------------------------- | ---------------------------------- | -------------------------------------------------------------- |
| Air-conditioning and refrigeration equipment circuit conductors | 440, Parts III, VI                 |                                                                |
| Capacitor circuit conductors                                    | 460                                | 460.8(B) and 460.25                                            |
| Control and instrumentation circuit conductors (Type ITC)       | 727                                | 727.9                                                          |
| Electric welder circuit conductors                              | 630                                | 630.12 and 630.32                                              |
| Fire alarm system circuit conductors                            | 760                                | 760.43, 760.45, 760.121, and Chapter 9, Tables 12(A) and 12(B) |
| Motor-operated appliance circuit conductors                     | 422, Part II                       |                                                                |
| Motor and motor-control circuit conductors                      | 430, Parts II, III, IV, V, VI, VII |                                                                |
| Phase converter supply conductors                               | 455                                | 455.7                                                          |
| Remote-control, signaling, and power-limited circuit conductors | 725                                | 725.43, 725.45, 725.121, and Chapter 9, Tables 11(A) and 11(B) |
| Secondary tie conductors                                        | 450                                | 450.6                                                          |

### 240.5 Protection of Flexible Cords, Flexible Cables, and Fixture Wires.

Flexible cord and flexible cable, including tinsel cord and extension cords, and fixture wires shall be protected against overcurrent by either 240.5(A) or (B).

#### 240.5(A) Ampacities.

Flexible cord and flexible cable
shall be protected by an overcurrent device
in accordance with their ampacity
as specified in Table 400.5(A)(1) and Table 400.5(A) (2).
Fixture wire shall be protected against overcurrent
in accordance with its ampacity as specified in Table 402.5.
Supplementary overcurrent protection, as covered in 240.10,
shall be permitted to be an acceptable means for providing this protection.

#### 240.5(B) Branch-Circuit Overcurrent Device.

Flexible cord shall be protected,
where supplied by a branch circuit,
in accordance with one of the methods
described in 240.5(B)(1),
(B)(3),
or (B)(4).
Fixture wire shall be protected,
where supplied by a branch circuit,
in accordance with 240.5(B)(2).

##### 240.5(B)(1) Supply Cord of Listed Appliance or Luminaire.

Where flexible cord or tinsel cord is approved for and used with a specific listed appliance or luminaire, it shall be considered to be protected when applied within the appliance or luminaire listing requirements. For the purposes of this section, a luminaire may be either portable or permanent.

##### 240.5(B)(2) Fixture Wire.

Fixture wire shall be permitted to be tapped to the branch-circuit conductor of a branch circuit in accordance with the following:

* (1) 15- or 20-ampere circuits --- 18 AWG, up to 15 m (50 ft) of run length
* (2) 15- or 20-ampere circuits --- 16 AWG, up to 30 m (100 ft) of run length
* (3) 20-ampere circuits --- 14 AWG and larger
* (4) 30-ampere circuits --- 14 AWG and larger
* (5) 40-ampere circuits --- 12 AWG and larger
* (6) 50-ampere circuits --- 12 AWG and larger

##### 240.5(B)(3) Extension Cord Sets.

Flexible cord used in listed extension cord sets shall be considered to be protected when applied within the extension cord listing requirements.

##### 240.5(B)(4) Field Assembled Extension Cord Sets.

Flexible cord used in extension cords made with separately listed and installed components shall be permitted to be supplied by a branch circuit in accordance with the following:
20-ampere circuits --- 16 AWG and larger

### 240.6 Standard Ampere Ratings.

#### 240.6(A) Fuses and Fixed-Trip Circuit Breakers.

The standard ampere ratings
for fuses and inverse time circuit breakers
shall be considered as shown in Table 240.6(A).
Additional standard ampere ratings for fuses
shall be 1, 3, 6, 10, and 601.
The use of fuses and inverse time circuit breakers
with nonstandard ampere ratings shall be permitted.

##### Table 240.6(A) Standard Ampere Ratings for Fuses and Inverse Time Circuit Breakers

| Standard Ampere Ratings |
| ----------------------- |
| 15                      |
| 20                      |
| 25                      |
| 30                      |
| 35                      |
| 40                      |
| 45                      |
| 50                      |
| 60                      |
| 70                      |
| 80                      |
| 90                      |
| 100                     |
| 110                     |
| 125                     |
| 150                     |
| 175                     |
| 200                     |
| 225                     |
| 250                     |
| 300                     |
| 350                     |
| 400                     |
| 450                     |
| 500                     |
| 600                     |
| 700                     |
| 800                     |
| 1000                    |
| 1200                    |
| 1600                    |
| 2000                    |
| 2500                    |
| 3000                    |
| 4000                    |
| 5000                    |
| 6000                    |

#### 240.6(B) Adjustable-Trip Circuit Breakers.

The rating of adjustable-trip circuit breakers
having external means for adjusting the current setting
(long-time pickup setting),
not meeting the requirements of 240.6(C),
shall be the maximum setting possible.

#### 240.6(C) Restricted Access Adjustable-Trip Circuit Breakers.

A circuit breaker(s) that has restricted access to the adjusting means shall be permitted to have an ampere rating(s) that is equal to the adjusted current setting (long-time pickup setting). Restricted access shall be achieved by one of the following methods:

* (1) Located behind removable and sealable covers over the adjusting means
* (2) Located behind bolted equipment enclosure doors
* (3) Located behind locked doors accessible only to qualified personnel
* (4) Password protected, with password accessible only to qualified personnel

### 240.8 Fuses or Circuit Breakers in Parallel.

Fuses and circuit breakers
shall be permitted to be connected in parallel
where they are factory assembled in parallel
and listed as a unit.
Individual fuses, circuit breakers,
or combinations thereof
shall not otherwise be connected in parallel.

### 240.9 Thermal Devices.

Thermal relays and other devices not designed to open short circuits or ground faults
shall not be used for the protection of conductors
against overcurrent due to short circuits or ground faults,
but the use of such devices
shall be permitted to protect motor branch-circuit conductors
from overload if protected in accordance with 430.40.

### 240.10 Supplementary Overcurrent Protection.

Where supplementary overcurrent protection is used for luminaires, appliances, and other equipment or for internal circuits and components of equipment, it shall not be used as a substitute for required branch-circuit overcurrent devices or in place of the required branch-circuit protection. Supplementary overcurrent devices shall not be required to be readily accessible.

### 240.12 Electrical System Coordination.

Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on the following two conditions shall be permitted:

* (1) Coordinated short-circuit protection
* (2) Overload indication based on monitoring systems or devices

> [!info] Informational Note:
> The monitoring system may cause the condition to go to alarm, allowing corrective action or an orderly shutdown, thereby minimizing personnel hazard and equipment damage.

### 240.13 Ground-Fault Protection of Equipment.

Ground-fault protection of equipment shall be provided in accordance with 230.95 for solidly grounded wye electrical systems of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase for each individual device used as a building or structure main disconnecting means rated 1000 amperes or more.

This section shall not apply to the disconnecting means for the following:

* (1) Continuous industrial processes
    where a nonorderly shutdown will introduce additional or increased hazard

* (2) Installations where ground-fault protection
    is provided by other requirements for services or feeders

* (3) Fire pumps

### 240.15 Ungrounded Conductors.

#### 240.15(A) Overcurrent Device Required.

A fuse or an overcurrent trip unit of a circuit breaker
shall be connected in series with each ungrounded conductor.
A combination of a current transformer and overcurrent relay
shall be considered equivalent to an overcurrent trip unit.

> [!info] Informational Note:
> For motor circuits, see Parts III, IV, V, and XI of [[nfpa-70_430_motors|Article 430]].

#### 240.15(B) Circuit Breaker as Overcurrent Device.

Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in
240.15(B)(1), (B)(2), (B)(3), and (B)(4).

##### 240.15(B)(1) Multiwire Branch Circuits.

Individual single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multiwire branch circuits that serve only single-phase line-to-neutral loads.

##### 240.15(B)(2) Grounded Single-Phase Alternating-Current Circuits.

In grounded systems, individual single-pole circuit breakers rated 120/240 volts ac, with identified handle ties, shall be permitted as the protection for each ungrounded conductor for line-to-line connected loads for single-phase circuits.

##### 240.15(B)(3) 3-Phase and 2-Phase Systems.

For line-to-line loads in 4-wire, 3-phase systems or 5-wire, 2-phase systems, individual single-pole circuit breakers rated 120/240 volts ac with identified handle ties shall be permitted as the protection for each ungrounded conductor, if the systems have a grounded neutral point and the voltage to ground does not exceed 120 volts.

##### 240.15(B)(4) 3-Wire Direct-Current Circuits.

Individual single-pole circuit breakers rated 125/250 volts dc with identified handle ties shall be permitted as the protection for each ungrounded conductor for line-to-line connected loads for 3-wire, direct-current circuits supplied from a system with a grounded neutral where the voltage to ground does not exceed 125 volts.

## Part II. Location

### 240.21 Location in Circuit.

Overcurrent protection shall be provided in each ungrounded circuit conductor
and shall be located at the point where the conductors receive their supply
except as specified in 240.21(A) through (H).
Conductors supplied under 240.21(A) through (H)
shall not supply another conductor
except through an overcurrent protective device
meeting the requirements of [[#240.4 Protection of Conductors.|240.4]].

#### 240.21(A) Branch-Circuit Conductors.

Branch-circuit tap conductors
meeting the requirements specified in 210.19
shall be permitted to have overcurrent protection as specified in 210.20.

#### 240.21(B) Feeder Taps.

Conductors shall be permitted to be tapped,
without overcurrent protection at the tap,
to a feeder as specified in 240.21(B)(1) through (B)(5).
The tap shall be permitted at any point
on the load side of the feeder overcurrent protective device.
Section 240.4(B) shall not be permitted for tap conductors.

##### 240.21(B)(1) Taps Not over 3 m (10 ft) Long.

If the length of the tap conductors does not exceed 3 m (10 ft)
and the tap conductors comply with all of the following:

* (1) The ampacity of the tap conductors is
  * a. Not less than the combined calculated loads on the circuits supplied by the tap conductors
  * b. Not less than the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors
    or not less than the rating of the overcurrent protective device at the termination of the tap conductors

    > [!important] Exception to b:
    > Where listed equipment, such as a surge-protective device(s) \[SPD(s)\],
    > is provided with specific instructions on minimum conductor sizing,
    > the ampacity of the tap conductors supplying that equipment
    > shall be permitted to be determined based on the manufacturer's instructions.

* (2) The tap conductors do not extend beyond the switchboard,
  switchgear, panelboard, disconnecting means,
  or control devices they supply.

* (3) Except at the point of connection to the feeder,
  the tap conductors are enclosed in a raceway,
  which extends from the tap to the enclosure of an enclosed switchboard,
  switchgear, a panelboard, or control devices,
  or to the back of an open switchboard.

* (4) For field installations,
  if the tap conductors leave the enclosure or vault in which the tap is made,
  the ampacity of the tap conductors is not less than one-tenth of the rating
  of the overcurrent device protecting the feeder conductors.

> [!info] Informational Note:
> For overcurrent protection requirements for panelboards, see 408.36.

##### 240.21(B)(2) Taps Not over 7.5 m (25 ft) Long.

Where the length of the tap conductors does not exceed 7.5 m (25 ft)
and the tap conductors comply with all the following:

* (1) The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors.

* (2) The tap conductors terminate in a single circuit breaker or a single set of fuses
    that limit the load to the ampacity of the tap conductors.
    This device shall be permitted to supply any number of additional overcurrent devices on its load side.

* (3) The tap conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

##### 240.21(B)(3) Taps Supplying a Transformer \[Primary Plus Secondary Not over 7.5 m (25 ft) Long\].

Where the tap conductors supply a transformer and comply with all the following conditions:

* (1) The conductors supplying the primary of a transformer have an ampacity
  at least one-third the rating of the overcurrent device
  protecting the feeder conductors.

* (2) The conductors supplied by the secondary of the transformer shall have an ampacity
  that is not less than the value of the primary-to-secondary voltage ratio
  multiplied by one-third of the rating of the overcurrent device
  protecting the feeder conductors.

* (3) The total length of one primary plus one secondary conductor,
  excluding any portion of the primary conductor that is protected at its ampacity,
  is not over 7.5 m (25 ft).

* (4) The primary and secondary conductors are protected from physical damage
  by being enclosed in an approved raceway or by other approved means.

* (5) The secondary conductors terminate in a single circuit breaker or set of fuses
  that limit the load current to not more than the conductor ampacity that is permitted by 310.14.

##### 240.21(B)(4) Taps over 7.5 m (25 ft) Long.

Where the feeder is in a high bay manufacturing building
over 11 m (35 ft) high at walls
and the installation complies with all the following conditions:

* (1) Conditions of maintenance and supervision
  ensure that only qualified persons service the systems.

* (2) The tap conductors are not over 7.5 m (25 ft) long horizontally
  and not over 30 m (100 ft) total length.

* (3) The ampacity of the tap conductors
  is not less than one-third the rating of the overcurrent device
  protecting the feeder conductors.

* (4) The tap conductors terminate at a single circuit breaker or a single set of fuses
  that limit the load to the ampacity of the tap conductors.
  This single overcurrent device
  shall be permitted to supply any number of additional overcurrent devices
  on its load side.

* (5) The tap conductors are protected from physical damage
  by being enclosed in an approved raceway or by other approved means.

* (6) The tap conductors are continuous from end-to-end and contain no splices.

* (7) The tap conductors are sized 6 AWG copper or 4 AWG aluminum or larger.

* (8) The tap conductors do not penetrate walls, floors, or ceilings.

* (9) The tap is made no less than 9 m (30 ft) from the floor.

##### 240.21(B)(5) Outside Taps of Unlimited Length.

Where the conductors are located outside of a building or structure,
except at the point of load termination,
and comply with all of the following conditions:

* (1) The tap conductors are protected from physical damage in an approved manner.

* (2) The tap conductors terminate at a single circuit breaker or a single set of fuses
  that limits the load to the ampacity of the tap conductors.
  This single overcurrent device
  shall be permitted to supply any number of additional overcurrent devices
  on its load side.

* (3) The overcurrent device for the tap conductors
  is an integral part of a disconnecting means
  or shall be located immediately adjacent thereto.

* (4) The disconnecting means for the tap conductors
  is installed at a readily accessible location complying with one of the following:

  * a. Outside of a building or structure
  * b. Inside, nearest the point of entrance of the tap conductors
  * c. Where installed in accordance with 230.6, nearest the point of entrance of the tap conductors

#### 240.21(C) Transformer Secondary Conductors.

A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). Section 240.4(B) shall not be permitted for transformer secondary conductors.

> [!info] Informational Note:
> For overcurrent protection requirements for transformers, see 450.3.

##### 240.21(C)(1) Protection by Primary Overcurrent Device.

Conductors supplied by the secondary side
of a single-phase transformer having a 2-wire (single-voltage) secondary,
or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary,
shall be permitted to be protected by overcurrent protection
provided on the primary (supply) side of the transformer,
provided this protection is in accordance with 450.3
and does not exceed the value determined by multiplying the secondary conductor ampacity
by the secondary-to-primary transformer voltage ratio.

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

##### 240.21(C)(2) Transformer Secondary Conductors Not over 3 m (10 ft) Long.

If the length of secondary conductor does not exceed 3 m (10 ft) and complies with all of the following:

* (1) The ampacity of the secondary conductors is
  * a. Not less than the combined calculated loads on the circuits supplied by the secondary conductors, and
  * b. Not less than the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductors
    or not less than the rating of the overcurrent protective device at the termination of the secondary conductors.

  > [!important] Exception:
  > Where listed equipment, such as a surge protective device(s) \[SPD(s)\],
  > is provided with specific instructions on minimum conductor sizing,
  > the ampacity of the tap conductors supplying that equipment
  > shall be permitted to be determined based on the manufacturer's instructions.

* (2) The secondary conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.

* (3) The secondary conductors are enclosed in a raceway, which shall extend from the transformer to the enclosure of an enclosed switchboard, switchgear, a panelboard, or control devices or to the back of an open switchboard.

* (4) For field installations where the secondary conductors leave the enclosure or vault in which the supply connection is made, the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary to secondary transformer voltage ratio, shall not exceed 10 times the ampacity of the secondary conductor.

> [!info] Informational Note:
> For overcurrent protection requirements for panelboards, see 408.36.

##### 240.21(C)(3) Industrial Installation Secondary Conductors Not over 7.5 m (25 ft) Long.

For the supply of switchgear or switchboards in industrial installations only, where the length of the secondary conductors does not exceed 7.5 m (25 ft) and complies with all of the following:

* (1) Conditions of maintenance and supervision ensure that only qualified persons service the systems.

* (2) The ampacity of the secondary conductors is not less than the secondary current rating of the transformer, and the sum of the ratings of the overcurrent devices does not exceed the ampacity of the secondary conductors.

* (3) All overcurrent devices are grouped.

* (4) The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

##### 240.21(C)(4) Outside Secondary Conductors.

Where the conductors are located outside of a building or structure, except at the point of load termination, and comply with all of the following conditions:

* (1) The conductors are protected from physical damage in an approved manner.

* (2) The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors.

This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.

* (3) The overcurrent device for the conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.

* (4) The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
  * a. Outside of a building or structure
  * b. Inside, nearest the point of entrance of the conductors
  * c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors

##### 240.21(C)(5) Secondary Conductors from a Feeder Tapped Transformer.

Transformer secondary conductors installed in accordance with 240.21(B)(3) shall be permitted to have overcurrent protection as specified in that section.

##### 240.21(C)(6) Secondary Conductors Not over 7.5 m (25 ft) Long.

Where the length of secondary conductor does not exceed 7.5 m (25 ft) and complies with all of the following:

* (1) The secondary conductors shall have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the primary of the transformer.

* (2) The secondary conductors terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity that is permitted by 310.14.

* (3) The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

#### 240.21(D) Service Conductors.

Service conductors shall be permitted to be protected by overcurrent devices in accordance with 230.91.

#### 240.21(E) Busway Taps.

Busways and busway taps shall be permitted to be protected against overcurrent in accordance with 368.17.

#### 240.21(F) Motor Circuit Taps.

Motor-feeder and branch-circuit conductors shall be permitted to be protected against overcurrent in accordance with 430.28 and 430.53, respectively.

#### 240.21(G) Conductors from Generator Terminals.

Conductors from generator terminals that meet the size requirement in 445.13 shall be permitted to be protected against overload by the generator overload protective device(s) required by 445.12.

#### 240.21(H) Battery Conductors.

Overcurrent protection shall be permitted to be installed as close as practicable to the storage battery terminals in an unclassified location. Installation of the overcurrent protection within a hazardous (classified) location shall also be permitted.

### 240.22 Grounded Conductor.

No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met:

* (1) The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.

* (2) Where required by 430.36 or 430.37 for motor overload protection.

### 240.24 Location in or on Premises.

#### 240.24(A) Accessibility.

Circuit breakers and switches containing fuses shall be readily accessible and installed so that the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, is not more than 2.0 m (6 ft 7 in.) above the floor or working platform, unless one of the following applies:

* (1) For busways, as provided in 368.17(C).

* (2) For supplementary overcurrent protection, as described in 240.10.

* (3) For overcurrent devices, as described in 225.40 and 230.92.

* (4) For overcurrent devices adjacent to utilization equipment that they supply, access shall be permitted to be by portable means.

> [!important] Exception:
> The use of a tool shall be permitted to access overcurrent devices located within listed industrial control panels or similar enclosures.

#### 240.24(B) Occupancy.

Each occupant shall have ready access to all overcurrent devices protecting the conductors supplying that occupancy, unless otherwise permitted in 240.24(B)(1) and (B) (2).

##### 240.24(B)(1) Service and Feeder Overcurrent Devices.

Where electric service and electrical maintenance are provided by the building management and where these are under continuous building management supervision, the service overcurrent devices and feeder overcurrent devices supplying more than one occupancy shall be permitted to be accessible only to authorized management personnel in the following:

* (1) Multiple-occupancy buildings
* (2) Guest rooms or guest suites

##### 240.24(B)(2) Branch-Circuit Overcurrent Devices.

Where electric service and electrical maintenance are provided by the building management and where these are under continuous building management supervision, the branch-circuit overcurrent devices supplying any guest rooms or guest suites without permanent provisions for cooking shall be permitted to be accessible only to authorized management personnel.

#### 240.24(C) Not Exposed to Physical Damage.

Overcurrent devices shall be located where they will not be exposed to physical damage.

> [!info] Informational Note:
> See [[nfpa-70_110_requirements#110.11 Deteriorating Agents.|110.11, Deteriorating Agents]]

#### 240.24(D) Not in Vicinity of Easily Ignitible Material.

Overcurrent devices shall not be located in the vicinity of easily ignitible material, such as in clothes closets.

#### 240.24(E) Not Located in Bathrooms.

In dwelling units, dormitory units, and guest rooms or guest suites, overcurrent devices, other than supplementary overcurrent protection, shall not be located in bathrooms.

#### 240.24(F) Not Located over Steps.

Overcurrent devices shall not be located over steps of a stairway.

## Part III. Enclosures

### 240.30 General.

#### 240.30(A) Protection from Physical Damage.

Overcurrent devices shall be protected from physical damage by one of the following:

* (1) Installation in enclosures, cabinets, cutout boxes, or equipment assemblies

* (2) Mounting on open-type switchboards, panelboards, or control boards that are in rooms or enclosures free from dampness and easily ignitible material and are accessible only to qualified personnel

#### 240.30(B) Operating Handle.

The operating handle of a circuit breaker shall be permitted to be accessible without opening a door or cover.

### 240.32 Damp or Wet Locations.

Enclosures for overcurrent devices in damp or wet locations shall comply with 312.2.

### 240.33 Vertical Position.

Enclosures for overcurrent devices shall be mounted in a vertical position.
Circuit breaker enclosures shall be permitted to be installed horizontally
where the circuit breaker is installed in accordance with 240.81.
Listed busway plug-in units shall be permitted to be mounted
in orientations corresponding to the busway mounting position.

## Part IV. Disconnecting and Guarding

### 240.40 Disconnecting Means for Fuses.

Cartridge fuses in circuits of any voltage,
and all fuses in circuits over 150 volts to ground,
shall be provided with a disconnecting means on their supply side
so that each circuit containing fuses
can be independently disconnected from the source of power.
A cable limiter without a disconnecting means
shall be permitted on the supply side of the service disconnecting means
as permitted by 230.82.

A single disconnecting means
shall be permitted on the supply side of more than one set of fuses
as permitted by 430.112, Exception,
for group operation of motors, 424.22(C) for fixed electric space-heating equipment, and 425.22(C) for fixed resistance and electrode industrial process heating equipment, or where specifically permitted elsewhere in this Code.

### 240.41 Arcing or Suddenly Moving Parts.

Arcing or suddenly moving parts shall comply with 240.41(A) and (B).

#### 240.41(A) Location.

Fuses and circuit breakers shall be located or shielded so that persons will not be burned or otherwise injured by their operation.

#### 240.41(B) Suddenly Moving Parts.

Handles or levers of circuit breakers, and similar parts that may move suddenly in such a way that persons in the vicinity are likely to be injured by being struck by them, shall be guarded or isolated.

## Part V. Plug Fuses, Fuseholders, and Adapters

### 240.50 General.

#### 240.50(A) Maximum Voltage.

Plug fuses shall be permitted to be used in the following circuits:

* (1) Circuits not exceeding 125 volts between conductors

* (2) Circuits supplied by a system having a grounded neutral point
  where the line-to-neutral voltage does not exceed 150 volts

#### 240.50(B) Marking.

Each fuse, fuseholder, and adapter shall be marked with its ampere rating.

#### 240.50(C) Hexagonal Configuration.

Plug fuses of 15-ampere and lower rating shall be identified by a hexagonal configuration of the window, cap, or other prominent part to distinguish them from fuses of higher ampere ratings.

#### 240.50(D) No Energized Parts.

Plug fuses, fuseholders, and adapters shall have no exposed energized parts after fuses or fuses and adapters have been installed.

#### 240.50(E) Screw Shell.

The screw shell of a plug-type fuseholder shall be connected to the load side of the circuit.

### 240.51 Edison-Base Fuses.

#### 240.51(A) Classification.

Plug fuses of the Edison-base type shall be classified at not over 125 volts and 30 amperes and below.

#### 240.51(B) Replacement Only.

Plug fuses of the Edison-base type shall be used only for replacements in existing installations where there is no evidence of overfusing or tampering.

### 240.52 Edison-Base Fuseholders.

Fuseholders of the Edison-base type shall be installed only where they are made to accept Type S fuses by the use of adapters.

### 240.53 Type S Fuses.

Type S fuses shall be of the plug type and shall comply with 240.53(A) and (B).

#### 240.53(A) Classification.

Type S fuses shall be classified at not over 125 volts and 0 to 15 amperes, 16 to 20 amperes, and 21 to 30 amperes.

#### 240.53(B) Noninterchangeable.

Type S fuses of an ampere classification as specified in 240.53(A) shall not be interchangeable with a lower ampere classification. They shall be designed so that they cannot be used in any fuseholder other than a Type S fuseholder or a fuseholder with a Type S adapter inserted.

### 240.54 Type S Fuses, Adapters, and Fuseholders.

#### 240.54(A) To Fit Edison-Base Fuseholders.

Type S adapters shall fit Edison-base fuseholders.

#### 240.54(B) To Fit Type S Fuses Only.

Type S fuseholders and adapters shall be designed so that either the fuseholder itself or the fuseholder with a Type S adapter inserted cannot be used for any fuse other than a Type S fuse.

#### 240.54(C) Nonremovable.

Type S adapters shall be designed so that once inserted in a fuseholder, they cannot be removed.

#### 240.54(D) Nontamperable.

Type S fuses, fuseholders, and adapters shall be designed so that tampering or shunting (bridging) would be difficult.

#### 240.54(E) Interchangeability.

Dimensions of Type S fuses, fuseholders, and adapters shall be standardized to permit interchangeability regardless of the manufacturer.

## Part VI. Cartridge Fuses and Fuseholders

### 240.60 General.

#### 240.60(A) Maximum Voltage --- 300-Volt Type.

Cartridge fuses and fuseholders of the 300-volt type shall be permitted to be used in the following circuits:

* (1) Circuits not exceeding 300 volts between conductors

* (2) Single-phase line-to-neutral circuits
  supplied from a 3-phase, 4-wire, solidly grounded neutral source
  where the line-to-neutral voltage does not exceed 300 volts

#### 240.60(B) Noninterchangeable --- 0--6000-Ampere Cartridge Fuseholders.

Fuseholders shall be designed so that it will be difficult to put a fuse of any given class into a fuseholder that is designed for a current lower, or voltage higher, than that of the class to which the fuse belongs. Fuseholders for current-limiting fuses shall not permit insertion of fuses that are not current-limiting.

#### 240.60(C) Marking.

Fuses shall be plainly marked, either by printing on the fuse barrel or by a label attached to the barrel showing the following:

* (1) Ampere rating
* (2) Voltage rating
* (3) Interrupting rating where other than 10,000 amperes
* (4) Current limiting where applicable
* (5) The name or trademark of the manufacturer

The interrupting rating shall not be required to be marked on fuses used for supplementary protection.

#### 240.60(D) Renewable Fuses.

Class H cartridge fuses of the renewable type shall be permitted to be used only for replacement in existing installations where there is no evidence of overfusing or tampering.

### 240.61 Classification.

Cartridge fuses and fuseholders shall be classified according to voltage and amperage ranges. Fuses rated 1000 volts, nominal, or less shall be permitted to be used for voltages at or below their ratings.

### 240.62 Reconditioned Equipment.

Low-voltage fuseholders and low-voltage nonrenewable fuses shall not be permitted to be reconditioned.

### 240.67 Arc Energy Reduction.

Where fuses rated 1200 A or higher are installed, 240.67(A) and (B) shall apply. This requirement shall become effective January 1,

#### 240.67(A) Documentation.

Documentation shall be available to those authorized to design, install, operate, or inspect the installation as to the location of the fuses.

Documentation shall also be provided to demonstrate that the method chosen to reduce clearing time is set to operate at a value below the available arcing current.

#### 240.67(B) Method to Reduce Clearing Time.

A fuse shall have a clearing time of 0.07 seconds or less at the available arcing current, or one of the following means shall be provided and shall be set to operate at less than the available arcing current:

* (1) Differential relaying
* (2) Energy-reducing maintenance switching with local status indicator
* (3) Energy-reducing active arc-flash mitigation system
* (4) Current-limiting, electronically actuated fuses
* (5) An approved equivalent means

> [!info] Informational Note No. 1:
> An energy-reducing maintenance switch
> allows a worker to set a disconnect switch
> to reduce the clearing time while the worker is working within an arc-flash boundary
> as defined in NFPA 70E-2018, Standard for Electrical Safety in the Workplace,
> and then to set the disconnect switch back to a normal setting
> after the potentially hazardous work is complete.

> [!info] Informational Note No. 2:
> An energy-reducing active arc-flash mitigation system
> helps in reducing arcing duration in the electrical distribution system.
> No change in the disconnect switch or the settings of other devices
> is required during maintenance when a worker is working within an arc-flash boundary
> as defined in NFPA 70E-2018, Standard for Electrical Safety in the Workplace.

> [!info] Informational Note No. 3:
> IEEE 1584-2002, IEEE Guide for Performing Arc Flash Hazard Calculations,
> is one of the available methods
> that provides guidance in determining arcing current.

#### 240.67(C) Performance Testing.

The arc energy reduction protection system
shall be performance tested by primary current injection testing
or another approved method when first installed on site.
This testing shall be conducted by a qualified person(s)
in accordance with the manufacturer's instructions.

A written record of this testing
shall be made and shall be available to the authority having jurisdiction.

> [!info] Informational Note:
> Some energy reduction protection systems cannot be tested
> using a test process of primary current injection
> due to either the protection method being damaged
> such as with the use of fuse technology
> or because current is not the primary method of arc detection.

## Part VII. Circuit Breakers

### 240.80 Method of Operation.

Circuit breakers shall be trip free and capable of being closed and opened by manual operation. Their normal method of operation by other than manual means, such as electrical or pneumatic, shall be permitted if means for manual operation are also provided.

### 240.81 Indicating.

Circuit breakers shall clearly indicate whether they are in the open "off" or closed "on" position.

Where circuit breaker handles are operated vertically rather than rotationally or horizontally, the "up" position of the handle shall be the "on" position.

### 240.82 Nontamperable.

A circuit breaker shall be of such design that any alteration of its trip point (calibration) or the time required for its operation requires dismantling of the device or breaking of a seal for other than intended adjustments.

### 240.83 Marking.

#### 240.83(A) Durable and Visible.

Circuit breakers shall be marked with their ampere rating in a manner that will be durable and visible after installation. Such marking shall be permitted to be made visible by removal of a trim or cover.

#### 240.83(B) Location.

Circuit breakers rated at 100 amperes or less and 1000 volts or less shall have the ampere rating molded, stamped, etched, or similarly marked into their handles or escutcheon areas.

#### 240.83(C) Interrupting Rating.

Every circuit breaker having an interrupting rating other than 5000 amperes shall have its interrupting rating shown on the circuit breaker. The interrupting rating shall not be required to be marked on circuit breakers used for supplementary protection.

#### 240.83(D) Used as Switches.

Circuit breakers used as switches in 120- volt and 277-volt fluorescent lighting circuits shall be listed and shall be marked SWD or HID.

Circuit breakers used as switches in high-intensity discharge lighting circuits shall be listed and shall be marked as HID.

#### 240.83(E) Voltage Marking.

Circuit breakers shall be marked with a voltage rating not less than the nominal system voltage that is indicative of their capability to interrupt fault currents between phases or phase to ground.

### 240.85 Applications.

A circuit breaker with a straight voltage rating, such as 240V or 480V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker's voltage rating. A two-pole circuit breaker shall not be used for protecting a 3- phase, corner-grounded delta circuit unless the circuit breaker is marked 1φ--3φ to indicate such suitability.

A circuit breaker with a slash rating, such as 120/240V or 480Y/277V, shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker's voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker's voltage rating.

> [!info] Informational Note:
> Proper application of molded case circuit breakers on 3-phase systems, other than solidly grounded wye, particularly on corner grounded delta systems, considers the circuit breakers' individual pole-interrupting capability.

### 240.86 Series Ratings.

Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an approved overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in 240.86(A) or (B), and (C).

#### 240.86(A) Selected Under Engineering Supervision in Existing Installations.

The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.

For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.

#### 240.86(B) Tested Combinations.

The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.

Informational Note to (A) and (B): See 110.22 for marking of series combination systems.

#### 240.86(C) Motor Contribution.

Series ratings shall not be used where

* (1) Motor circuits are connected between the higher-rated overcurrent device of a series-rated combination and on the lower-rated circuit breaker, and

* (2) The sum of these motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.

### 240.87 Arc Energy Reduction.

Where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher, 240.87(A) and (B) shall apply.

#### 240.87(A) Documentation.

Documentation shall be available to those authorized to design, install, operate, or inspect the installation as to the location of the circuit breaker(s). Documentation shall also be provided to demonstrate that the method chosen to reduce clearing time is set to operate at a value below the available arcing current.

#### 240.87(B) Method to Reduce Clearing Time.

One of the following means shall be provided
and shall be set to operate at less than the available arcing current:

* (1) Zone-selective interlocking

* (2) Differential relaying

* (3) Energy-reducing maintenance switching with local status indicator

* (4) Energy-reducing active arc flash mitigation system

* (5) An instantaneous trip setting.
  Temporary adjustment of the instantaneous trip setting
  to achieve arc energy reduction shall not be permitted.

* (6) An instantaneous override

* (7) An approved equivalent means

> [!info] Informational Note No. 1:
> An energy-reducing maintenance switch allows a worker to set a circuit breaker trip unit to "no intentional delay" to reduce the clearing time while the worker is working within an arc-flash boundary as defined in NFPA 70E-2018, Standard for Electrical Safety in the Workplace, and then to set the trip unit back to a normal setting after the potentially hazardous work is complete.

> [!info] Informational Note No. 2:
> An energy-reducing active arc-flash mitigation system helps in reducing arcing duration in the electrical distribution system. No change in the circuit breaker or the settings of other devices is required during maintenance when a worker is working within an arc-flash boundary as defined in NFPA 70E-2018, Standard for Electrical Safety in the Workplace.

> [!info] Informational Note No. 3:
> An instantaneous trip is a function that causes a circuit breaker to trip with no intentional delay when currents exceed the instantaneous trip setting or current level. If arcing currents are above the instantaneous trip level, the circuit breaker will trip in the minimum possible time.

> [!info] Informational Note No. 4:
> IEEE 1584-2002, IEEE Guide for Performing Arc Flash Hazard Calculations, is one of the available methods that provide guidance in determining arcing current.

#### 240.87(C) Performance Testing.

The arc energy reduction protection system shall be performance tested by primary current injection testing or another approved method when first installed on site. This testing shall be conducted by a qualified person(s) in accordance with the manufacturer's instructions.

A written record of this testing shall be made and shall be available to the authority having jurisdiction.

> [!info] Informational Note:
> Some energy reduction protection systems cannot be tested using a test process of primary current injection due to either the protection method being damaged such as with the use of fuse technology or because current is not the primary method of arc detection.

### 240.88 Reconditioned Equipment.

Reconditioned equipment shall be listed as "reconditioned" and the original listing mark removed.

#### 240.88(A) Circuit Breakers.

The use of reconditioned circuit breakers shall comply with (1) through (3):

* (1) Molded-case circuit breakers shall not be permitted to be reconditioned.

* (2) Low- and medium-voltage power circuit breakers shall be permitted to be reconditioned.

* (3) High-voltage circuit breakers shall be permitted to be reconditioned.

#### 240.88(B) Components.

The use of reconditioned trip units, protective relays, and current transformers shall comply with (1) and (2):

* (1) Low-voltage power circuit breaker electronic trip units shall not be permitted to be reconditioned.

* (2) Electromechanical protective relays and current transformers shall be permitted to be reconditioned.

## Part VIII. Supervised Industrial Installations

### 240.90 General.

Overcurrent protection in areas of supervised industrial installations shall comply with all of the other applicable provisions of this article, except as provided in Part VIII. Part VIII shall be permitted to apply only to those portions of the electrical system in the supervised industrial installation used exclusively for manufacturing or process control activities.

### 240.91 Protection of Conductors.

Conductors shall be protected in accordance with 240.91(A) or 240.91(B).

#### 240.91(A) General.

Conductors shall be protected in accordance with 240.4.

#### 240.91(B) Devices Rated Over 800 Amperes.

Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than
95 percent of the rating of the overcurrent device in accordance with the following:

* (1) The conductors are protected within recognized time vs. current limits for short-circuit currents

> [!info] Informational Note:
> Table 240.92(B) provides time vs. short-circuit current formulas to determine limits for copper and aluminum conductors.

* (2) All equipment in which the conductors terminate is listed and marked for the application.

### 240.92 Location in Circuit.

An overcurrent device shall be connected in each ungrounded circuit conductor as required in 240.92(A) through (E).

#### 240.92(A) Feeder and Branch-Circuit Conductors.

Feeder and branch-circuit conductors shall be protected at the point the conductors receive their supply as permitted in 240.21 or as otherwise permitted in 240.92(B), (C), (D), or (E).

#### 240.92(B) Feeder Taps.

For feeder taps specified in 240.21(B)(2), (B)(3), and (B)(4), the tap conductors shall be permitted to be sized in accordance with Table
240.92(B).

##### Table 240.92(B) Tap Conductor Short-Circuit Current Ratings

Tap conductors are considered to be protected under short-circuit conditions when their short-circuit temperature limit is not exceeded.

Conductor heating under short-circuit conditions is determined by (1) or (2):

* (1) _Short-Circuit Formula for Copper Conductors_

    $$
    (I^2|A^2)t = 0.0297 \log_{10} [(T_{2} + 234)/(T_{1} + 234)]
    $$

* (2) _Short-Circuit Formula for Aluminum Conductor_

    $$
    (I^2|A^2)t = 0.0125 \log_{10} [(T_{2} + 228)/(T_{1} + 228)]
    $$

where:

$I$ = short-circuit current in amperes

$A$ = conductor area in circular mils

$t$ = time of short circuit in seconds
(for times less than or equal to 10 seconds)

$T_{1}$ = initial conductor temperature in degrees Celsius

$T_{2}$ = final conductor temperature in degrees Celsius

Copper conductor with paper, rubber, varnished cloth insulation, $T_{2}$ = 200

Copper conductor with thermoplastic insulation, $T_{2}$ = 150

Copper conductor with cross-linked polyethylene insulation, $T_{2}$ = 250

Copper conductor with ethylene propylene rubber insulation, $T_{2}$ = 250

Aluminum conductor with paper, rubber, varnished cloth insulation, $T_{2}$ = 200

Aluminum conductor with thermoplastic insulation, $T_{2}$ = 150

Aluminum conductor with cross-linked polyethylene insulation, $T_{2}$ = 250

Aluminum conductor with ethylene propylene rubber insulation, $T_{2}$ = 250

#### 240.92(C) Transformer Secondary Conductors of Separately Derived Systems.

Conductors shall be permitted to be connected to a transformer secondary of a separately derived system, without overcurrent protection at the connection, where the conditions of 240.92(C)(1), (C)(2), and (C)(3) are met.

##### 240.92(C)(1) Short-Circuit and Ground-Fault Protection.

The conductors shall be protected from short-circuit and ground-fault conditions by complying with one of the following conditions:

* (1) The length of the secondary conductors does not exceed 30 m (100 ft), and the transformer primary overcurrent device has a rating or setting that does not exceed 150 percent of the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

* (2) The conductors are protected by a differential relay with a trip setting equal to or less than the conductor ampacity.

  > [!info] Informational Note:
  > A differential relay is connected to be sensitive only to short-circuit or fault currents within the protected zone and is normally set much lower than the conductor ampacity. The differential relay is connected to trip protective devices that deenergize the protected conductors if a short-circuit condition occurs.

* (3) The conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors within recognized time vs. current limits for all short-circuit and groundfault conditions.

##### 240.92(C)(2) Overload Protection.

The conductors shall be protected against overload conditions by complying with one of the following:

* (1) The conductors terminate in a single overcurrent device that will limit the load to the conductor ampacity.

* (2) The sum of the overcurrent devices at the conductor termination limits the load to the conductor ampacity. The overcurrent devices shall consist of not more than six circuit breakers or sets of fuses mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard or switchgear. There shall be no more than six overcurrent devices grouped in any one location.

* (3) Overcurrent relaying is connected \[with a current transformer(s), if needed\] to sense all of the secondary conductor current and limit the load to the conductor ampacity by opening upstream or downstream devices.

* (4) Conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors from overload conditions.

##### 240.92(C)(3) Physical Protection.

The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

#### 240.92(D) Outside Feeder Taps.

Outside conductors shall be permitted to be tapped to a feeder or to be connected at a transformer secondary, without overcurrent protection at the tap or connection, where all the following conditions are met:

* (1) The conductors are protected from physical damage in an approved manner.

* (2) The sum of the overcurrent devices at the conductor termination limits the load to the conductor ampacity. The overcurrent devices shall consist of not more than six circuit breakers or sets of fuses mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard or switchgear. There shall be no more than six overcurrent devices grouped in any one location.

* (3) The tap conductors are installed outdoors of a building or structure except at the point of load termination.

* (4) The overcurrent device for the conductors is an integral part of a disconnecting means or is located immediately adjacent thereto.

* (5) The disconnecting means for the conductors are installed at a readily accessible location complying with one of the following:
  * a. Outside of a building or structure
  * b. Inside, nearest the point of entrance of the conductors
  * c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors

#### 240.92(E) Protection by Primary Overcurrent Device.

Conductors supplied by the secondary side of a transformer shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided the primary device time--current protection characteristic, multiplied by the maximum effective primary-to-secondary transformer voltage ratio, effectively protects the secondary conductors.

## Part IX. Overcurrent Protection over 1000 Volts, Nominal

### 240.100 Feeders and Branch Circuits.

#### 240.100(A) Location and Type of Protection.

Feeder and branch-circuit conductors
shall have overcurrent protection in each ungrounded conductor
located at the point where the conductor receives its supply
or at an alternative location in the circuit
when designed under engineering supervision
that includes but is not limited to considering the appropriate fault studies and time--current coordination analysis of the protective devices and the conductor damage curves.
The overcurrent protection shall be permitted to be provided by either 240.100(A)(1) or (A)(2).

##### 240.100(A)(1) Overcurrent Relays and Current Transformers.

Circuit breakers used for overcurrent protection of 3-phase circuits shall have a minimum of three overcurrent relay elements operated from three current transformers. The separate overcurrent relay elements (or protective functions) shall be permitted to be part of a single electronic protective relay unit.

On 3-phase, 3-wire circuits, an overcurrent relay element in the residual circuit of the current transformers shall be permitted to replace one of the phase relay elements.

An overcurrent relay element, operated from a current transformer that links all phases of a 3-phase, 3-wire circuit, shall be permitted to replace the residual relay element and one of the phase-conductor current transformers. Where the neutral conductor is not regrounded on the load side of the circuit as permitted in 250.184(B), the current transformer shall be permitted to link all 3-phase conductors and the grounded circuit conductor (neutral).

##### 240.100(A)(2) Fuses.

A fuse shall be connected in series with each ungrounded conductor.

#### 240.100(B) Protective Devices.

The protective device(s) shall be capable of detecting and interrupting all values of current that can occur at their location in excess of their trip-setting or melting point.

#### 240.100(C) Conductor Protection.

The operating time of the protective device, the available short-circuit current, and the conductor used shall be coordinated to prevent damaging or dangerous temperatures in conductors or conductor insulation under short-circuit conditions.

### 240.101 Additional Requirements for Feeders.

#### 240.101(A) Rating or Setting of Overcurrent Protective Devices.

The continuous ampere rating of a fuse shall not exceed three times the ampacity of the conductors. The long-time trip element setting of a breaker or the minimum trip setting of an electronically actuated fuse shall not exceed six times the ampacity of the conductor. For fire pumps, conductors shall be permitted to be protected for overcurrent in accordance with 695.4(B)(2).

#### 240.101(B) Feeder Taps.

Conductors tapped to a feeder shall be permitted to be protected by the feeder overcurrent device where that overcurrent device also protects the tap conductor.

### 240.102 Reconditioned Equipment.

Medium-voltage fuseholders and medium-voltage nonrenewable fuses shall not be permitted to be reconditioned.
