This article covers the installation requirements, overcurrent protection requirements, minimum size, and ampacity of conductors for
feeders.
This article covers the installation requirements, overcurrent protection requirements, minimum size, and ampacity of conductors for feeders.
Exception:
Feeders for electrolytic cells as covered in 668.3(C)(1) and (C)(4).
> [!important] Exception:
> Feeders for electrolytic cells as covered in 668.3(C)(1) and (C)(4).
#### 215.2 Minimum Rating and Size.
@@ -35,54 +34,180 @@ and shall comply with 110.14(C).
or any combination of continuous and noncontinuous loads,
the minimum feeder conductor size shall have an ampacity
not less than the noncontinuous load plus 125 percent of the continuous load.
> [!important] Exception No. 1:
> If the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
> [!important] Exception No. 2:
> Where a portion of a feeder is connected at both its supply and load ends to separately installed pressure connections as covered in 110.14(C)(2), it shall be permitted to have an ampacity not less than the sum of the continuous load plus the noncontinuous load. No portion of a feeder installed under this exception shall extend into an enclosure containing either the feeder supply or the feeder load terminations, as covered in 110.14(C)(1).
> [!important] Exception No. 3:
> Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.
* (b) The minimum feeder conductor size
shall have an ampacity not less than the maximum load to be served
after the application of any adjustment or correction factors
in accordance with
[[nfpa-70_310_conductors_for_general_wiring#310.14 Ampacities for Conductors Rated 0 Volts -- 2000 Volts.|310.14]].
Exception No. 1:
If the assembly, including the overcurrent devices protecting the feeder(s),
is listed for operation at 100 percent of its rating,
the ampacity of the feeder conductors shall be permitted to be not less
than the sum of the continuous load plus the noncontinuous load.
> [!info] Informational Note No. 1:
> See Examples D1 through D11 in Informative Annex D.
Exception No. 2:
Where a portion of a feeder is connected at both its supply and load ends
to separately installed pressure connections as covered in 110.14(C)(2),
it shall be permitted to have an ampacity
not less than the sum of the continuous load plus the noncontinuous load.
No portion of a feeder installed under this exception
shall extend into an enclosure containing either the feeder supply
or the feeder load terminations, as covered in 110.14(C)(1).
> [!info] Informational Note No. 2:
> Conductors for feeders, as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation.
Exception No. 3:
Grounded conductors that are not connected to an overcurrent device
shall be permitted to be sized at 100 percent
of the continuous and noncontinuous load.
> [!info] Informational Note No. 3:
> See 210.19(A), Informational Note No. 4, for voltage drop for branch circuits.
Informational Note No. 1:
See Examples D1 through D11 in Informative Annex D.
##### 215.2(A)(2) Grounded Conductor.
Informational Note No. 2:
Conductors for feeders, as defined in Article 100,
sized to prevent a voltage drop exceeding 3 percent
at the farthest outlet of power, heating, and lighting loads,
or combinations of such loads,
and where the maximum total voltage drop on both feeders and branch circuits
to the farthest outlet does not exceed 5 percent,
will provide reasonable efficiency of operation.
The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.
Informational Note No. 3:
See 210.19(A), Informational Note No. 4, for voltage drop for branch circuits.
Additional minimum sizes shall be as specified in 215.2(A)(3) under the conditions stipulated.
#### 215.2(A)(2) Grounded Conductor.
##### 215.2(A)(3) Ampacity Relative to Service Conductors.
The size of the feeder circuit grounded conductor
shall not be smaller than that required by 250.122,
except that 250.122(F) shall not apply
where grounded conductors are run in parallel.
The feeder conductor ampacity shall not be less than that of the service conductors where the feeder conductors carry the total load supplied by service conductors with an ampacity of 55 amperes or less.
Additional minimum sizes shall be as specified in 215.2(A)(3)
under the conditions stipulated.
#### 215.2(B) Feeders over 1000 Volts.
The ampacity of conductors shall be in accordance with 310.14 and 311.60 as applicable. Where installed, the size of the feeder-circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel. Feeder conductors over 1000 volts shall be sized in accordance with 215.2(B)(1), (B)(2), or (B)(3).
##### 215.2(B)(1) Feeders Supplying Transformers.
The ampacity of feeder conductors
shall not be less than the sum of the nameplate ratings of the transformers supplied
when only transformers are supplied.
##### 215.2(B)(2) Feeders Supplying Transformers and Utilization Equipment.
The ampacity of feeders supplying a combination
of transformers and utilization equipment
shall not be less than the sum of the nameplate ratings of the transformers
and 125 percent of the designed potential load of the utilization equipment
that will be operated simultaneously.
##### 215.2(B)(3) Supervised Installations.
For supervised installations, feeder conductor sizing shall be permitted to be determined by qualified persons under engineering supervision in accordance with 310.14(B) or 311.60(B). Supervised installations are defined as those portions of a facility where all of the following conditions are met:
* (1) Conditions of design and installation are provided under engineering supervision.
* (2) Qualified persons with documented training and experience in over 1000-volt systems provide maintenance, monitoring, and servicing of the system.
### 215.3 Overcurrent Protection.
Feeders shall be protected against overcurrent in accordance with Part I of Article 240. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.
> [!important] Exception:
> Where the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
### 215.4 Feeders with Common Neutral Conductor.
#### 215.4(A) Feeders with Common Neutral.
Up to three sets of 3-wire feeders or two sets of 4-wire or 5-wire feeders shall be permitted to utilize a common neutral.
#### 215.4(B) In Metal Raceway or Enclosure.
Where installed in a metal raceway or other metal enclosure, all conductors of all feeders using a common neutral conductor shall be enclosed within the same raceway or other enclosure as required in 300.20.
### 215.5 Diagrams of Feeders.
If required by the authority having jurisdiction, a diagram showing feeder details shall be provided prior to the installation of the feeders.
Such a diagram shall show the area in square feet of the building or other structure supplied by each feeder, the total calculated load before applying demand factors, the demand factors used, the calculated load after applying demand factors, and the size and type of conductors to be used.
### 215.6 Feeder Equipment Grounding Conductor.
Where a feeder supplies branch circuits
in which equipment grounding conductors are required,
the feeder shall include or provide an equipment grounding conductor,
to which the equipment grounding conductors of the branch circuits shall be connected.
Where the feeder supplies a separate building or structure,
the requirements of [[nfpa-70_250_grounding-and-bonding#250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).|250.32]] shall apply.
### 215.7 Ungrounded Conductors Tapped from Grounded Systems.
Two-wire dc circuits and ac circuits of two or more ungrounded conductors shall be permitted to be tapped from the ungrounded conductors of circuits having a grounded neutral conductor. Switching devices in each tapped circuit shall have a pole in each ungrounded conductor.
### 215.9 Ground-Fault Circuit-Interrupter Protection for Personnel.
Feeders shall be permitted to be protected by a ground-fault circuit interrupter installed in a readily accessible location in lieu of the provisions for such interrupters as specified in 210.8 and 590.6(A).
### 215.10 Ground-Fault Protection of Equipment.
Each feeder disconnect rated 1000 amperes or more and installed on solidly grounded wye electrical systems of more than 150 volts to ground, but not exceeding 600 volts phase-to-phase, shall be provided with ground-fault protection of equipment in accordance with 230.95.
> [!info] Informational Note:
> For buildings that contain health care occupancies, see 517.17.
> [!important] Exception No. 1:
> This section shall not apply to a disconnecting means for a continuous industrial process where a nonorderly shutdown will introduce additional or increased hazards.
> [!important] Exception No. 2:
> This section shall not apply if ground-fault protection of equipment is provided on the supply side of the feeder and on the load side of any transformer supplying the feeder.
> [!important] Exception No. 3:
> If temporary feeder conductors are used to connect a generator to a facility for repair, maintenance, or emergencies, ground-fault protection of equipment shall not be required. Temporary feeders without ground-fault protection shall be permitted for the time period necessary but shall not exceed 90 days.
### 215.11 Circuits Derived from Autotransformers.
Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.
> [!important] Exception No. 1:
> An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts.
> [!important] Exception No. 2:
> In industrial occupancies, where conditions of maintenance and supervision ensure that only qualified persons service the installation, autotransformers shall be permitted to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from nominal 600-volt systems, without the connection to a similar grounded conductor.
### 215.12 Identification for Feeders.
#### 215.12(A) Grounded Conductor.
The grounded conductor of a feeder, if insulated, shall be identified in accordance with 200.6.
#### 215.12(B) Equipment Grounding Conductor.
The equipment grounding conductor shall be identified in accordance with 250.119.
#### 215.12(C) Identification of Ungrounded Conductors.
Ungrounded conductors shall be identified in accordance with 215.12(C)(1) or (C)(2), as applicable.
##### 215.12(C)(1) Feeders Supplied from More Than One Nominal Voltage System.
Where the premises wiring system has feeders supplied from more than one nominal voltage system, each ungrounded conductor of a feeder shall be identified by phase or line and system at all termination, connection, and splice points in compliance with 215.12(C)(1)
* (a) and (b).
* (a) Means of Identification. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means.
* (b) Posting of Identification Means. The method utilized for conductors originating within each feeder panelboard or similar feeder distribution equipment shall be documented in a manner that is readily available or shall be permanently posted at each feeder panelboard or similar feeder distribution equipment.
##### 215.12(C)(2) Feeders Supplied from Direct-Current Systems.
Where a feeder is supplied from a dc system operating at more than 60 volts, each ungrounded conductor of 4 AWG or larger shall be identified by polarity at all termination, connection, and splice points by marking tape, tagging, or other approved means; each ungrounded conductor of 6 AWG or smaller shall be identified by polarity at all termination, connection, and splice points in compliance with 215.12(C)(2)(a) and (b). The identification methods utilized for conductors originating within each feeder panelboard or similar feeder distribution equipment shall be documented in a manner that is readily available or shall be permanently posted at each feeder panelboard or similar feeder distribution equipment.
* (a) Positive Polarity, Sizes 6 AWG or Smaller. Where the positive polarity of a dc system does not serve as the connection for the grounded conductor, each positive ungrounded conductor shall be identified by one of the following means:
* (1) A continuous red outer finish
* (2) A continuous red stripe durably marked along the conductor’s entire length on insulation of a color other than green, white, gray, or black
* (3) Imprinted plus signs (+) or the word POSITIVE or POS durably marked on insulation of a color other than green, white, gray, or black, and repeated at intervals not exceeding 610 mm (24 in.) in accordance with 310.8(B)
* (4) An approved permanent marking means such as sleeving or shrink-tubing that is suitable for the conductor size, at all termination, connection, and splice points, with imprinted plus signs (+) or the word POSITIVE or POS durably marked on insulation of a color other than green, white, gray, or black
* (b) Negative Polarity, Sizes 6 AWG or Smaller. Where the negative polarity of a dc system does not serve as the connection for the grounded conductor, each negative ungrounded conductor shall be identified by one of the following means:
* (1) A continuous black outer finish
* (2) A continuous black stripe durably marked along the conductor’s entire length on insulation of a color other than green, white, gray, or red
* (3) Imprinted minus signs (–) or the word NEGATIVE or NEG durably marked on insulation of a color other than green, white, gray, or red, and repeated at intervals not exceeding 610 mm (24 in.) in accordance with 310.8(B)
* (4) An approved permanent marking means such as sleeving or shrink-tubing that is suitable for the conductor size, at all termination, connection, and splice points, with imprinted minus signs (–) or the word NEGATIVE or NEG durably marked on insulation of a color other than green, white, gray, or red
title: Article 450 Transformers and Transformer Vaults (Including Secondary Ties)
@@ -19,45 +19,550 @@ title: Article 450 Transformers and Transformer Vaults (Including Secondary Ties
This article covers the installation of all transformers.
Exception No. 1:
Current transformers.
> [!important] Exception No. 1:
> Current transformers.
Exception No. 2:
Dry-type transformers
that constitute a component part of other apparatus
and comply with the requirements for such apparatus.
> [!important] Exception No. 2:
> Dry-type transformers that constitute a component part of other apparatus and comply with the requirements for such apparatus.
Exception No. 3:
Transformers that are an integral part
of an X-ray, high-frequency, or electrostatic-coating apparatus.
> [!important] Exception No. 3:
> Transformers that are an integral part of an X-ray, high-frequency, or electrostatic-coating apparatus.
Exception No. 4:
Transformers used with Class 2 and Class 3 circuits
that comply with Article 725.
> [!important] Exception No. 4:
> Transformers used with Class 2 and Class 3 circuits that comply with Article 725.
Exception No. 5:
Transformers for sign and outline lighting
that comply with Article 600.
> [!important] Exception No. 5:
> Transformers for sign and outline lighting that comply with Article 600.
Exception No. 6:
Transformers for electric-discharge lighting
that comply with Article 410.
> [!important] Exception No. 6:
> Transformers for electric-discharge lighting that comply with Article 410.
Exception No. 7:
Transformers used for power-limited fire alarm circuits
that comply with Part III of Article 760.
> [!important] Exception No. 7:
> Transformers used for power-limited fire alarm circuits that comply with Part III of Article 760.
Exception No. 8:
Transformers used for research, development, or testing,
where effective arrangements are provided
to safeguard persons from contacting energized parts.
> [!important] Exception No. 8:
> Transformers used for research, development, or testing, where effective arrangements are provided to safeguard persons from contacting energized parts.
This article covers the installation of transformers
dedicated to supplying power to a fire pump installation
as modified by Article 695.
This article covers the installation of transformers dedicated to supplying power to a fire pump installation as modified by Article 695.
This article also covers the installation of transformers
in hazardous (classified) locations
as modified by Articles 501 through 504.
This article also covers the installation of transformers in hazardous (classified) locations as modified by Articles 501 through 504.
<!-- TODO: TEXT OMITTED -->
### 450.2 Definition.
The definitions in this section shall apply only within this article.
Transformer.
An individual transformer, single or polyphase, identified by a single nameplate, unless otherwise indicated in this article.
### 450.3 Overcurrent Protection.
Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit.
> [!info] Informational Note No. 1:
> See 240.4, 240.21, 240.100, and 240.101 for overcurrent protection of conductors.
> [!info] Informational Note No. 2:
> Nonlinear loads can increase heat in a transformer without operating its overcurrent protective device.
#### 450.3(A) Transformers Over 1000 Volts, Nominal.
Overcurrent protection shall be provided in accordance with Table 450.3(A).
#### 450.3(B) Transformers 1000 Volts, Nominal, or Less.
Overcurrent protection shall be provided in accordance with Table 450.3(B).
> [!important] Exception:
> Where the transformer is installed as a motor control circuit transformer in accordance with 430.72(C)(1) through (C)(5).
Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts
(as a Percentage of Transformer-Rated Current)
%% TODO %%
Notes:
1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the following shall be permitted:
* a. The next higher standard rating or setting for fuses and circuit breakers 1000 volts and below, or
* b. The next higher commercially available rating or setting for fuses and circuit breakers above 1000 volts.
1. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses.
2. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the transformer installation.
3. Electronically actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers.
4. A transformer equipped with a coordinated thermal overload protection by the manufacturer shall be permitted to have separate secondary protection omitted.
Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 1000 Volts and Less (as a Percentage of Transformer-Rated Current)
%% TODO %%
Notes:
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.
2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device.
3. A transformer equipped with coordinated thermal overload protection by the manufacturer and arranged to interrupt the primary current shall be permitted to have primary overcurrent protection rated or set at a current value that is not more than six times the rated current of the transformer for transformers having not more than 6 percent impedance and not more than four times the rated current of the transformer for transformers having more than 6 percent but not more than 10 percent impedance.
#### 450.3(C) Voltage (Potential) Transformers.
Voltage (potential) transformers installed indoors or enclosed shall be protected with primary fuses.
> [!info] Informational Note:
> For protection of instrument circuits including voltage transformers, see 408.52.
### 450.4 Autotransformers 1000 Volts, Nominal, or Less.
#### 450.4(A) Overcurrent Protection.
Each autotransformer 1000 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. Where this calculation does not correspond to a standard rating of a fuse or nonadjustable circuit breaker and the rated input current is 9 amperes or more, the next higher standard rating described in 240.6 shall be permitted. An overcurrent device shall not be installed in series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between Points A and B as shown in Figure 450.4(A).
> [!info] Figure 450.4(A) Autotransformer.
> [!important] Exception:
> Where the rated input current of the autotransformer is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the input current shall be permitted.
#### 450.4(B) Transformer Field-Connected as an Autotransformer.
A transformer field-connected as an autotransformer shall be identified for use at elevated voltage.
> [!info] Informational Note:
> For information on permitted uses of autotransformers, see 210.9 and 215.11.
### 450.5 Grounding Autotransformers.
Grounding autotransformers covered in this section are zigzag or T-connected transformers connected to 3-phase, 3-wire ungrounded systems for the purpose of creating a 3-phase, 4-wire distribution system or providing a neutral point for grounding purposes. Such transformers shall have a continuous per-phase current rating and a continuous neutral current rating. Zigzag-connected transformers shall not be installed on the load side of any system grounding connection, including those made in accordance with 250.24(B),
250.30(A)(1), or 250.32(B), Exception No. 1.
> [!info] Informational Note:
> The phase current in a grounding autotransformer is one-third the neutral current.
#### 450.5(A) Three-Phase, 4-Wire System.
A grounding autotransformer used to create a 3-phase, 4-wire distribution system from a 3-phase, 3-wire ungrounded system shall conform to 450.5(A) (1) through (A)(4).
##### 450.5(A)(1) Connections.
The transformer shall be directly connected to the ungrounded phase conductors and shall not be switched or provided with overcurrent protection that is independent of the main switch and common-trip overcurrent protection for the 3-phase, 4-wire system.
##### 450.5(A)(2) Overcurrent Protection.
An overcurrent sensing device shall be provided that will cause the main switch or common-trip overcurrent protection referred to in
450.5(A)(1) to open if the load on the autotransformer reaches or exceeds 125 percent of its continuous current per-phase or neutral rating. Delayed tripping for temporary overcurrents sensed at the autotransformer overcurrent device shall be permitted for the purpose of allowing proper operation of branch or feeder protective devices on the 4-wire system.
##### 450.5(A)(3) Transformer Fault Sensing.
A fault-sensing system that causes the opening of a main switch or common-trip overcurrent device for the 3-phase, 4-wire system shall be provided to guard against single-phasing or internal faults.
> [!info] Informational Note:
> This can be accomplished by the use of two subtractive-connected donut-type current transformers installed to sense and signal when an unbalance occurs in the line current to the autotransformer of 50 percent or more of rated current.
##### 450.5(A)(4) Rating.
The autotransformer shall have a continuous neutral-current rating that is not less than the maximum possible neutral unbalanced load current of the 4-wire system.
#### 450.5(B) Ground Reference for Fault Protection Devices.
A grounding autotransformer used to make available a specified magnitude of ground-fault current for operation of a ground-responsive protective device on a 3-phase, 3-wire ungrounded system shall conform to 450.5(B)(1) and (B)(2).
##### 450.5(B)(1) Rating.
The autotransformer shall have a continuous neutral-current rating not less than the specified ground-fault current.
##### 450.5(B)(2) Overcurrent Protection.
Overcurrent protection shall comply with 450.5(B)(2)(a) and (B)(2)(b).
* (a) Operation and Interrupting Rating. An overcurrent protective device having an interrupting rating in compliance with 110.9 and that will open simultaneously all ungrounded conductors when it operates shall be applied in the grounding autotransformer branch circuit.
* (b) Ampere Rating. The overcurrent protection shall be rated or set at a current not exceeding 125 percent of the autotransformer continuous per-phase current rating or 42 percent of the continuous-current rating of any series-connected devices in the autotransformer neutral connection. Delayed tripping for temporary overcurrents to permit the proper operation of groundresponsive tripping devices on the main system shall be permitted but shall not exceed values that would be more than the short-time current rating of the grounding autotransformer or any series connected devices in the neutral connection thereto.
> [!important] Exception:
> For high-impedance grounded systems covered in 250.36, where the maximum ground-fault current is designed to be not more than 10 amperes, and where the grounding autotransformer and the grounding impedance are rated for continuous duty, an overcurrent device rated not more than 20 amperes that will simultaneously open all ungrounded conductors shall be permitted to be installed on the line side of the grounding autotransformer.
#### 450.5(C) Ground Reference for Damping Transitory Overvoltages.
A grounding autotransformer used to limit transitory overvoltages shall be of suitable rating and connected in accordance with 450.5(A)(1).
### 450.6 Secondary Ties.
As used in this article, a secondary tie is a circuit operating at 1000 volts, nominal, or less between phases that connects two power sources or power supply points, such as the secondaries of two transformers. The tie shall be permitted to consist of one or more conductors per phase or neutral. Conductors connecting the secondaries of transformers in accordance with 450.7 shall not be considered secondary ties.
As used in this section, the word transformer means a transformer or a bank of transformers operating as a unit.
#### 450.6(A) Tie Circuits.
Tie circuits shall be provided with overcurrent protection at each end as required in Parts I, II, and VIII of Article 240.
Under the conditions described in 450.6(A)(1) and 450.6(A)(2), the overcurrent protection shall be permitted to be in accordance with 450.6(A)(3).
##### 450.6(A)(1) Loads at Transformer Supply Points Only.
Where all loads are connected at the transformer supply points at each end of the tie and overcurrent protection is not provided in accordance with Parts I, II, and VIII of Article 240, the ampacity of the tie shall not be less than 67 percent of the rated secondary current of the highest rated transformer supplying the secondary tie system.
##### 450.6(A)(2) Loads Connected Between Transformer Supply Points.
Where load is connected to the tie at any point between transformer supply points and overcurrent protection is not provided in accordance with Parts I, II, and VIII of Article 240, the ampacity of the tie shall not be less than 100 percent of the rated secondary current of the highest rated transformer supplying the secondary tie system.
> [!important] Exception:
> Tie circuits comprised of multiple conductors per phase shall be permitted to be sized and protected in accordance with 450.6(A)(4).
##### 450.6(A)(3) Tie Circuit Protection.
Under the conditions described in 450.6(A)(1) and (A)(2), both supply ends of each ungrounded tie conductor shall be equipped with a protective device that opens at a predetermined temperature of the tie conductor under short-circuit conditions. This protection shall consist of one of the following: (1) a fusible link cable connector, terminal, or lug, commonly known as a limiter, each being of a size corresponding with that of the conductor and of construction and characteristics according to the operating voltage and the type of insulation on the tie conductors or (2) automatic circuit breakers actuated by devices having comparable time--current characteristics.
##### 450.6(A)(4) Interconnection of Phase Conductors Between Transformer Supply Points.
Where the tie consists of more than one conductor per phase or neutral, the conductors of each phase or neutral shall comply with
450.6(A)(4)(a) or (A)(4)(b).
* (a) Interconnected. The conductors shall be interconnected in order to establish a load supply point, and the protective device specified in 450.6(A)(3) shall be provided in each ungrounded tie conductor at this point on both sides of the interconnection.
The means of interconnection shall have an ampacity not less than the load to be served.
* (b) Not Interconnected. The loads shall be connected to one or more individual conductors of a paralleled conductor tie without interconnecting the conductors of each phase or neutral and without the protection specified in 450.6(A)(3) at load connection points. Where this is done, the tie conductors of each phase or neutral shall have a combined capacity ampacity of not less than 133 percent of the rated secondary current of the highest rated transformer supplying the secondary tie system, the total load of such taps shall not exceed the rated secondary current of the highest rated transformer, and the loads shall be equally divided on each phase and on the individual conductors of each phase as far as practicable.
##### 450.6(A)(5) Tie Circuit Control.
Where the operating voltage exceeds 150 volts to ground, secondary ties provided with limiters shall have a switch at each end that, when open, de-energizes the associated tie conductors and limiters. The current rating of the switch shall not be less than the rated current ampacity of the conductors connected to the switch. It shall be capable of interrupting its rated current, and it shall be constructed so that it will not open under the magnetic forces resulting from short-circuit current.
#### 450.6(B) Overcurrent Protection for Secondary Connections.
Where secondary ties are used, an overcurrent device rated or set at not more than 250 percent of the rated secondary current of the transformers shall be provided in the secondary connections of each transformer supplying the tie system. In addition, an automatic circuit breaker actuated by a reverse-current relay set to open the circuit at not more than the rated secondary current of the transformer shall be provided in the secondary connection of each transformer.
#### 450.6(C) Grounding.
Where the secondary tie system is grounded, each transformer secondary supplying the tie system shall be grounded in accordance with the requirements of 250.30 for separately derived systems.
### 450.7 Parallel Operation.
Transformers shall be permitted to be operated in parallel and switched as a unit, provided the overcurrent protection for each transformer meets the requirements of 450.3(A) for primary and secondary protective devices over 1000 volts, or 450.3(B) for primary and secondary protective devices 1000 volts or less.
### 450.8 Guarding.
Transformers shall be guarded as specified in 450.8(A) through (D).
#### 450.8(A) Mechanical Protection.
Appropriate provisions shall be made to minimize the possibility of damage to transformers from external causes where the transformers are exposed to physical damage.
#### 450.8(B) Case or Enclosure.
Dry-type transformers shall be provided with a noncombustible moisture-resistant case or enclosure that provides protection against the accidental insertion of foreign objects.
#### 450.8(C) Exposed Energized Parts.
Switches or other equipment operating at 1000 volts, nominal, or less and serving only equipment within a transformer enclosure shall be permitted to be installed in the transformer enclosure if accessible to qualified persons only. All energized parts shall be guarded in accordance with 110.27 and 110.34.
#### 450.8(D) Voltage Warning.
The operating voltage of exposed live parts of transformer installations shall be indicated by signs or visible markings on the equipment or structures.
### 450.9 Ventilation.
The ventilation shall dispose of the transformer full-load heat losses without creating a temperature rise that is in excess of the transformer rating.
> [!info] Informational Note No. 1:
> See IEEE C57.12.00-2015, General Requirements for Liquid-Immersed Distribution, Power, and Regulating
Transformers, and IEEE C57.12.01-2015, General Requirements for Dry-Type Distribution and Power Transformers.
> [!info] Informational Note No. 2:
> Additional losses occur in some transformers where nonsinusoidal currents are present, resulting in increased heat in the transformer above its rating. See IEEE C57.110-2008, Recommended Practice for Establishing Liquid-Filled and Dry-Type
Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents, where transformers are utilized with nonlinear loads.
Transformers with ventilating openings shall be installed so that the ventilating openings are not blocked by walls or other obstructions.
The required clearances shall be clearly marked on the transformer. Transformer top surfaces that are horizontal and readily accessible shall be marked to prohibit storage.
### 450.10 Grounding.
#### 450.10(A) Dry-Type Transformer Enclosures.
Where separate equipment grounding conductors and supply-side bonding jumpers are installed, a terminal bar for all grounding and bonding conductor connections shall be secured inside the transformer enclosure. The terminal bar shall be bonded to the enclosure in accordance with 250.12 and shall not be installed on or over any vented portion of the enclosure.
> [!important] Exception:
> Where a dry-type transformer is equipped with wire-type connections (leads), the grounding and bonding connections shall be permitted to be connected together using any of the methods in 250.8 and shall be bonded to the enclosure if of metal.
#### 450.10(B) Other Metal Parts.
Exposed non--current-carrying metal parts of transformer installations, including fences, guards, and so forth, shall be grounded and bonded under the conditions and in the manner specified for electrical equipment and other exposed metal parts in Parts V, VI, and VII of Article 250.
### 450.11 Marking.
#### 450.11(A) General.
Each transformer shall be provided with a nameplate giving the following information:
* (1) Name of manufacturer
* (2) Rated kilovolt-amperes
* (3) Frequency
* (4) Primary and secondary voltage
* (5) Impedance of transformers 25 kVA and larger
* (6) Required clearances for transformers with ventilating openings
* (7) Amount and kind of insulating liquid where used
* (8) For dry-type transformers, temperature class for the insulation system
#### 450.11(B) Source Marking.
A transformer shall be permitted to be supplied at the marked secondary voltage, provided that the installation is in accordance with the manufacturer's instructions.
### 450.12 Terminal Wiring Space.
The minimum wire-bending space at fixed, 1000-volt and below terminals of transformer line and load connections shall be as required in 312.6. Wiring space for pigtail connections shall conform to Table 314.16(B).
### 450.13 Accessibility.
All transformers and transformer vaults shall be readily accessible to qualified personnel for inspection and maintenance or shall meet the requirements of 450.13(A) or 450.13(B).
#### 450.13(A) Open Installations.
Dry-type transformers 1000 volts, nominal, or less, located in the open on walls, columns, or structures, shall not be required to be readily accessible.
#### 450.13(B) Hollow Space Installations.
Dry-type transformers 1000 volts, nominal, or less and not exceeding 50 kVA shall be permitted in hollow spaces of buildings not permanently closed in by structure, provided they meet the ventilation requirements of 450.9 and separation from combustible materials requirements of 450.21(A). Transformers so installed shall not be required to be readily accessible.
### 450.14 Disconnecting Means.
Transformers, other than Class 2 or Class 3 transformers, shall have a disconnecting means located either in sight of the transformer or in a remote location. Where located in a remote location, the disconnecting means shall be lockable open in accordance with 110.25, and its location shall be field marked on the transformer.
## Part II. Specific Provisions Applicable to Different Types of Transformers
Dry-type transformers installed indoors and rated 1121⁄ 2 kVA or less shall have a separation of at least 300 mm (12 in.) from combustible material unless separated from the combustible material by a fire-resistant, heat-insulated barrier.
> [!important] Exception:
> This rule shall not apply to transformers rated for 1000 volts, nominal, or less that are completely enclosed, except for ventilating openings.
#### 450.21(B) Over 1121⁄ 2 kVA.
Individual dry-type transformers of more than 1121⁄ 2 kVA rating shall be installed in a transformer room of fire-resistant construction having a minimum fire rating of 1 hour.
> [!important] Exception No. 1:
> Transformers with Class 155 or higher insulation systems and separated from combustible material by a fire-resistant, heat-insulating barrier or by not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically.
> [!important] Exception No. 2:
> Transformers with Class 155 or higher insulation systems and completely enclosed except for ventilating openings.
> [!info] Informational Note:
> See ASTM E119-18a, Standard Test Methods for Fire Tests of Building Construction and Materials.
#### 450.21(C) Over 35,000 Volts.
Dry-type transformers rated over 35,000 volts shall be installed in a vault complying with Part III of this article.
Dry-type transformers installed outdoors shall have a weatherproof enclosure.
Transformers exceeding 1121⁄ 2 kVA shall not be located within 300 mm (12 in.) of combustible materials of buildings unless the transformer has Class 155 insulation systems or higher and is completely enclosed except for ventilating openings.
Transformers insulated with listed less-flammable liquids that have a fire point of not less than 300°C shall be permitted to be instal led in accordance with 450.23(A) or 450.23(B).
#### 450.23(A) Indoor Installations.
Indoor installations shall be permitted in accordance with one of the following:
* (1) In Type I or Type II buildings, in areas where all of the following requirements are met:
* a. The transformer is rated 35,000 volts or less.
* b. No combustible materials are stored.
* c. A liquid confinement area is provided.
* d. The installation complies with all the restrictions provided for in the listing of the liquid.
> [!info] Informational Note:
> Such restrictions can include, but are not limited to, maximum pressure of the tank, use of a pressure relief valve, appropriate fuse types, and proper sizing of overcurrent protection.
* (2) If an automatic fire extinguishing system and a liquid confinement area is present, provided the transformer is rated 35,000 volts or less
* (3) If the installation complies with 450.26
#### 450.23(B) Outdoor Installations.
Less-flammable liquid-filled transformers shall be permitted to be installed outdoors, attached to, adjacent to, or on the roof of buildings, if instal led in accordance with (1) or (2).
* (1) For Type I and Type II buildings, the installation shall comply with all the restrictions provided for in the listing of the liquid.
> [!info] Informational Note No. 1:
> Installations adjacent to combustible material, fire escapes, or door and window openings can require additional safeguards such as those listed in 450.27.
> [!info] Informational Note No. 2:
> Such restrictions can include, but are not limited to: maximum pressure of the tank, use of a pressure relief valve, appropriate fuse types, and proper sizing of overcurrent protection.
* (2) In accordance with 450.27.
> [!info] Informational Note No. 1:
> As used in this section, Type I and Type II buildings refers to Type I and Type II building construction as defined in NFPA 220-2018, Standard on Types of Building Construction. Combustible materials refers to those materials not classified as noncombustible or limited-combustible as defined in NFPA 220-2018, Standard on Types of Building Construction.
Transformers insulated with a dielectric fluid identified as nonflammable shall be permitted to be installed indoors or outdoors. Such transformers installed indoors and rated over 35,000 volts shall be installed in a vault. Such transformers installed indoors shall be furnished with a liquid confinement area and a pressure-relief vent. The transformers shall be furnished with a means for absorbing any gases generated by arcing inside the tank, or the pressure-relief vent shall be connected to a chimney or flue that will carry such gases to an environmentally safe area.
> [!info] Informational Note:
> Safety may be increased if fire hazard analyses are performed for such transformer installations.
For the purposes of this section, a nonflammable dielectric fluid is one that does not have a flash point or fire point and is not flammable in air.
Askarel-insulated transformers installed indoors and rated over 25 kVA shall be furnished with a pressure-relief vent. Where installed in a poorly ventilated place, they shall be furnished with a means for absorbing any gases generated by arcing inside the case, or the pressure-relief vent shall be connected to a chimney or flue that carries such gases outside the building. Askarel-insulated transformers rated over 35,000 volts shall be installed in a vault.
Oil-insulated transformers installed indoors shall be installed in a vault constructed as specified in Part III of this article.
> [!important] Exception No. 1:
> Where the total capacity does not exceed 1121⁄ 2 kVA, the vault specified in Part III of this article shall be permitted to be constructed of reinforced concrete that is not less than 100 mm (4 in.) thick.
> [!important] Exception No. 2:
> Where the nominal voltage does not exceed 1000, a vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting other materials and the total capacity in one location does not exceed 10 kVA in a section of the building classified as combustible or 75 kVA where the surrounding structure is classified as fire-resistant construction.
> [!important] Exception No. 3:
> Electric furnace transformers that have a total rating not exceeding 75 kVA shall be permitted to be installed without a vault in a building or room of fire-resistant construction, provided suitable arrangements are made to prevent a transformer oil fire from spreading to other combustible material.
> [!important] Exception No. 4:
> A transformer that has a total rating not exceeding 75 kVA and a supply voltage of 1000 volts or less that is an integral part of charged-particle-accelerating equipment shall be permitted to be installed without a vault in a building or room of noncombustible or fire-resistant construction, provided suitable arrangements are made to prevent a transformer oil fire from spreading to other combustible material.
> [!important] Exception No. 5:
> Transformers shall be permitted to be installed in a detached building that does not comply with Part III of this article if neither the building nor its contents present a fire hazard to any other building or property, and if the building is used only in supplying electric service and the interior is accessible only to qualified persons.
> [!important] Exception No. 6:
> Oil-insulated transformers shall be permitted to be used without a vault in portable and mobile surface mining equipment (such as electric excavators) if each of the following conditions is met:
* (1) Provision is made for draining leaking fluid to the ground.
* (2) Safe egress is provided for personnel.
* (3) A minimum 6-mm (1⁄ 4-in.) steel barrier is provided for personnel protection.
Combustible material, combustible buildings, and parts of buildings, fire escapes, and door and window openings shall be safeguarded from fires originating in oil-insulated transformers installed on roofs, attached to or adjacent to a building or combustible material.
In cases where the transformer installation presents a fire hazard, one or more of the following safeguards shall be applied according to the degree of hazard involved:
* (1) Space separations
* (2) Fire-resistant barriers
* (3) Automatic fire suppression systems
* (4) Enclosures that confine the oil of a ruptured transformer tank
Oil enclosures shall be permitted to consist of fire-resistant dikes, curbed areas or basins, or trenches filled with coarse, crushed stone.
Oil enclosures shall be provided with trapped drains where the exposure and the quantity of oil involved are such that removal of oil is important.
> [!info] Informational Note:
> For additional information on transformers installed on poles or structures or under ground, see ANSI/IEEE C22017,
National Electrical Safety Code.
### 450.28 Modification of Transformers.
When modifications are made to a transformer in an existing installation that change the type of the transformer with respect to Part II of this article, such transformer shall be marked to show the type of insulating liquid installed, and the modified transformer installation shall comply with the applicable requirements for that type of transformer.
## Part III. Transformer Vaults
### 450.41 Location.
Vaults shall be located where they can be ventilated to the outside air without using flues or ducts wherever such an arrangement is practicable.
### 450.42 Walls, Roofs, and Floors.
The walls and roofs of vaults shall be constructed of materials that have approved structural strength for the conditions with a minimum fire resistance of 3 hours. The floors of vaults in contact with the earth shall be of concrete that is not less than 100 mm (4 in.) thick, but, where the vault is constructed with a vacant space or other stories below it, the floor shall have approved structural strength for the load imposed thereon and a minimum fire resistance of 3 hours. For the purposes of this section, studs and wallboard construction shall not be permitted.
> [!important] Exception:
> Where transformers are protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted.
> [!info] Informational Note No. 1:
> For additional information, see ASTM E119-18a, Methods for Fire Tests of Building Construction and Materials.
> [!info] Informational Note No. 2:
> A typical 3-hour construction is 150 mm (6 in.) thick reinforced concrete.
### 450.43 Doorways.
Vault doorways shall be protected in accordance with 450.43(A), (B), and (C).
#### 450.43(A) Type of Door.
Each doorway leading into a vault from the building interior shall be provided with a tight-fitting door that has a minimum fire rating of
3 hours. The authority having jurisdiction shall be permitted to require such a door for an exterior wall opening where conditions warrant.
> [!important] Exception:
> Where transformers are protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted.
> [!info] Informational Note:
> For additional information, see NFPA 80-2016, Standard for Fire Doors and Other Opening Protectives.
#### 450.43(B) Sills.
A door sill or curb that is of an approved height that will confine the oil from the largest transformer within the vault shall be provided, and in no case shall the height be less than 100 mm (4 in.).
#### 450.43(C) Locks.
Doors shall be equipped with locks, and doors shall be kept locked, with access being allowed only to qualified persons. Personnel doors shall open in the direction of egress and be equipped with listed fire exit hardware.
### 450.45 Ventilation Openings.
Where required by 450.9, openings for ventilation shall be provided in accordance with 450.45(A) through (F).
#### 450.45(A) Location.
Ventilation openings shall be located as far as possible from doors, windows, fire escapes, and combustible material.
#### 450.45(B) Arrangement.
A vault ventilated by natural circulation of air shall be permitted to have roughly half of the total area of openings required for ventilation in one or more openings near the floor and the remainder in one or more openings in the roof or in the sidewalls near the roof, or all of the area required for ventilation shall be permitted in one or more openings in or near the roof.
#### 450.45(C) Size.
For a vault ventilated by natural circulation of air to an outdoor area, the combined net area of all ventilating openings, after deducting the area occupied by screens, gratings, or louvers, shall not be less than 1900 mm (3 in.) per kVA of transformer capacity in service, and in no case shall the net area be less than 0.1 m (1 ft) for any capacity under 50 kVA.
#### 450.45(D) Covering.
Ventilation openings shall be covered with durable gratings, screens, or louvers, according to the treatment required in order to avoid unsafe conditions.
#### 450.45(E) Dampers.
All ventilation openings to the indoors shall be provided with automatic closing fire dampers that operate in response to a vault fire.
Such dampers shall possess a standard fire rating of not less than 1 1⁄2 hours.
> [!info] Informational Note:
> See ANSI/UL 555-2016, Standard for Fire Dampers.
#### 450.45(F) Ducts.
Ventilating ducts shall be constructed of fire-resistant material.
### 450.46 Drainage.
Where practicable, vaults containing more than 100 kVA transformer capacity shall be provided with a drain or other means that will carry off any accumulation of oil or water in the vault unless local conditions make this impracticable. The floor shall be pitched to the drain where provided.
### 450.47 Water Pipes and Accessories.
Any pipe or duct system foreign to the electrical installation shall not enter or pass through a transformer vault. Piping or other facilities provided for vault fire protection, or for transformer cooling, shall not be considered foreign to the electrical installation.
### 450.48 Storage in Vaults.
Materials shall not be stored in transformer vaults.
The formula for effective impedance $Z$ is given by
$$
Z = R \cos(\theta) + X \sin(\theta)
$$
where
* $Z$ = Effective impedance
* $R$ = AC resistance
* $X$ = Reactance
* $\theta$ = Power factor angle = $\arccos(PF)$
* $\theta$ = Power factor angle = $\arccos(\text{PF})$
### Parallel Runs
@@ -85,7 +86,7 @@ $$
### 3-Phase Loads
$$
\Delta V_{3\phi} = \sqrt{3} \times \left( I \times Z \times L )
\Delta V_{3\phi} = \sqrt{3} ( I \times Z \times L )
$$
where
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