---
id:
aliases: []
tags:
  - destiny/fleeting
  - status/incomplete
  - topic/construction/electrical
  - type/encyclopedia
  - authorship/original
title: Conductor Sizing
---
# Conductor Sizing

Conductors are sized to be suitable for the load,
overcurrent protection is sized to protect the conductors.

## "The 80% Rule"

> [!danger]
> This description is provided _for reference only_.
> 
> Like every NEC "rule"
> that isn't preceded by a section reference,
> it is _not_ code.
> 
> It is my opinion that this should never be repeated,
> even as shorthand.

"The 80% Rule" is a rule of thumb
referring to a common convention of several articles
including:

* [[nfpa-70_210_branch-circuits#210.19(A)(1) General.]]
* [[nfpa-70_215_feeders#215.2(A)(1) General.]]
* [[nfpa-70_430_motors#430.22 Single Motor.]]

which paraphrased states:

> ... the minimum conductor size shall have an ampacity
> not less than the noncontinuous load
> plus 125 percent of the continuous load

When the rule is repeated,
the noncontinuous load is ignored
and it is stated that conductors are suitable
for 80% their listed rating,
since 80% is the reciprocal of 125%.

The rule neglects important context and common exceptions,
namely transformers, whose feeder conductors are sized at 100%.

![[nfpa-70_215_feeders#215.2(B)(1) Feeders Supplying Transformers.]]

## Branch Circuits

### Receptacle Branch

There is no maximum number of receptacles per circuit _in any occupancy_.

It is a common misconception that the limit
can be calculated with a formula like

$$
\frac{1.25(180VA)}{120V} = 1.875A, \quad \frac{20A}{1.875A} = 10.\bar{6}
$$

but the 180VA per yoke load specified in
[[nfpa-70_220_load-calculations#220.14(I) Receptacle Outlets.|220.14(I)]]
is specifically for calculating service and feeder sizing.

Per [[nfpa-70_210_branch-circuits#210.19(A)(1) General.|210.19(A)(1)]]
a receptacle branch circuit's load
is the load of the equipment intended to be served by it.

Where general-use receptacles are provided
without specific equipment in mind,
circuits will be engineered at the minimum load.
If a receptacle circuit's load is a whole multiple of 180VA
there's a good chance that's the number of devices,
or at least was at some point in the design.

## Feeders

![[nfpa-70_250_grounding-and-bonding#250.122(A) General.]]

Apparently in the 2026 NEC First Draft Meetings,
Code Making Panel 5 clarified that the equipment grounding conductor (EGC)
never needs to be larger than the largest ungrounded conductor in any raceway
when installed in parallel.
I can not find a source to verify this.
Statements from other reputable sources including Mike Holt
are in contradiction to this idea.

***

Given a minimum ampacity, find all valid configurations.

[[nfpa-70_310_conductors_for_general_wiring]]

> [!cite] NEC Article 310 (emphasis added)
> #### 310.10(H) Conductors in Parallel.
> ##### (1) General.
> Aluminum, copper-clad aluminum, or copper conductors,
> for each phase, polarity, neutral, or grounded circuit
> shall be permitted to be connected in parallel
> (electrically joined at both ends)
> _only in sizes 1/0 AWG and larger_
> where installed in accordance with 310.10(H)(2) through (H)(6).

Rank by total cost of install.

### Complexity to Ignore

#### Conductor Material

Tinned copper and copper-clad aluminum conductors
can be assumed out of scope.

### Complexity to Respect

#### Equipment Grounding Conductor Material

Wire and EGC conductors are usually assumed to match,
but it is sometimes necessary to use a copper EGC with aluminum wires,
either for spec requirements or conduit fill considerations.

## Voltage Drop

[[voltage-drop]]

## Transformers

$$
I = \frac{S}{ \sqrt{3} \times V \times E }
$$

* $I$ = nameplate current rating
* $S$ = nameplate kVA rating
* $V$ = feeder voltage
* $E$ = efficiency

## Motors

1 electric horsepower = 746 watts

full-load current (FLC) / full-load amperes (FLA)

minimum circuit ampacity (MCA)

$$
\text{MCA} = 1.25 \times \text{FLC}
$$
