Conductor Sizing

"The 80% Rule"

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

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%.

Branch Circuits

Receptacle Branch

Important

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_article-220_load-calculations#220.14(I) Receptacle Outlets. is specifically for calculating service and feeder sizing.

Per nfpa-70_article-210_branch-circuits#210.19(A)(1) General. a receptacle branch circuit's load is the load of the equipment intended to be served by it.

Feeders

[!cite] 250.122 Size of Equipment Grounding Conductors (A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment...

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.

[!cite] 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


V_d = \frac{ I \times R \times L \times M }{ P }

where

V_d = Voltage Drop in volts (V)
I = Current in Amperes (A)
R = Linear resistance in ohms per foot (\Omega\text{ft}^{-1})
L = Length of wire one way in feet (\text{ft})
M = Multiplier
- 2 for 1-phase
- \sqrt{3} for 3-phase
P = Number of parallel runs

It is often more useful to know the maximum length a certain wiring configuration is suitable for.


L = \frac{ V_d }{ I \times M } \times \frac{ P }{ R }

L = Max length of wire one way in feet (\text{ft})
\frac{ V_d }{ I \times M } = Max circuit resistance in ohms (\Omega)
\frac{ P }{ R } = ??? in feet per ohm (\text{ft}\Omega^{-1})

[!info] Ohm's Law


V = I \times R, \quad R = \frac{ V }{ I }, \quad I = \frac{ V }{ R }

3.7 KiB Raw Blame History