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Conductor Sizing |
Conductor Sizing
It is tempting to size circuit conductors based on overcurrent protection, but they are sized independently.
Conductors are sized to be suitable for the load, overcurrent protection is sized to protect the conductors.
Allowable conductor ampacity restrictions are not based on a wire's ability to carry current, they are intended to protect its insulation from damage due to excessive heating.
"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 one should never be repeated, even as shorthand.
"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%.
The rule neglects important context and common exceptions, namely transformers, whose feeder conductors are sized at 100%1 .
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. is specifically for calculating service and feeder sizing.
Per nfpa-70_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.
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#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.
Dwelling Unit Services
!nfpa-70_310_conductors_for_general_wiring#Table 310.12 Single-Phase Dwelling Services and Feeders
Voltage Drop
"Bundling"
It is permissible but uncommon to run more than three c
Transformers
I = \frac{S}{ \sqrt{3} \times V \times E }
I= nameplate current ratingS= nameplate kVA ratingV= feeder voltageE= efficiency (95--99%)
Motors
1~\text{electric horsepower} \equiv 746~\text{watts}
full-load current (FLC) / full-load amperes (FLA)
minimum circuit ampacity (MCA)
\text{MCA} = 1.25 \times \text{FLC}