What is the NEC 310.12 83% rule?

NEC 310.12 lets single-phase dwelling service and main feeder conductors be sized for 83 percent of the service rating rather than 100 percent. This reflects the natural load diversity in homes and is why a 200 A service is commonly run with 4/0 aluminum instead of a much larger conductor.

When should I use the general 230.42 method instead?

Use the general method for commercial services, three-phase services, or any case that does not qualify for the 310.12 dwelling allowance. There the conductor ampacity must equal or exceed the full calculated service load, including the 125 percent factor on continuous loads.

Why does the calculator use the 75 C column?

Most service equipment terminations are rated 75 C, and NEC 110.14(C) prohibits loading a conductor above its termination rating even if the insulation is rated 90 C. Using the 75 C ampacity keeps the result compliant with typical lugs and breakers.

What size is a 200 A copper service conductor?

Under the 310.12 dwelling allowance a 200 A service needs about 166 A of ampacity (83 percent), which is met by 2/0 copper at 175 A. In aluminum it is 4/0 at 180 A. These are the classic dwelling service sizes the tool returns.

Does this size the neutral and grounding conductors too?

No. The tool sizes the ungrounded service conductors. The grounded neutral is sized per 250.24(C) and 220.61 from the maximum unbalanced load, and the grounding electrode conductor comes from Table 250.66. Size those separately.

Service Entrance Conductor Sizing Calculator

Service-entrance conductors carry the entire load of a building, so the NEC gives them specific sizing rules. For single-phase dwellings, NEC 310.12 grants an 83 percent allowance that accounts for load diversity; for everything else, NEC 230.42 requires sizing to the full calculated load. This calculator handles both paths and returns a compliant copper or aluminum conductor.

How it works

In dwelling mode the tool applies the 310.12 allowance:

required ampacity = 0.83 × service rating

In general mode it sizes directly to your calculated load per 230.42. In either case it scans NEC Table 310.16 at the 75 C column and returns the smallest conductor whose ampacity meets the requirement. The 75 C column is used because NEC 110.14(C) caps loading at the termination temperature rating, which is 75 C for typical service equipment.

Worked example

A 200 A single-phase dwelling service needs 0.83 × 200 = 166 A. The smallest 75 C copper conductor meeting that is 2/0 AWG at 175 A; in aluminum it is 4/0 AWG at 180 A. These match the conductors electricians routinely pull for a 200 A residential service.

Notes

Size the grounded neutral from the maximum unbalanced load per 250.24(C) and 220.61, and the grounding electrode conductor from Table 250.66 — these are not the same as the ungrounded conductor size. Apply ambient correction if the service conductors pass through hot attic or rooftop spaces, and always confirm your termination temperature ratings before finalizing.