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.

What is the Service Entrance Conductor Sizing Calculator?

Apply the NEC 310.12 dwelling 83% allowance or the NEC 230.42 full-load rule to recommend the service-entrance conductor AWG or kcmil size in copper or aluminum. For electricians designing new services and service upgrades. It runs free in your browser on Gera Tools, with nothing uploaded.

Service Entrance Conductor Sizing Calculator

Name: Service Entrance Conductor Sizing Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Common service sizes and their conductor requirements

Service rating	Dwelling method (83%)	Min. copper (75°C)	Min. aluminum (75°C)
100 A	83 A required	4 AWG (95 A)	2 AWG (90 A)
150 A	124.5 A required	1 AWG (130 A)	2/0 AWG (135 A)
200 A	166 A required	2/0 AWG (175 A)	4/0 AWG (180 A)
320 A	265.6 A required	350 kcmil (310 A)	600 kcmil (310 A)
400 A	332 A required	600 kcmil (420 A)	1000 kcmil (375 A)

These values assume a 75°C termination rating and no ambient correction. Confirm against the current edition of NEC Table 310.16 and your jurisdiction’s amendments.

The 83 percent allowance explained

The dwelling 83% rule in NEC 310.12 reflects the reality that residential loads are highly diverse — the range, dryer, water heater, AC, and EV charger rarely all run simultaneously at rated load. Allowing 83% of the service rating as the sizing basis accounts for this natural demand diversity without requiring a detailed load calculation. It is limited to single-phase, 120/240 V dwelling services; commercial and three-phase installations must use the full calculated load per 230.42.

Why aluminum is common for service entrances

Aluminum conductors are significantly larger in diameter than copper for a given ampacity, but service-entrance runs are typically short (from the meter to the main breaker) and do not bend repeatedly. Aluminum’s lower cost-per-foot for large conductors makes it the material of choice for 200 A and larger services. Connections must be made to AL-CU rated lugs and must use anti-oxidant compound; aluminum-to-copper direct terminations require rated connectors.

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. Verify compliance with the authority having jurisdiction before rough-in on any permit work.