How to use the NEC 310.12 83% rule for dwelling services

The 83% rule in plain language

Look up 310.12 and Table 310.12 in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how to use the rule; it does not reproduce the table.

A house almost never pulls its full service rating for long — the range, dryer, HVAC, and water heater don’t all peak together. The NEC recognizes that diversity in 310.12: for a single-family dwelling or an individual dwelling unit, the service conductors, and a feeder that carries the entire load of the unit, only need an ampacity of 83% of the service or feeder rating. It applies across the common residential range of service sizes (100–400 A in the 2023 text).

Worked example. A 200 A single-family service needs conductors rated at least 200 × 0.83 = 166 A. In the 75 °C column of Table 310.16, 2/0 copper (175 A) clears it, and so does 4/0 aluminum (180 A). Now run the same conductors through a hot attic: the shortcut table is off the table, so start from the 310.16 ampacity, apply the temperature correction factor, and check that the result still meets 166 A.

What it replaced. Older electricians learned the dwelling table at 310.15(B)(7) — a straight lookup of service rating to wire size. The 2017 NEC removed that table and replaced it with the 83% ampacity rule; the 2020 NEC moved the rule to 310.12 and added back a convenience table for installations with no correction or adjustment; the 2023 NEC kept that structure. Pocket charts printed before 2017 can steer you wrong, especially where derating applies.

Once the phase conductors are set, the GEC comes from the NEC 250.66 grounding electrode conductor guide, and the minimum service neutral from the Table 250.102(C)(1) guide. Check long service laterals with the voltage drop calculator. Heads-up for the next cycle: a later edition reportedly limits the rule to conductors rated 75 °C or higher, which matters for NM cable feeders — check when your state adopts it.