Sizing the Service: Minimum Rating and the 83% Dwelling Conductor Rule

Once you know a home's calculated load (previous lesson), two code rules decide the service: how big the disconnect must be, and how large the conductors must be. Dwellings get a special break on conductor size — the "83% rule" — which is one of the most-tested items on the exam.

1. Minimum service size — 100 amperes

For a one-family dwelling, the service disconnecting means must be rated at least 100 amperes, 3-wire. (CEC 230.79(C))

This is a floor, not a calculation result. Even if the calculated load works out to, say, 92 A, you still install at least a 100 A service disconnect for a house.

Read the rule precisely: 230.79(C)'s 100 A minimum applies to the rating of the service disconnecting means — not directly to the service conductors, the grounding electrode conductor, or a feeder's overcurrent device. Exam questions test that distinction.

2. The 83% rule — smaller conductors allowed for dwellings

Normally a conductor must be rated for the full load it carries. But for a 120/240-volt, single-phase service or feeder that carries the entire load of a one-family dwelling (or an individual dwelling unit of a two-family/multifamily building), the code lets you size the ungrounded (hot) conductors at not less than 83% of the service rating. (CEC 310.12)