Quick Hits

EPISODE 7 — The 83% Rule: Single-Phase Dwelling Services & Feeders (310.12)

Slide 1 — Hook "One of the MOST commonly misapplied rules in the whole code — today we clear up exactly when the '83% shortcut' for dwelling service/feeder sizing actually applies."

Slide 2 — What it is

• For one-family dwellings (and individual units of two-family/multifamily dwellings) on a single-phase 120/240V system (or a 208Y/120V single-phase feeder with two ungrounded conductors + neutral): services AND feeders rated 100A-400A can be sized at just 83% of the service/feeder rating — as long as those conductors carry the ENTIRE dwelling unit load.

• Table 310.12(A) gives you the shortcut sizes directly (for 75°C-rated conductors, no adjustment/correction needed) — e.g., a 200A service can use 2/0 AWG copper or 4/0 AWG aluminum under this table.

• A feeder's ampacity can never be REQUIRED to exceed what 310.12(A)/(B) specifies, even under otherwise-applicable rules.

Slide 3 — The misapplication trap — READ THIS CAREFULLY

• This table ONLY applies where the conductors carry the ENTIRE load of the dwelling unit. It does NOT apply to feeders for detached garages, pole barns, pool houses, or anything that's only PART of the dwelling's total load — those still need standard Table 310.16-style sizing.

• Grounded conductors CAN be sized smaller than the ungrounded conductors here, as long as the sizing rules in 120.61/230.42 (services) or 215.4/120.61 (feeders) are satisfied.

Slide 4 — Quiz + CTA Q: "The 310.12 83% dwelling sizing rule applies to a feeder ONLY if that feeder does what?" A) Serves any residential structure B) Carries the entire load of the dwelling unit C) Is rated under 100A D) Is copper, not aluminum

✅ Answer: B — 310.12: the reduced sizing only applies where the feeder carries the dwelling unit's ENTIRE load — not a partial load like a detached garage.

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