UNIT 2.1 — CONDUCTORS

Conductor sizing and ampacity

CERT 2 · Residential Wiring · ~11 min

Community content — not verified by a licensed electrician. Lessons and questions here are written for study practice and pinned to NEC 2023. Code editions and local amendments vary by jurisdiction. Always confirm against your adopted code book and your licensing board's published exam outline before test day.

Ampacity is the current a conductor can carry continuously without exceeding its insulation's temperature rating. It depends on the wire's cross-section (AWG — smaller number, bigger wire), the insulation temperature column (60 °C / 75 °C / 90 °C), and correction factors for ambient heat and bundling.

Residential exams live on the small-conductor trio: 14 AWG copper → 15 A breaker, 12 AWG → 20 A, 10 AWG → 30 A. These caps come from the small-conductor rule (NEC 240.4(D)) and apply regardless of what the ampacity table might otherwise suggest.

Continuous loads (3 hours or more, like water heaters or EV chargers) must be computed at 125%: a 16 A continuous load requires 16 × 1.25 = 20 A of circuit capacity. This multiplier is one of the most-tested numbers in residential exams.

Exam focus: the trio above, the 125% continuous rule, and recognizing that voltage drop (Formula Lab, Solver 02) can force a *larger* conductor than ampacity alone requires on long runs.

14 AWG Cu
15 A max
12 AWG Cu
20 A max
Continuous loads
× 1.25
Knowledge check

Quiz — 3 questions

1. The maximum overcurrent protection for 12 AWG copper in a dwelling is:

2. A continuous 24 A load requires a circuit rated at least:

3. On a very long branch-circuit run, the conductor size is most often increased because of:

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