How to Size a Circuit Breaker — 5 Steps with the 125% Rule
Breaker sizing looks like reading a chart, but the chart is the last step. Get the load and the 125% rule right and the standard sizes and wire pairing fall out mechanically.
Quick answer: Breaker sizing in 5 steps: (1) amps = watts ÷ volts; (2) if the load runs 3+ hours, multiply by 1.25 (NEC 210.20(A)); (3) round UP to the next standard size (15/20/25/30/40/50/60…); (4) pair wire by ampacity — 15 A→14 AWG, 20 A→12, 30 A→10, 40 A→8, 50 A→8, 60 A→6 at 75°C; (5) verify long runs against 3% voltage drop. Example: 2,400 W at 120 V continuous = 20 A × 1.25 = 25 A breaker → 10 AWG.
Step 1 — Get the true load current
Nameplate watts ÷ volts, at the actual circuit voltage (a '120 V' circuit at the panel often measures 118). Resistive loads (heat, water heating) use PF = 1; generic plug loads are close enough. Motors and HVAC nameplates are the exceptions: they publish MCA/MOCP that already embed their own math — use those, not watts ÷ volts.
Step 2 — Apply the 125% continuous rule
NEC 210.20(A): a branch circuit supplying a continuous load (3 hours or more at max) needs an overcurrent device rated at least 125% of the load. Water heaters, baseboard heat, EV charging and many lighting circuits are continuous. A 2,400 W, 120 V water heater draws 20 A → design at 25 A. The headroom isn't bureaucracy: breakers age under sustained heat near their rating.
Step 3 — Round up, once
- Standard sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A.
- Design current 21 A → 25 A breaker. Not 30. The breaker protects the wire, and oversizing protects nothing.
- If your load exceeds 400 A class, that's a feeder/service design conversation, not a branch-circuit calculator.
Step 4 — Pair the wire
The conductor's effective ampacity must meet or beat the breaker: 15 A → 14 AWG, 20 A → 12, 30 A → 10, 40–50 A → 8, 60 A → 6 (75°C copper column, with the 240.4(D) caps: 14→15 A, 12→20 A, 10→30 A for everyday circuits). The breaker-size calculator on this site performs steps 1–4 together and shows which table row decided.
Step 5 — Check distance
Ampacity says nothing about distance. A 20 A circuit at 100 ft on 12 AWG drops 6.6% — triple the 3% target — and shows up later as dim lights and hot motor windings. Runs over ~50 ft at high load deserve the voltage-drop check, and often one or two wire sizes up.
The four mistakes that fail inspection
- Wire smaller than the breaker calls for (12 AWG on a 30 A) — the classic.
- Continuous loads sized at 100% — the water heater trips every January evening.
- Rounding up twice — a 21 A calc on a 30 A breaker 'because it was in the truck.'
- Ignoring the 90°C trap — 90°C cable still lands on 75°C lugs; the working number is 75°C.