Circuit Breaker Sizing Calculator Formula

Understand the math behind the circuit breaker sizing calculator. Each variable explained with a worked example.

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Formulas Used

Actual Current Draw

actual_amps = load_amps

Design Current

design_amps = load_amps * safety_multiplier

Recommended Breaker

breaker_amps = ceil(load_amps * safety_multiplier / 5) * 5

Variables

VariableDescriptionDefault
total_wattsTotal Connected Load(W)2400
voltageVoltage(V)120
is_continuousContinuous Load?1
load_ampsDerived value= voltage > 0 ? total_watts / voltage : 0calculated
safety_multiplierDerived value= is_continuous == 1 ? 1.25 : 1calculated

How It Works

Circuit Breaker Sizing

Amps = Watts / Volts

Continuous load: multiply by 1.25 per NEC 210.20(A). Then round up to the next standard breaker size (15A, 20A, 30A, 40A, 50A, etc.).

Worked Example

2,400W continuous load on 120V.

total_watts = 2400voltage = 120is_continuous = 1
  1. 01Amps = 2400 / 120 = 20A
  2. 02Design amps = 20 x 1.25 = 25A
  3. 03Next standard size = 25A, or use 30A breaker

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