Circuit Breaker Size Calculator

A breaker has one job with two hands: let the circuit run what it must, and cut the power before the cable burns. The tension between those two is why sizing is a calculation and not a guess.

Circuit Breaker Size Calculator turns load current plus continuous-load rules into a suggested breaker rating, showing the required conductor ampacity and checking the motor-start surge where it applies.

It is a planning aid. Final selection must follow the code and the equipment manufacturer requirements in force.

• Load in kW, A or VA: Enter what the circuit really carries.
• Continuous load handling: 125% applied to continuous parts.
• Phase and voltage: Single and three-phase formulas.
• Power factor: For reactive loads like motors.
• Conductor check: Minimum ampacity shown for the cable.
• NEC or IEC: Select the sizing convention.
• Motor surge note: Where starting current is entered.
• Export: Copy or download as text, CSV or JSON.

1. Enter the load current (or kW + voltage).
2. Enter the continuous portion of the load.
3. Select phase, voltage and standard.
4. Optional: enter the motor starting current.
5. Read the suggested breaker size and minimum cable ampacity.
6. Export the sizing note if needed.

Example 1 — Heater. 3.68 kW continuous at 230 V, 1P: 16 A at design current, a 20 A breaker.

Example 2 — Mixed. 20 A of continuous and 10 A non-continuous: 35 A design, a 40 A breaker, minimum cable 35 A.

Example 3 — Motor. 11 kW, 20.8 A FLA with a 125 A start: the frame must ride the surge, so a D-curve 32 A breaker protects the running current.

Example 4 — 3-phase. 15 kW at 400 V, pf 0.9: 24.1 A running, 32 A breaker with the right curve.

• Design current correct: Continuous 125% handled automatically.
• Frame picked: Next standard rating chosen for you.
• Wire not forgotten: Minimum conductor ampacity shown.
• Standards aware: NEC or IEC selected explicitly.
• Free and private: No uploads, no accounts, no logging.