Circuit Breaker Size Calculator
Calculate the correct circuit breaker (overcurrent protective device) size for any circuit. Enter the load current and load type, and the calculator applies the NEC continuous-load and motor rules to find the next standard breaker rating.
Breaker size
20A
Minimum rating
20A
Use a 20 A breaker (next standard size)
Continuous load (NEC 210.20(A)): OCPD sized at 125 % of the load.
Always confirm the conductor ampacity supports the chosen OCPD; check our Wire Size calculator. Motor circuits also require separate overload protection (NEC 430.32).
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How breaker sizing works
The overcurrent protective device (OCPD) protects the conductor and equipment from overload and short-circuit. The minimum rating depends on the load:
- Non-continuous load: OCPD ≥ 100% of the load.
- Continuous load (3+ hours): OCPD ≥ 125% of the load (NEC 210.20(A)).
- Motor branch circuit: inverse-time breaker up to 250% of full-load current (NEC 430.52), with separate overload protection.
The result is rounded up to the next standard breaker size from NEC 240.6(A).
Worked example
A 16 A continuous lighting load:
- Minimum rating = 16 × 1.25 = 20 A
- Next standard size = 20 A breaker, on 12 AWG copper
Frequently Asked Questions
- How do I size a breaker for a continuous load?
- NEC 210.20(A) requires the OCPD to be rated at least 125% of a continuous load (one expected to run for three hours or more). A 16 A continuous load needs a 20 A breaker.
- Why are motor breakers sized so much larger than the motor current?
- Motors draw a large inrush at start-up. NEC 430.52 allows an inverse-time breaker up to 250% of the motor full-load current so it doesn't trip on starting. Overload protection is provided separately.
- What are the standard breaker sizes?
- NEC 240.6(A) lists standard ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and up. You round up to the next standard size above the calculated minimum.
- Does the breaker size depend on the wire size?
- They're linked. The breaker protects the conductor, so the wire ampacity must be at least the breaker rating (with the small-conductor limits of 240.4(D)). Always check both together.
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