About

Selecting the correct circuit breaker is a matter of fire safety. A breaker that is too small will trip unnecessarily (nuisance tripping), while a breaker that is too large may fail to interrupt a fault before the wire insulation melts. This tool calculates the required breaker size based on the connected load (Watts), Voltage, and Power Factor.

Crucially, this calculator adheres to the 80% Rule (NEC Standard). For continuous loads (running 3+ hours), the circuit breaker should be sized so that the load does not exceed 80% of the breaker's rating. Inversely, the minimum breaker size must be 125% of the continuous load current. This tool outputs both the raw calculated amps and the next standard commercial breaker size available.

Formulas

First, calculate Load Current (I_load). For Single Phase AC:

I_load = PV × PF

For Three Phase AC:

I_load = P√3 × V × PF

Then, determine Minimum Breaker Rating (I_cb):

I_cb ≥ I_load × 1.25

Reference Data

Breaker Size (Amps)	Typical Wire Gauge (Copper)	Max Continuous Load (80%)
15 A	14 AWG	12 A
20 A	12 AWG	16 A
30 A	10 AWG	24 A
40 A	8 AWG	32 A
50 A	6 AWG	40 A
60 A	4 AWG	48 A
100 A	3 AWG	80 A
200 A	2/0 AWG	160 A

Frequently Asked Questions

The National Electrical Code (NEC) states that a circuit breaker should not handle a continuous load (3 hours or more) higher than 80% of its rating. Effectively, this means you multiply your load by 1.25 to find the breaker size.

You must round up. For example, if the calculation calls for a 17 Amp breaker, you cannot buy one. You must use the next standard size up, which is 20 Amps, provided the wire gauge is also upgraded to match.

A lower Power Factor (e.g., 0.8) draws more current (Amps) to do the same amount of work (Watts) compared to a PF of 1.0. This "wasted" current flows through the breaker, so the breaker must be sized larger to handle it.