About the Circuit Breaker Size Calculator
This circuit breaker size calculator tells you which standard breaker rating a load needs. Enter the load in amps, or in watts with the voltage, say whether it runs continuously, and it applies the 125% rule (the same thing as loading a breaker to no more than 80%) and rounds up to the next standard rating from NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60 A and so on.
It is useful when adding a dedicated circuit for a water heater, EV charger, space heater, air conditioner or workshop tool, or when checking whether an existing breaker is adequate. It also shows the smallest copper wire that the breaker can protect, so you do not end up with a breaker larger than the wire can safely carry.
A continuous load is one expected to run at full current for three hours or more (heaters, EV charging, lighting in commercial spaces). Motor, HVAC and welder circuits follow special nameplate rules (MCA/MOCP), so use the equipment label for those and treat this result as a cross-check. Breaker work should be done or checked by a licensed electrician.
How to use the circuit breaker size calculator
- 1Enter the load in amps, or switch to watts and enter the voltage.
- 2Turn on "continuous" if the load runs at full power for 3 hours or more.
- 3Read the breaker size — the next standard rating above the minimum.
- 4Check the suggested wire gauge so the wire is protected by the breaker.
Formula and method
If you enter watts, the load current is found from I = P ÷ (V × PF), or I = P ÷ (√3 × V × PF) for three-phase. For continuous loads the NEC requires the overcurrent device to be rated at least 125% of the load — equivalently, the load may use only 80% of the breaker rating. Non-continuous loads can use the full rating.
The minimum rating is then rounded up to the next standard size listed in NEC 240.6(A). The wire suggestion is the smallest copper conductor whose 75 °C ampacity (with the 15/20/30 A limits for 14/12/10 AWG) is at least the breaker rating, so the breaker always protects the wire.
- I
- Load current in amps
- P
- Load power in watts
- V
- Circuit voltage (line-to-line for three-phase)
- PF
- Power factor (1 for resistive loads)
Worked examples
16 A continuous load
16 A × 1.25 = 20 A, so a 20 A breaker is exactly right: 16 A is 80% of its rating. 12 AWG copper is the matching wire.
4,500 W water heater on 240 V
I = 4,500 ÷ 240 = 18.75 A. Water heaters are continuous loads, so the minimum is 18.75 × 1.25 = 23.44 A — the next standard size is 25 A, which needs 10 AWG copper (12 AWG is limited to 20 A). Many installers simply use a 30 A breaker with 10 AWG.
42 A non-continuous load
Without the 125% factor the minimum is 42 A, so the next standard size is 45 A. 8 AWG copper (50 A at 75 °C) is the smallest wire it protects.
20 kW three-phase heater at 480 V
I = 20,000 ÷ (1.732 × 480) ≈ 24.06 A per line. × 1.25 = 30.07 A, which just exceeds 30 A, so a 35 A three-pole breaker is required.
Frequently asked questions
What is the 80% rule for breakers?+
Standard breakers may carry only 80% of their rating continuously (3+ hours). A 20 A breaker should see no more than 16 A of continuous load. Sizing the breaker at 125% of the load is the same rule.
What size breaker do I need for 1500 watts?+
At 120 V, 1500 W is 12.5 A. As a continuous load that needs 15.6 A of capacity, so use a 20 A breaker with 12 AWG wire. A 15 A breaker would be loaded to 83%.
Can I put a bigger breaker on the same wire?+
No. The breaker protects the wire, so its rating must not exceed the wire’s allowable ampacity. A 30 A breaker on 12 AWG wire lets the wire overheat before the breaker trips.
What are the standard breaker sizes?+
NEC 240.6(A) lists standard ratings of 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and larger. Residential panels most often use 15, 20, 30, 40 and 50 A.
How do I size a breaker for an air conditioner or motor?+
Use the nameplate: MCA (minimum circuit ampacity) sets the wire size and MOCP (maximum overcurrent protection) sets the largest breaker. Motor circuits may use breakers well above the running current to handle starting surges.