LumaQuote Electrical calculators Breaker size

Breaker Size Calculator

Find the right breaker size for a load. Enter the watts or amps, and the calculator applies the 125% rule for continuous loads and picks the next standard breaker size.

How to size a breaker

Find the load current. Use the nameplate amps, or convert watts with the watts to amps calculator.
Decide if it is continuous. A load that runs at its maximum current for 3 hours or more is continuous. Lighting, heaters, EV chargers and most commercial loads count.
Multiply continuous loads by 1.25. The NEC requires the breaker to be at least 125% of the continuous load (NEC 210.20(A) for branch circuits, 215.3 for feeders). It is the same as keeping the load at or under 80% of the breaker.
Round up to the next standard size. Standard ratings are listed in NEC 240.6(A). Never round down.
Match the wire to the breaker. The breaker protects the conductor, so the wire must be rated for at least the breaker size.

Breaker ≥ continuous load × 1.25 + non-continuous load

Examples

1,500 W heater at 120 V: 12.5 A × 1.25 = 15.63 A, so a 20 A breaker.
5,000 W heater at 240 V: 20.83 A × 1.25 = 26.04 A, so a 30 A 2-pole breaker.
EV charger set to 40 A: 40 × 1.25 = 50 A, so a 50 A breaker. A 48 A charger needs 60 A.
2,000 W of LED lighting at 277 V, power factor 0.9: 8.02 A × 1.25 = 10.03 A, so a 15 A breaker, with plenty of room. The lighting circuit calculator shows how many fixtures fit.

Standard breaker sizes

Standard ampere ratings for breakers and fuses up to 600 A (NEC 240.6(A)):

15 A	20 A	25 A	30 A	35 A
40 A	45 A	50 A	60 A	70 A
80 A	90 A	100 A	110 A	125 A
150 A	175 A	200 A	225 A	250 A
300 A	350 A	400 A	450 A	500 A
600 A

Larger standard sizes: 700, 800, 1,000, 1,200, 1,600, 2,000, 2,500, 3,000, 4,000, 5,000 and 6,000 A.

Breaker and wire size

For small copper conductors, the NEC caps the overcurrent protection no matter what the wire's ampacity table says (NEC 240.4(D)):

Copper wire	Maximum breaker
14 AWG	15 A
12 AWG	20 A
10 AWG	30 A

Above 30 A, size the wire from NEC Table 310.16 (CEC Table 2 in Canada) using the terminal temperature rating, then correct for ambient temperature and for more than three current-carrying conductors in a raceway. Long runs may need a larger wire for voltage drop.

Canada: the CEC 80% rule

The Canadian Electrical Code gets to the same answer from the other side: a continuous load must not exceed 80% of the rating of a standard breaker (CEC Rule 8-104). Breakers marked for continuous operation at 100% can be loaded to their full rating. The calculator's continuous option covers both codes.

When this calculator does not apply

Motors, compressors and HVAC equipment. They follow their own rules (NEC Articles 430 and 440) and allow larger breakers for starting current. Use the nameplate MOCP value.
Welders and transformers. They have their own protection tables.
Services and feeders with many loads. Those need a load calculation with demand factors.

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Breaker size FAQ

What size breaker do I need for 30 amps?

For a continuous 30 A load, 40 A (30 × 1.25 = 37.5 A). For a non-continuous 30 A load, a 30 A breaker.

What size breaker do I need for a 5000 watt heater?

At 240 V a 5,000 W heater draws 20.83 A. Heaters are continuous, so 20.83 × 1.25 = 26.04 A: use a 30 A 2-pole breaker.

What is the 80% rule for breakers?

A continuous load should be no more than 80% of the breaker rating. It is the same rule as sizing the breaker at 125% of the continuous load, written the other way round.

Can I use a bigger breaker than I need?

Only if the wire is rated for it. The breaker protects the conductor, so it must not be larger than the wire allows. A 12 AWG copper circuit is limited to 20 A.

Does this work for motors and air conditioners?

No. Motors and HVAC equipment use different rules for starting current. Use the maximum overcurrent protection (MOCP) printed on the nameplate.

What is a 100% rated breaker?

A breaker, usually in larger frames, listed to carry its full rating continuously. With one, a continuous load can be sized at 100% instead of 125%. Standard breakers are 80% rated.