Breaker Size Calculator
Size a circuit breaker for a continuous load: enter the load in amps and get the minimum breaker, using the NEC 80% rule (breaker = load ÷ 0.8).
How this calculator works
A circuit breaker protects a circuit, but for a continuous load it cannot be loaded to its full rating. The code requires the load to stay at or below 80% of the breaker rating, so the breaker must be sized larger than the load itself.
This calculator takes your continuous load in amps, divides it by 0.8 to find the minimum rating, then rounds up to the next standard breaker size.
Formula: A breaker on a continuous load must be at least load ÷ 0.8 (the load may not exceed 80% of the breaker rating). Round up to the next standard breaker (15, 20, 25, 30, 40, 50, 60…).
Worked example
A 40 A continuous load:
- Minimum rating: 40 ÷ 0.8 = 50 A
- Next standard breaker at or above 50 A: 50 A
- Result: use a 50 A breaker
Reference (continuous load to breaker)
| Load | Min rating | Breaker |
|---|---|---|
| 12 A | 15 A | 15 A |
| 16 A | 20 A | 20 A |
| 24 A | 30 A | 30 A |
| 40 A | 50 A | 50 A |
| 64 A | 80 A | 80 A |
How to use
Enter your continuous load in amps. The calculator divides it by 0.8 and rounds up to the next standard breaker rating. Confirm the circuit conductors are sized to carry that breaker’s current, and verify the result against your local code.
Electrical work is code-governed and safety-critical. Confirm circuit, breaker, and conductor sizing with a licensed electrician and your local code.
Frequently asked questions
What is the NEC 80% rule for breakers?
The National Electrical Code says a continuous load may not exceed 80% of a breaker's rating. A continuous load is one expected to run for three hours or more. To size the breaker, you divide the load by 0.8, which is the same as making the load no more than 80% of the rating.
Why can't a continuous load use the full breaker rating?
A breaker running near its rating for hours builds up heat, and sustained heat can cause nuisance trips or wear on the device. The 80% margin leaves headroom so the breaker and conductors stay within safe temperatures during long, steady operation. It is a safety buffer, not wasted capacity.
What are standard breaker sizes?
Common residential and light commercial breakers come in fixed ratings: 15, 20, 25, 30, 40, 50, and 60 amps, continuing to larger sizes. Because breakers only exist in these steps, you always round the calculated minimum up to the next standard size rather than using an in-between value.
Does this replace conductor sizing?
No. Sizing the breaker is only part of the job. The wire feeding the circuit must be rated to carry the breaker's current safely, and long runs may need a thicker gauge for voltage drop. Always check conductor ampacity and voltage drop alongside the breaker size, and confirm with an electrician.
Estimates only. Verify quantities with your supplier before purchasing.