Breaker Size Calculator
Enter the load current and whether it's continuous to get the design current and the next standard breaker rating — plus the conductor requirement that goes with it.
Open calculatorLast updated: 2026-09-15
How the calculation works
- Continuous loads (3 h+) multiply by 1.25 per NEC 210.20(A) so the breaker doesn't sit at 100% for hours.
- The design current is matched to the next standard rating from the 15–400 A series.
- The conductor must have ampacity (after derating) at least equal to the design current.
Worked example
Interpreting the result
The breaker protects the conductor, not the appliance — so conductor and breaker size together. Going one breaker size up without upsizing the wire is the classic code violation. Standard ratings exist because devices are manufactured in fixed sizes; the next-size-up rule (240.4(B)) has strict conditions.
Assumptions
- General branch-circuit rules; motor circuits follow NEC 430 with different multipliers.
- Standard thermal-magnetic breaker trip characteristics.
Limitations
- Motor branch/short-circuit protection can be 2.5× FLC (inverse-time) — this tool is not for motor circuits.
- Specific appliance rules (AC units, ranges) override general sizing.
Breaker size by load, for continuous and non-continuous loads
| Load current | Non-continuous: breaker | Continuous: design current (125 %) | Continuous: breaker |
|---|---|---|---|
| 8 A | 15 A | 10 A | 15 A |
| 12 A | 15 A | 15 A | 15 A |
| 16 A | 20 A | 20 A | 20 A |
| 20 A | 20 A | 25 A | 25 A |
| 24 A | 25 A | 30 A | 30 A |
| 32 A | 35 A | 40 A | 40 A |
| 40 A | 40 A | 50 A | 50 A |
| 48 A | 50 A | 60 A | 60 A |
Rounded up to the next standard size (the list in NEC 240.6(A)). Always confirm the result against the code edition your jurisdiction has adopted.
What counts as a continuous load
The electrical code treats a load as continuous when it is expected to run for three hours or more. EV charging, lighting in stores and many heating loads fall in that group. For these, the breaker and the conductor are sized at 125 % of the load.
Worked scenario: a 32 A electric-vehicle charger
Entered as a continuous load, 32 A becomes a design current of 40 A and the calculator selects a 40 A breaker. The conductors must be rated for at least 40 A.
Limits of the result
- The breaker protects the conductor, not the appliance. It can never be larger than the conductor's ampacity allows.
- Terminal temperature ratings, ambient temperature and the number of conductors in a conduit can reduce the ampacity you may use.
- Motors, welders and some other loads have their own sizing rules. This tool covers the general case.
Frequently asked questions
What breaker for a 20 amp load?
A 20 A breaker for non-continuous; if it runs 3+ hours, 25 A design → 25 A breaker with matching conductor.
Can I use a bigger breaker than the wire rating?
Generally no — the breaker protects the wire. Limited exceptions exist (240.4(B) next-size-up, motor circuits), but they're rule-specific, not general.