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.
Last 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.
Formula
Design A = load × 1.25 (continuous) Breaker = next standard rating ≥ design current
| Symbol | Meaning | Unit |
|---|---|---|
I_design | Design current | A |
I_breaker | Standard breaker rating | A |
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.
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.