Circuit Breaker Size Calculator
Breaker ≥ 1.25 × continuous amps + non-continuous amps, rounded up to the next standard size (15, 20, 25, 30, 35, 40, 45, 50, 60 A …). Example: a 48 A EV charger needs 60 A and at least 6 AWG copper with 75 °C terminations. Have a licensed electrician confirm local code.
Use the Circuit Breaker Size Calculator
Circuit Parameters
Continuous = maximum current for 3 hours or more (NEC Art. 100): EV chargers, water heaters, heating, store lighting.
1.0 for heaters; used only with watts.
Examples
Enter the load above to see the required breaker size.
Recommended Breaker
Minimum wire
Max continuous load on this breaker
Calculation Breakdown
Standard Breaker Sizes (NEC 240.6(A))
Breaker Size and Minimum Wire Chart
NEC Table 310.16 ampacity ≥ breaker rating, with 240.4(D). 30 °C ambient, up to 3 current-carrying conductors.
| Breaker | Max continuous | Copper 60 °C | Copper 75 °C | Aluminum 75 °C |
|---|---|---|---|---|
| 15 A | 12 A | 14 AWG | 14 AWG | 12 AWG |
| 20 A | 16 A | 12 AWG | 12 AWG | 10 AWG |
| 25 A | 20 A | 10 AWG | 10 AWG | 10 AWG |
| 30 A | 24 A | 10 AWG | 10 AWG | 8 AWG |
| 40 A | 32 A | 8 AWG | 8 AWG | 8 AWG |
| 50 A | 40 A | 6 AWG | 8 AWG | 6 AWG |
| 60 A | 48 A | 4 AWG | 6 AWG | 4 AWG |
| 70 A | 56 A | 4 AWG | 4 AWG | 3 AWG |
| 80 A | 64 A | 3 AWG | 4 AWG | 2 AWG |
| 90 A | 72 A | 2 AWG | 3 AWG | 2 AWG |
| 100 A | 80 A | 1 AWG | 3 AWG | 1 AWG |
| 125 A | 100 A | — | 1 AWG | 2/0 AWG |
| 150 A | 120 A | — | 1/0 AWG | 3/0 AWG |
| 175 A | 140 A | — | 2/0 AWG | 4/0 AWG |
| 200 A | 160 A | — | 3/0 AWG | 250 kcmil |
Use the 60 °C column for circuits of 100 A or less unless the breaker and equipment terminals are marked 75 °C, and always for NM (Romex) cable. Above 100 A use 75 °C. A 120/240 V dwelling service or main feeder may be sized at 83% of its rating (310.12), e.g. 2/0 copper or 4/0 aluminum for 200 A. Voltage drop on long runs may need a larger wire. Not a substitute for a licensed electrician or your local code.
Summary
A branch-circuit or feeder breaker must be rated at least 125% of the continuous load (anything running 3 hours or more) plus 100% of the non-continuous load (NEC 210.20(A) for branch circuits, 215.3 for feeders). The result is rounded up to the next standard rating in NEC 240.6(A). The same 125% rule is why a breaker is said to carry only 80% of its rating continuously. This calculator accepts loads in watts or amps on 120, 208, 240, 277 or 480 V, single- or three-phase with a power factor, then gives the breaker size, the pole count, the largest continuous load it allows, and the smallest copper or aluminum conductor whose Table 310.16 ampacity at the correct termination temperature covers the breaker, including the 240.4(D) limits (14 AWG copper 15 A, 12 AWG 20 A, 10 AWG 30 A).
How it works
- Enter the continuous load and the non-continuous load in watts or amps. Single-phase current = W ÷ (V × PF); three-phase current = W ÷ (√3 × V × PF).
- Required rating = 1.25 × continuous amps + 1.00 × non-continuous amps (NEC 210.20(A), 215.3).
- The breaker is the next standard rating at or above that value from NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and up.
- The conductor must have an ampacity of at least the required rating (210.19(A)(1)) and must be protected by the breaker (240.4), so the tool picks the smallest wire with ampacity ≥ the breaker rating that also meets 240.4(D).
- Ampacity uses the 60 °C column for circuits of 100 A or less and the 75 °C column above (110.14(C)), unless you select 75 °C terminations. NM (Romex) cable is always limited to 60 °C.
- Ampacities assume no more than three current-carrying conductors and 30 °C (86 °F) ambient. Adjust for more conductors, heat, or long runs (voltage drop) separately.
Use cases
- Size the breaker and wire for an EV charger (48 A continuous → 60 A breaker).
- Check the breaker for a water heater, which NEC 422.13 treats as a continuous load.
- Verify an existing breaker is not undersized after adding loads.
- Plan a subpanel feeder with mixed continuous and non-continuous loads.
- Size breakers for 208 V or 480 V three-phase commercial equipment with a power factor.
- Look up the minimum wire for a 20, 30, 40, 50 or 60 amp breaker.