Breaker Size Calculator
Enter the load, split into continuous and noncontinuous portions, and get the standard NEC 240.6(A) breaker plus the minimum conductor that breaker can legally protect. Applies the 210.20(A) 125% continuous rule, the 240.4(B) next-size-up allowance, and the 240.4(D) small-conductor limits.
How to use this calculator
- Enter the load in amperes, split into noncontinuous and continuous (3+ hours) portions. Or switch to watts + voltage and the calculator converts for you (single-phase).
- Set the conductor material, insulation and termination ratings, ambient temperature, and conductor count; these drive the conductor recommendation, not the breaker size.
- Read the breaker rating and the minimum conductor. The notes explain any 240.4(B) next-size-up or 240.4(D) small-conductor interactions, which are where most sizing mistakes happen.
- For motors, air conditioners, and welders, use the equipment nameplate and the applicable article instead (430/440/630); see the FAQ.
NEC reference
Protection is sized per NEC 2020 210.20(A): 100% of the noncontinuous load plus 125% of the continuous load, rounded up to a standard rating from 240.6(A). The conductor is sized to the same requirement per 210.19(A)(1) using Table 310.16 with ambient correction, conductor-count adjustment, and the 110.14(C) termination cap, then checked against the 240.4(D) small-conductor limits so the recommended breaker can actually protect it, with 240.4(B) next-size-up flagged when used.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Common breaker sizes: copper conductors, standard conditions
| Circuit | Breaker | Copper conductor | Governing rule |
|---|---|---|---|
| General lighting/receptacles | 15 A | 14 AWG | 240.4(D): 14 AWG capped at 15 A |
| Kitchen small-appliance, bath | 20 A | 12 AWG | 240.4(D): 12 AWG capped at 20 A |
| Electric dryer | 30 A | 10 AWG | 220.54; 240.4(D): 10 AWG capped at 30 A |
| Water heater 4500 W / 240 V | 25-30 A | 10 AWG | 422.13 continuous → 23.4 A required |
| EV charger, 32 A continuous | 40 A | 8 AWG | 210.20(A): 32 × 1.25 = 40 A |
| Range (per 220.55 demand) | 40-50 A | 8-6 AWG | Table 220.55 demand factors |
Standard conditions (86°F, ≤3 current-carrying conductors, 75°C terminations). Derating, long runs (voltage drop), and aluminum change the conductor; use the calculator above with your actual conditions.
The three rules that decide breaker size
- 210.20(A): the 125% rule. Continuous loads (3+ hours) count at 125% toward the required protection. Miss this and an EV charger or water heater circuit ends up undersized.
- 240.6(A): standard ratings. Breakers come in fixed sizes; you round the requirement up to the next one.
- 240.4(D): small-conductor caps. Regardless of computed ampacity, copper 14/12/10 AWG are capped at 15/20/30 A (aluminum 12/10 at 15/25 A). This is why "12 AWG is good for 25 A at 90°C" still doesn't permit a 25 A breaker.
Frequently asked questions
What size breaker for a 4500W water heater?
4500 W ÷ 240 V = 18.75 A, treated as continuous per NEC 422.13, so the protection requirement is 23.4 A → a 25 A breaker minimum, with 30 A on 10 AWG copper the common field practice. Enter 18.75 A in the continuous field above to see the full arithmetic.
Can I put a 25 A breaker on 12 AWG copper?
No. NEC 240.4(D) caps 12 AWG copper protection at 20 A regardless of the conductor's computed ampacity (25 A at 75°C, 30 A at 90°C). The 240.4(B) next-size-up allowance explicitly does not override the small-conductor rule. A 25 A breaker requires 10 AWG copper.
When does next-size-up (240.4(B)) apply?
When the conductor's ampacity doesn't correspond to a standard breaker rating, the breaker is 800 A or less, and the circuit doesn't supply multiple cord-and-plug receptacles. Classic example: 6 AWG copper at 65 A protected by a 70 A breaker. Above 800 A the allowance disappears; the conductor must be upsized instead. The calculator flags when its recommendation uses this allowance.
Why doesn't this work for motors and AC units?
Motor branch circuits (Article 430) size conductors at 125% of full-load current from the NEC tables (not nameplate) and allow much larger short-circuit protection (up to 250% for inverse-time breakers) because the overload relay protects the motor. HVAC equipment (Article 440) puts the answer on the nameplate as MCA and MOP. Feeding those loads through a general-purpose calculation gives wrong (usually nuisance-tripping) answers, so this tool stays out of that territory.
Sizing guides
- How to Size a Breaker for a Continuous Load: the 210.20(A) 125 percent rule with EV, water heater, and mixed-load examples worked step by step.
- What Size Wire for a 50 Amp Breaker: the 240.4 conductor pairing once the breaker is set, and why 8 AWG copper is a trap.
Related calculators
- Wire Size Calculator: full conductor sizing with voltage drop and OCPD together.
- Wire Gauge Calculator: minimum AWG for a required ampacity.
- Wire Ampacity Calculator: derated ampacity of a specific conductor.
- Voltage Drop Calculator: check the run length before finalizing the circuit.
Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.
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For deeper NEC training on circuit protection and breaker sizing, Mike Holt's NEC courses are the industry standard.
Working from the printed code? NEC code books and tab sets are available from Mike Holt.
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