This breaker size calculator finds the right circuit breaker rating for a load in amps, watts or kilowatts. It applies the NEC 125% rule for continuous loads, rounds up to the next standard breaker rating, and suggests the minimum copper wire for that breaker.
How breaker size is calculated
- Find the load current. Single-phase: I = P ÷ (V × PF). Three-phase: I = P ÷ (√3 × V × PF).
- Apply 125% for continuous loads. NEC 210.20(A) and 215.3 require the breaker to be at least 125% of a continuous load (one that runs for 3 hours or more), unless the breaker is listed for 100% operation.
- Round up to a standard rating. NEC 240.6(A) lists standard sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and up.
- Make sure the wire is protected. The conductor ampacity must suit the breaker. Small conductors are limited by NEC 240.4(D): 14 AWG copper to 15 A, 12 AWG to 20 A and 10 AWG to 30 A. For circuits of 100 A or less, NEC 110.14(C) requires the 60 °C ampacity column unless the breaker and equipment terminals are marked 75 °C.
Under IEC practice (IEC 60364, BS 7671) the rule is Ib ≤ In ≤ Iz: design current ≤ breaker rating ≤ cable capacity. Common MCB ratings are 6, 10, 16, 20, 25, 32, 40, 50 and 63 A. Read more about breaking capacity (Icu and Ics) and how to read an MCB nameplate.
Worked examples
- 1,500 W space heater, 120 V: 1500 ÷ 120 = 12.5 A; × 1.25 = 15.6 A → 20 A 1-pole breaker with 12 AWG copper.
- 32 A EV charger (continuous), 240 V: 32 × 1.25 = 40 A → 40 A 2-pole breaker with 8 AWG copper.
- 277 V LED lighting circuit, 16 A continuous: 16 × 1.25 = 20 A → 20 A 1-pole breaker with 12 AWG copper.
- 4,500 W water heater, 240 V: 4500 ÷ 240 = 18.75 A; × 1.25 = 23.4 A → 25 A 2-pole breaker with 10 AWG copper (30 A is also common).
- 7 kW cooker, 230 V (IEC): 7000 ÷ 230 = 30.4 A → 32 A MCB (or 40 A if treated as continuous).
Common breaker and wire sizes (NEC, copper)
| Breaker | Copper wire – 60 °C terminals | Copper wire – 75 °C terminals | Typical uses |
|---|---|---|---|
| 15 A (120 V) | 14 AWG | 14 AWG | Lighting, general receptacles |
| 20 A (120 V) | 12 AWG | 12 AWG | Kitchen, bathroom and laundry receptacles |
| 30 A (240 V) | 10 AWG | 10 AWG | Water heater, dryer |
| 40 A (240 V) | 8 AWG | 8 AWG | Range, EV charger (32 A) |
| 50 A (240 V) | 6 AWG | 8 AWG | Range, EV charger (40 A), welder |
| 60 A (240 V) | 4 AWG | 6 AWG | Sub-panel, EV charger (48 A) |
| 100 A | 1 AWG | 3 AWG | Sub-panel, small service |
| 200 A | – | 3/0 AWG | Service or large feeder (above 100 A the 75 °C column applies) |
Frequently asked questions
What size breaker do I need for 40 amps?
If the 40 A load is continuous, multiply by 1.25 to get 50 A and use a 50 A breaker. If it is non-continuous, a 40 A breaker is enough. Make sure the wire ampacity is at least equal to the breaker rating: 6 AWG copper for 50 A with 60 °C terminals, or 8 AWG if all terminals are rated 75 °C.
What is the 80% rule for breakers?
The 80% rule is the same as the 125% rule seen from the other side: a standard breaker should carry no more than 80% of its rating continuously. A 20 A breaker should carry at most 16 A of continuous load.
Can I put a bigger breaker on the same wire?
No. The breaker protects the wire. Installing a larger breaker than the wire’s ampacity allows can let the wire overheat before the breaker trips, creating a fire risk.
What breaker size for a 5 kW load?
At 230 V single-phase, 5 kW draws about 21.7 A, so a 25 A or 32 A MCB is typical. At 240 V it draws 20.8 A; as a continuous load that becomes 26 A, so use a 30 A breaker with 10 AWG copper.
Related calculators and guides
These calculators give engineering estimates for planning and learning. Final designs must follow your local code (NEC, IEC/BS 7671, PEC or the authority having jurisdiction) and be checked by a qualified electrical engineer or licensed electrician.