Breaker size calculator

Introduction

A circuit breaker must be big enough not to trip under normal load, and small enough to protect the wire behind it. Under the US National Electrical Code (NEC), a load that runs for three hours or more counts as continuous, and the breaker must be rated at 125 % of it. That is why a 30 A EV charger sits on a 40 A breaker. Under IEC rules, used in Europe and most of the world, the breaker must be at least the load current and no more than the current the cable can carry.

This calculator finds the design current from the load in amps or watts, applies the NEC or IEC rule, and picks the next standard breaker rating. If you enter the ampacity of the wire from your code table, it checks that the wire and the breaker match. It is for electricians, engineers and homeowners planning a new circuit.

Formulas

Load current from power:

DC:            I_b = P / V
single-phase:  I_b = P / (V × cos φ)
three-phase:   I_b = P / (√3 × V × cos φ)

NEC minimum rating, for continuous and non-continuous loads:

I_min = 1.25 × I_continuous + I_noncontinuous

IEC coordination of the breaker and the cable:

I_b ≤ I_n ≤ I_z

• I_b — design (load) current; I_n — rated current of the breaker; I_z — ampacity of the cable
• NEC standard ratings (240.6(A)): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and up
• IEC preferred ratings (IEC 60898-1): 6, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A, then 160, 200, 250, 315, 400 A and up

Assumptions and limitations

• The rules here are for general feeders and branch circuits. Motor circuits are sized differently: the NEC allows an inverse-time breaker up to 250 % of the motor current, and IEC practice uses motor-protection breakers.
• Under the NEC, heaters, EV chargers, lighting, water heaters and servers are usually continuous. Breakers listed for 100 % of their rating, used in some large panels, are not covered.
• The calculator does not size the wire. Wire ampacity depends on insulation, installation and temperature, and comes from NEC Table 310.16 or IEC 60364-5-52. For small copper wire, NEC 240.4(D) caps 14 AWG at 15 A, 12 AWG at 20 A and 10 AWG at 30 A.
• NEC 240.4(B) allows the next standard rating above the wire's ampacity, up to 800 A, except on circuits with several receptacles for cord-and-plug loads. IEC has no such allowance.

Worked example

A 7.2 kW Level 2 EV charger runs on 240 V single-phase. What breaker does it need under the NEC?

1. Load current: I = 7 200 / 240 = 30.0 A.
2. EV charging is a continuous load (NEC 625.41), so the minimum rating is 1.25 × 30 = 37.5 A.
3. The next standard rating at or above 37.5 A is 40 A.
4. The charger loads the breaker to 30 / 40 = 75 %.

So the charger needs a 40 A breaker and a wire rated for at least 40 A, typically 8 AWG copper with 75 °C insulation.

FAQ

Related calculators

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