How do I calculate breaker size?

Find the load current, multiply by 1.25 for a continuous load, then round up to the next standard overcurrent device rating. Standard ratings are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175 and 200 amps and upward.

Why multiply the load by 125%?

A continuous load is one expected to run for three hours or more. The overcurrent device and conductors must be sized at 125% of that load so the breaker is not operating at its thermal limit indefinitely.

What breaker size do I need for 5000 watts at 240 volts?

5000 W / 240 V is 20.8 A. At 125% that is 26 A, so the next standard breaker is 30 A, typically on 10 AWG copper.

Can a breaker be larger than the wire rating?

Generally no. The overcurrent device protects the conductor, so the breaker rating must not exceed the conductor's ampacity after derating. Limited exceptions exist for motor circuits and taps, which are outside what this calculator covers.

What is the 80% rule for breakers?

A standard breaker should carry no more than 80% of its rating continuously, which is the inverse of the 125% sizing multiplier. A 20 A breaker is therefore good for a 16 A continuous load.

Breaker Size Calculator

Load in watts or amps, the 125% continuous multiplier, the next standard breaker rating, and the smallest conductor that carries it.

Quick answer

To size a breaker: multiply the load current by 1.25 for a continuous load, then round up to the next standard rating (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and up). A 5,000 W load at 240 V draws 20.8 A, becomes 26 A at 125%, and lands on a 30 A breaker with 10 AWG copper.

Load

Input asLoad (W)Voltage (V)PhaseConductor materialTermination rating

Continuous load (runs three hours or more) — applies the 125% multiplier

I = W / (V × 1) · design = I × 1.25

Result

Load current

20.8A

Design current

26.0A

Breaker size

30 A

Minimum conductor

10 AWG

10 AWG copper is estimated at 32 A at 75 °C, against a design current of 26.0 A.

Check voltage drop separately — on long runs the conductor usually has to grow beyond what ampacity alone requires. Run the voltage drop calculator.

Formula estimate

Breaker size and conductor by load current

Load (A)	Design at 125% (A)	Breaker (A)	Min. copper conductor
10	12.5	15	14 AWG
12	15.0	15	14 AWG
16	20.0	20	12 AWG
20	25.0	25	10 AWG
24	30.0	30	10 AWG
30	37.5	40	8 AWG
40	50.0	50	6 AWG
48	60.0	60	4 AWG
60	75.0	80	4 AWG
70	87.5	90	3 AWG
80	100.0	100	2 AWG
100	125.0	125	1 AWG
125	156.3	175	2/0 AWG
150	187.5	200	3/0 AWG

Copper at 75 °C, continuous load. Conductor is the smallest whose estimated ampacity covers the design current.

Source & method: Standard overcurrent device ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and upward. Conductor selection uses the ampacity model on the methodology page.

Typical circuit ampacity headroom

Copper size	Est. ampacity 60 °C	Est. ampacity 75 °C	Est. ampacity 90 °C	Max continuous load at 75 °C
14	15 A	18 A	20 A	14 A
12	20 A	24 A	27 A	19 A
10	27 A	32 A	37 A	25 A
8	37 A	44 A	49 A	35 A
6	50 A	59 A	67 A	47 A
4	68 A	80 A	90 A	64 A
3	79 A	93 A	105 A	74 A
2	92 A	108 A	122 A	86 A
1	107 A	126 A	142 A	100 A
1/0	124 A	146 A	165 A	116 A
2/0	144 A	170 A	192 A	136 A
3/0	168 A	198 A	223 A	158 A
4/0	195 A	230 A	260 A	184 A

Frequently asked

How do I calculate breaker size?: Find the load current, multiply by 1.25 for a continuous load, then round up to the next standard overcurrent device rating. Standard ratings are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175 and 200 amps and upward.
Why multiply the load by 125%?: A continuous load is one expected to run for three hours or more. The overcurrent device and conductors must be sized at 125% of that load so the breaker is not operating at its thermal limit indefinitely.
What breaker size do I need for 5000 watts at 240 volts?: 5000 W / 240 V is 20.8 A. At 125% that is 26 A, so the next standard breaker is 30 A, typically on 10 AWG copper.
Can a breaker be larger than the wire rating?: Generally no. The overcurrent device protects the conductor, so the breaker rating must not exceed the conductor's ampacity after derating. Limited exceptions exist for motor circuits and taps, which are outside what this calculator covers.
What is the 80% rule for breakers?: A standard breaker should carry no more than 80% of its rating continuously, which is the inverse of the 125% sizing multiplier. A 20 A breaker is therefore good for a 16 A continuous load.