ElectricalCharts

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

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 — source: 125% continuous-load multiplier with standard device ratings; conductor from the site ampacity model

Breaker size and conductor by load current

Load (A)Design at 125% (A)Breaker (A)Min. copper conductor
1012.51514 AWG
1215.01514 AWG
1620.02012 AWG
2025.02510 AWG
2430.03010 AWG
3037.5408 AWG
4050.0506 AWG
4860.0604 AWG
6075.0804 AWG
7087.5903 AWG
80100.01002 AWG
100125.01251 AWG
125156.31752/0 AWG
150187.52003/0 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 sizeEst. ampacity 60 °CEst. ampacity 75 °CEst. ampacity 90 °CMax continuous load at 75 °C
1415 A18 A20 A14 A
1220 A24 A27 A19 A
1027 A32 A37 A25 A
837 A44 A49 A35 A
650 A59 A67 A47 A
468 A80 A90 A64 A
379 A93 A105 A74 A
292 A108 A122 A86 A
1107 A126 A142 A100 A
1/0124 A146 A165 A116 A
2/0144 A170 A192 A136 A
3/0168 A198 A223 A158 A
4/0195 A230 A260 A184 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.

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Gear for panel work

Tools for measuring load and terminating conductors.

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