Calculators · Breaker Size Calculator

Electrical

Breaker Size Calculator

Find the minimum breaker rating for a load in amps, watts or kW, with the 125% rule for continuous loads. Or check what a breaker can carry.

Type of circuit

DC, single-phase AC or three-phase AC.

Continuous load

Runs at its maximum for 3 hours or more. Amps, watts or kW.

Noncontinuous load

Optional. Runs for less than 3 hours at a time.

Voltage (V)

Only needed if you type watts or want watts in the result.

Power factor (PF)

Only used for AC watts. Above 0 and up to 1. Use 1 for a purely resistive load.

The breaker and its enclosure are listed for 100% operation

Leave this off unless the breaker is marked as 100% rated. With it on, the continuous load is not increased by 25%.

How to size a breaker for a load

Start with the current the load draws. If you only know watts, divide by the voltage. For a purely resistive single-phase or DC load, amps = watts ÷ volts. For AC loads, also divide by the power factor, and for three-phase by √3 as well (see the watts to amps calculator). Then apply the rule for continuous loads.

Load	Minimum breaker rating
Noncontinuous only (under 3 hours at a time)	100% of the load
Continuous only (3 hours or more)	125% of the load
Both on the same circuit	Noncontinuous load + 125% of the continuous load
Breaker and enclosure listed for 100% operation	Continuous load + noncontinuous load

This is the rule for continuous and noncontinuous loads that appears in the National Electrical Code (NEC). We did not copy the code. We checked the rule and the 3-hour definition in a technical paper from ABB (2017, quoting the 2014 edition) and in an article from IAEI Magazine, and for EV chargers in the City of Philadelphia's EZ permit standard (2020 edition). We opened them in October 2026. The edition your area has adopted and any local changes can differ, so your local building department has the final say.

Example 1: a continuous load of 16 amps

A single-phase load runs at 16 A for hours. In the calculator, type 16 as the continuous load and leave the noncontinuous load empty.

Step	Operation	Result
Continuous load counted at 125%	16 × 1.25	20 A
Noncontinuous load	none	0 A
Minimum breaker rating	0 + 20	20 A

Example 2: continuous and noncontinuous loads together

A circuit carries 30 A of continuous load and 10 A of noncontinuous load.

Step	Operation	Result
Continuous load counted at 125%	30 × 1.25	37.5 A
Add the noncontinuous load	37.5 + 10	47.5 A

The minimum is 47.5 A. A breaker rated lower than that does not meet the rule. Pick a rating at or above 47.5 A that exists for your panel.

Example 3: a load given in watts

A continuous load of 9,600 W on a 240 V single-phase circuit with a power factor of 1. Choose "W" as the unit and type 240 as the voltage.

Step	Operation	Result
Current	9,600 ÷ 240	40 A
Counted at 125%	40 × 1.25	50 A

Continuous load to minimum rating

Multiply the continuous load by 1.25 to check any row. This is only the arithmetic of the rule. It is not a list of standard breaker sizes.

Continuous load	Minimum rating (× 1.25)
8 A	10 A
12 A	15 A
16 A	20 A
20 A	25 A
24 A	30 A
32 A	40 A
40 A	50 A
48 A	60 A
60 A	75 A
80 A	100 A

Breaker rating to largest continuous load

The other direction. A breaker supports a continuous load of at most 80% of its rating. The watts are for a single-phase circuit at 240 V with a power factor of 1. The ratings in the first column are examples for the arithmetic.

Breaker rating	Continuous load (× 0.8)	Watts at 240 V
15 A	12 A	2,880 W
20 A	16 A	3,840 W
30 A	24 A	5,760 W
40 A	32 A	7,680 W
50 A	40 A	9,600 W
60 A	48 A	11,520 W
100 A	80 A	19,200 W

The 100% rated exception

The ABB paper explains that the rule has an exception. If the assembly, the breaker together with its enclosure, is listed for operation at 100% of its rating, the rating only has to cover the sum of the continuous and the noncontinuous load. Such breakers carry markings with requirements such as a minimum enclosure size and the temperature rating of the wire. In the paper's own example, a noncontinuous load of 150 A plus a continuous load of 600 A calls for 150 + 1.25 × 600 = 900 A with an ordinary breaker, and for 150 + 600 = 750 A with a 100% rated one. Use the checkbox in the calculator only if your breaker is marked that way.

What this calculator does not do

It does not choose the wire. It does not apply temperature or bundling corrections. It does not cover motors, air conditioners or EV chargers beyond the 125% rule, and it does not round up to a standard breaker size, because we did not find an open source with that list. For the loads of a whole home, see the electrical load calculator, and for apparent power see the kVA calculator. The electrical code adopted where you live sets the final rules, and a licensed electrician must size and install the circuit. Nothing you type leaves your browser.

Frequently asked questions

How do you calculate the correct breaker size?

Find the load in amps. Divide watts by volts for a DC or single-phase circuit, or watts by (√3 × volts × power factor) for three-phase. Then add the noncontinuous load to 125% of the continuous load. That is the minimum rating of the breaker. For example, 30 amps of continuous load plus 10 amps of noncontinuous load gives 10 + 1.25 × 30 = 47.5 amps. Choose a breaker rated at or above that number, and have a licensed electrician check the wire and the rest of the circuit.

What is a continuous load?

It is a load where the maximum current is expected to continue for 3 hours or more. A load that runs for less than 3 hours at a time is noncontinuous and is counted at 100%. The sources we read for this page state the 3-hour definition and the 125% rule. If you are not sure which kind your load is, treat it as continuous and ask an electrician.

Can I replace a 20 amp breaker with a 30 amp breaker?

Not on the basis of this calculator. The breaker rating is tied to the wire it protects, and one manufacturer paper we read says the cable size is based on the breaker rating. A bigger breaker on wire that was sized for a smaller one can leave the wire unprotected. Have a licensed electrician check the wire, the equipment and the panel first.

What is the biggest breaker you can put in a 200 amp panel?

This page does not give a number, because we did not find a source we could check. The answer depends on the markings of the panel and on the electrical code adopted where you live. Read the label inside the panel and ask a licensed electrician.

Is 25 amp a standard breaker size?

We did not find an open source with the list of standard ratings, so this calculator gives the minimum rating and does not round it up to a standard size. Look at the ratings that the maker of your panel and breakers sells, and pick one at or above the result.

What is the 80% rule for breakers?

It is the 125% rule seen from the breaker side. Because 1 ÷ 1.25 = 0.8, a breaker should carry a continuous load of no more than 80% of its rating. A 20 amp breaker supports 16 amps of continuous load, and a 50 amp breaker supports 40 amps. A noncontinuous load can use the full rating. The "What a breaker can carry" tab does this calculation.

Does this work for motors, air conditioners or EV chargers?

It applies one rule, the 125% rule for continuous loads, to the load you type. Motors, air conditioners and EV chargers have installation instructions and code rules of their own. For example, the city of Philadelphia says an EV charger must be installed per the manufacturer's instructions, and that its overcurrent protection is not less than 125% of the maximum load of the equipment. Follow the instructions of the equipment and ask a licensed electrician.