Breaker Size Calculator

Breaker size calculator: load ÷ 0.8, then round up

This breaker size calculator finds the smallest standard breaker for a continuous load. The rule is minimum rating = continuous amps ÷ 0.8, then round up to the next standard size. A 40 A load works out to 40 ÷ 0.8 = 50 A, so it takes a 50 A breaker. The calculator shows both the raw minimum and the standard breaker it lands on.

Why the load is divided by 0.8

A continuous load runs at full current for three hours or more: an EV charger, electric baseboard heat, a pool pump, a commercial lighting circuit. The code requires its overcurrent device to be at least 125% of that load. Dividing by 0.8 and multiplying by 1.25 are the same thing. Either way, the load ends up using no more than 80% of the breaker.

Breakers only come in fixed steps, so the second step picks the first rating at or above the minimum. If the minimum is 23.4 A, the answer is 25 A, not 23.4.

Loads that are not continuous, such as a microwave or a saw, are sized at 100% of the load. If your load is not continuous, this tool will give a conservative (larger) answer.

Worked examples

1. Default: 40 A continuous load

• Minimum: 40 ÷ 0.8 = 50.00 A
• Next standard size at or above 50: 50 A breaker

2. 13 A continuous lighting circuit

• Minimum: 13 ÷ 0.8 = 16.25 A
• 15 A is too small, so the next size is a 20 A breaker

3. 48 A Level 2 EV charger

• Minimum: 48 ÷ 0.8 = 60.00 A
• Result: 60 A breaker

Continuous load to breaker reference

Continuous load	Minimum rating	Standard breaker
12 A	15.00 A	15 A
16 A	20.00 A	20 A
20 A	25.00 A	25 A
24 A	30.00 A	30 A
32 A	40.00 A	40 A
40 A	50.00 A	50 A
48 A	60.00 A	60 A
64 A	80.00 A	80 A
80 A	100.00 A	100 A

The tool’s list stops at 200 A, so it is meant for branch circuits and small feeders. Any continuous load above 160 A needs a larger device and an engineered service design.

How to find your continuous load

• Read the nameplate. Many appliances list full-load amps directly.
• Convert watts. Amps = watts ÷ volts. A 4,800 W heater at 240 V draws 20 A. The watts to amps calculator does this step.
• Add everything on the circuit. Two 1,500 W heaters on one 120 V circuit draw 12.5 A each, 25 A together.
• Check the voltage. The same 3,600 W load draws 30 A at 120 V but only 15 A at 240 V, which changes the breaker by several sizes.
• Use the manufacturer’s breaker size when given. Equipment instructions take priority over a generic rule.

Breakers protect the wire, not the appliance

A breaker’s main job is to keep the wire from overheating. That is why the breaker and wire are chosen together, and why a breaker must never be larger than the wire is rated for. Appliances have their own internal protection.

In practice, sizing goes in this order. First, find the load and decide whether it is continuous. Second, size the breaker with this tool. Third, choose wire whose ampacity is at least the breaker rating, then check it for voltage drop over the run length. Finally, confirm the panel has room and enough capacity for the new circuit. A load calculation on the whole panel is a separate step an electrician can do.

Common breaker sizing mistakes

• Sizing the breaker without the wire. A 30 A breaker on 12 AWG wire is a fire risk. Size the conductor with the wire size calculator.
• Forgetting motor rules. Motors, compressors, and heat pumps have their own sizing rules and nameplate maximums.
• Rounding down. If the minimum is 20.5 A, a 20 A breaker is too small.
• Ignoring voltage drop. A long run to a detached shop can need heavier wire even with the right breaker. Check it with the voltage drop calculator.
• Upsizing a tripping breaker. A breaker that trips is often doing its job. Find the cause rather than swapping in a bigger one.

Electrical work is code-governed and safety-critical. Have circuit, breaker, and conductor sizing confirmed by a licensed electrician and your local inspector.