Breaker Size Calculator

Q: What size breaker do I need for a 1500W heater?

At 120V: 1500W ÷ 120V = 12.5A. Since this is a continuous load, multiply by 1.25 = 15.6A. You need a 20A breaker (next standard size above 15.6A).

Q: What is the 125% rule for circuit breakers?

The NEC requires that a continuous load (running 3+ hours) must not exceed 80% of the breaker rating. This is equivalent to sizing the breaker at 125% of the calculated current. For example, a 16A continuous load needs a 20A breaker (16 × 1.25 = 20A).

Q: What are standard circuit breaker sizes?

Standard residential breaker sizes are: 15A, 20A, 25A, 30A, 35A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, 125A, 150A, 175A, 200A.

Find the right circuit breaker size for your load. Accounts for the NEC 125% continuous load rule and gives you the next standard breaker size.

Calculate Breaker Size

Load in Watts (W)

Voltage (V)

Is this a continuous load? (running 3+ hours)

Result

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Calculated Current

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Minimum Breaker

—

Recommended Standard

Formula

Current (A) = Watts ÷ Voltage
If continuous load: Minimum Breaker = Current × 1.25
Round up to the next standard breaker size

Standard breaker sizes: 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200A

Example Calculation

Sizing a breaker for a 3,000W electric water heater at 240V (continuous load):

Current = 3000W ÷ 240V = 12.5A
Continuous load: 12.5A × 1.25 = 15.6A minimum
Next standard breaker size = 20A

A 20A double-pole breaker at 240V is the correct choice for this water heater.

Practical Notes

A continuous load is any load that runs for 3 or more hours without interruption.
The NEC 125% rule means your load should not exceed 80% of the breaker rating continuously.
Always size the wire to match or exceed the breaker — not just the load.
For 240V loads, you need a double-pole breaker (two slots in your panel).
This calculator provides guidance only. Have a licensed electrician verify sizing before installation.

Frequently Asked Questions

What size breaker do I need for a 1500W heater?

At 120V: 1500W ÷ 120V = 12.5A. Since it runs continuously, multiply by 1.25 = 15.6A. You need a 20A breaker — the next standard size above 15.6A. Do not use a 15A breaker for a continuous 12.5A load.

What is the 125% rule for circuit breakers?

The NEC requires continuous loads to not exceed 80% of the breaker's rating. Equivalently, size the breaker at 125% of the load current. Example: a 16A continuous load needs a 20A breaker (16A × 1.25 = 20A minimum rating).

What are standard circuit breaker sizes?

Common residential and commercial breaker sizes: 15A, 20A, 25A, 30A, 35A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, 125A, 150A, 175A, 200A. You always round up to the next available size above your minimum requirement.