Breaker Size Calculator

What the breaker size calculator does

A circuit breaker must be sized to carry a circuit's load safely while protecting the wiring. This calculator estimates the required breaker rating from the power, voltage, current type, and power factor, and applies a safety factor for continuous loads.

Below is what sizing a breaker means, the equations behind it, why the safety factor matters, and a worked example.

How to use it

1. Choose the current type: DC, single-phase AC, or three-phase AC.
2. Enter the voltage and power, adding the power factor for AC, and set the safety factor.
3. Press Calculate for the required breaker rating, or Reset to clear it.

What sizing a breaker means

A circuit breaker is a safety device that automatically cuts off the electrical supply if the current exceeds a safe level, protecting the wiring from overheating and preventing fires. Sizing a breaker means choosing one with the right current rating: large enough to carry the normal load without nuisance tripping, but small enough to trip before the wires it protects are overloaded. Too small a breaker trips constantly; too large a breaker fails to protect the circuit, which is the more dangerous mistake.

The starting point for sizing is the current the load actually draws, which follows from its power and voltage. But the breaker is not set to exactly that current, because circuits that run for long periods need a margin to handle the sustained heating safely. This calculator works out the load current for direct current, single-phase, or three-phase systems, then applies a safety factor to give a required breaker rating. It is a first step in a process that, for real installations, also involves codes and conductor checks.

The equations it uses

The calculator first finds the load current from the power and voltage. For direct current it is the power divided by the voltage; for single-phase alternating current the power factor enters, I = P ÷ (V × PF); and for three-phase the square root of three appears as well. It then applies the safety factor to that current:

breaker rating = current × (1 + safety factor)

So a 25 percent safety factor multiplies the current by 1.25. This gives the minimum rating the breaker must have to serve the load with the required margin. The result is a current in amperes, which you then match to a real breaker, choosing the next standard size at or above it.

The safety factor and continuous loads

The safety factor exists because of how circuits heat up under sustained load. A load that runs only briefly produces brief heating that the wiring shrugs off, but a load that runs for hours produces sustained heating that builds up. To handle this safely, electrical codes require that circuits serving continuous loads, those running for three hours or more, be sized for more than the actual current, typically 125 percent of it, which is the same as adding a 25 percent margin.

This is why the calculator defaults to a 25 percent safety factor, matching the common rule for continuous loads. For loads that are not continuous, a smaller margin may be appropriate, and the calculator lets you adjust the factor accordingly. The margin ensures the breaker does not sit right at the edge of its capacity during long operation, where it might trip unexpectedly or run hot. Applying the safety factor is one of the most important steps in breaker sizing, and getting it right is essential for both safety and reliable operation.

Rounding up to a standard size

The figure this calculator produces is the minimum rating the breaker must meet, but breakers are not made in every possible size. They come in standard ratings such as 15, 20, 25, 30, 40, and 50 amperes, and you must choose an actual breaker from this list. The rule is to round up to the next standard size at or above the calculated minimum, never down, so that the breaker comfortably covers the required rating.

For example, a calculated minimum of 12.5 amperes would be served by a 15-ampere breaker, the next standard size up. Rounding down to a smaller breaker would risk nuisance tripping and might not match the wiring properly. It is also vital to remember that the breaker must protect the wire, so the conductor feeding the circuit has to be rated for at least as much current as the breaker. This calculator gives the minimum required current; selecting the real breaker, and matching it to suitable wiring, is the next step in a complete design that should follow the applicable electrical code.

Units and precision

The calculator takes the power in watts, the voltage in volts, the power factor as a value between 0 and 1 for alternating current, and the safety factor as a percentage. It returns the minimum required breaker rating in amperes. It applies the correct current relationship for each system type and the safety margin you choose. The result is a starting point for design; real installations must also check conductor ampacity, terminal temperature ratings, and the rules of the governing electrical code, ideally with a licensed professional.

A worked example

Suppose a single-phase AC load of 2,400 watts runs at 240 volts with a power factor of one, and you apply a 25 percent safety factor for a continuous load.

The load current is I = P ÷ (V × PF) = 2,400 ÷ 240 = 10 amperes. Applying the safety factor, the required breaker rating is 10 × 1.25 = 12.5 amperes. Since breakers come in standard sizes, you would round up to the next one, a 15-ampere breaker, and make sure the wiring feeding the circuit is rated to match.

Questions people ask

How do you size a circuit breaker?

Find the load current from power and voltage, apply a safety factor, typically 25 percent for continuous loads, then round up to the next standard breaker size.

Why multiply by 1.25?

Because circuits serving continuous loads, running three hours or more, must be sized for 125 percent of the current to handle sustained heating safely. That is the same as a 25 percent margin.

What are standard breaker sizes?

Common ratings include 15, 20, 25, 30, 40, and 50 amperes. You round the calculated minimum up to the next standard size at or above it, never down.

Does the breaker have to match the wire?

Yes. The breaker protects the wire, so the conductor must be rated for at least the breaker's current. Sizing the breaker and the wire go together in a complete design.

References

A quick note on where this comes from. Breaker sizing, the continuous-load rule, and standard ratings follow the National Electrical Code, published by the National Fire Protection Association, with the underlying current relationships standard in OpenStax's University Physics. Real installations should follow the applicable code and a licensed professional.

1. National Fire Protection Association (NFPA), NFPA 70 National Electrical Code, Articles 210 and 240. https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
2. OpenStax, University Physics Volume 2, Section 9.5, Electrical Energy and Power. https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power
3. HyperPhysics, Electric Power. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepow.html