Breaker Size Calculator

Our breaker size calculator allows you to estimate the breaker size you need to protect your electrical appliances while withstanding their load without using a breaker size chart.

If you're wondering, 'What breaker size does a dishwasher need?'; 'What's the correct breaker size for a microwave?'; or 'What does a circuit breaker do?', we will answer all those questions in a few paragraphs below.

You will also learn how to size a breaker and use our breaker size calculator to obtain an appropriate value.

A circuit breaker protects electrical appliances when an overload or fault is produced in the circuit. Devices connected to the same circuit lose power when the breaker trips, preventing the excess current from reaching them.

Common causes that make a circuit breaker trip are:

• Overload. When too many devices are connected, the load may be greater than the breaker can withstand. In return, the breaker trips and cuts the power to avoid damaging the equipment.
• Short circuit. When the hot wire touches the neutral wire somewhere in the circuit, the current increases significantly and overloads the circuit. If the breaker didn't trip, this short circuit could produce a fire.
• Ground fault surges. When the hot wire comes into contact with a conductive surface instead. This again produces an overload of the circuit, tripping the breaker.

Let's learn how to correctly size a breaker next.

Generally, you should size a breaker for 125% of the load (or 25% extra capacity) and no less. Oversized breakers can allow wires to heat above safety levels without stopping the current. On the other hand, undersized breakers may continuously trip under normal operation.

A circuit breaker is rated in amperes (A), and the rating tells us how much current can safely flow through the breaker without causing it to trip. For example, a 15 A breaker allows up to 15 A of total load to be connected simultaneously.

If we know the wattage of each appliance connected in the circuit, we can obtain their respective loads with different equations based on the type of current.

Load in DC

where:

• $I$ – Resulting current expressed in A;
• $W$ – Appliance's wattage in watts (W); and
• $V$ – System's voltage in volts (V).

Load in AC single-phase

Here $pf$ is the appliance's power factor (ratio between real power and apparent power).

Load in AC three-phase

This is similar to the single-phase equation, but we also divide by $\sqrt{3}$.

We covered how to size a breaker based on load. What if we want to go the other way around and figure out how many appliances you can connect to a single circuit breaker?

For example, 'How many watts can a 15 amp breaker handle?'. To answer that, we use the previous equations.

Suppose we're dealing with a $400\ \text{V}$ DC circuit, then the calculation is straightforward:

1. We multiply both sides of the breaker size for DC formula and obtain: $W = I \cdot V$.
2. Then we input our $15\ \text{A}$ and voltage to get: $W = 6000\ \text{W}$.

To calculate circuit breaker and wire size:

1. Write down an approximation of the total load you will connect to the circuit breaker.
2. Get a circuit breaker rated for 125% of this load.
3. Make sure the wire it will be paired with has a higher ampacity than the circuit breaker's rating. Otherwise, the current may heat the wire above safety levels under normal operation.
4. Always ask a certified electrician to do the work for you.

After using our breaker size calculator, input the result in the wire size calculator to obtain matching breaker-wire sizes. Alternatively, a wire-breaker size chart may provide the same information.

So far, we've covered:

• A breaker's function within a circuit;
• How to size a breaker based on load and the reverse example (watts from breaker size); and
• How to calculate circuit breaker and wire size.

You already have all the knowledge you need to use the breaker size calculator! Feel free to use our tool or keep reading to view some practical examples.

We've seen the example 'How many watts can a 15 amp breaker handle?'. Let's solve some similar problems with the breaker size calculator.

Microwave breaker size

Suppose we need to build a dedicated circuit for a microwave, so we need to pair it with an appropriate circuit breaker.

First, we need to check the current type at home (usually AC single-phase) and its voltage. Assume we do that and realize we have 230 V AC single-phase.

Then, we must find the microwave's power requirement or wattage and its power factor. You can find this information on a label on the back of the appliance. Let's say this example microwave has a $1000 \ \text{W}$ wattage paired with a $0.72$ power factor.

Last, we input the data in the breaker size calculator (or in the respective equation if we're solving manually) using the advanced mode of the calculator.

The calculator will output the microwave's load on the circuit $6.04 \ \text{A}$ and the closest standard breaker size suitable for that load $15 \ \text{A}$. This means we can probably add a few lights to that circuit as well.

What breaker size does a dishwasher need?

Again, we need to check the current type at home and the appliance's wattage and power factor.

Suppose it's a $1800 \ \text{W}$ dishwasher with a $0.88$ power factor operating with $230 \ \text{V}$ AC single-phase supply.

The formula for the load on this type of circuit says we need to divide the wattage by the power factor times the voltage. As a result, we will get a load of $8.89 \ \text{A}$, which becomes $11.11 \ \text{A}$ when we add the extra 25%.

The closest standard breaker size is $15 \ \text{A}$, so we would pick that one for our dishwasher circuit. We've already explained how to calculate circuit breaker and wire size, so remember to choose a wire with greater ampacity than the breaker's size to connect the outlets.

12A is the maximum load you should connect to a 15A circuit breaker. Your load should use at most 80% of the breaker's capacity to avoid power loss.

Some of the standard breaker sizes are: 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 400A.

You can replace a breaker with a bigger breaker, but it's not recommended. You should make sure the wiring is rated for a greater load to avoid overheating and potential fires. You should always ask a certified electrician to replace it for you.