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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.

⚠️ Remember to use this tool as a starting point only, and allow a licensed electrician to get and install the correct breaker size for your needs.

What does a circuit breaker do?

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.

🙋 Our breaker size calculator allows you to plan ahead and input up to 5 different appliances to estimate the total load the breaker will need to protect.

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

How do I size a breaker?

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

I=WV\quad I = \frac{W}{V}


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

Load in AC single-phase

I=WVpf\quad I = \frac{W}{V \cdot pf}

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

Load in AC three-phase

I=WVpf3\quad I = \frac{W}{V \cdot pf \cdot \sqrt{3}}

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

How many watts can a 15 amp breaker handle?

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 V400\ \text{V} DC circuit, then the calculation is straightforward:

  1. We multiply both sides of the breaker size for DC formula and obtain: W=IVW = I \cdot V.
  2. Then we input our 15 A15\ \text{A} and voltage to get: W=6000 WW = 6000\ \text{W}.

How do I calculate circuit breaker and wire size?

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.

🔎 If you want to learn more about wires used in electrical circuits, check our wire gauge calculator!

Microwave breaker size and other 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 W1000 \ \text{W} wattage paired with a 0.720.72 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 A6.04 \ \text{A} and the closest standard breaker size suitable for that load 15 A15 \ \text{A}. This means we can probably add a few lights to that circuit as well.

🙋 If you're solving the equation manually, remember to add an extra 25% capacity to the breaker.

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 W1800 \ \text{W} dishwasher with a 0.880.88 power factor operating with 230 V230 \ \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 A8.89 \ \text{A}, which becomes 11.11 A11.11 \ \text{A} when we add the extra 25%.

The closest standard breaker size is 15 A15 \ \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.

🙋 Now let's find out the monthly cost of running all your appliances using our appliance wattage calculator.


How much load can I connect to a 15A circuit breaker?

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.

What are the standard breaker sizes?

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.

Can I replace a breaker with a bigger breaker?

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.

Luciano Miño
Current type
AC single-phase
Input up to 5 appliances:
Appliance A
Appliance type
Appliance B (leave empty if unused)
Appliance type
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