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 the 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 determine 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.
How do I size a breaker?
To size a breaker, you follow these steps:
- Add up the power (in watts) of all the appliances that will share the circuit.
- Divide the total power by the circuit voltage to obtain the load current:
I = P / V. - Multiply the load current by 1.25 (the 125% safety factor) if the load is continuous — running for 3 hours or more. This is equivalent to never loading a breaker beyond 80% of its rating.
- Round the result up to the next standard breaker size (15 A, 20 A, 25 A, 30 A, 40 A, 50 A, 60 A, and so on).
How do I calculate circuit breaker and wire size?
The breaker and the wire must be matched: the wire has to safely carry at least as much current as the breaker is rated for. Once the calculator recommends a breaker size, it also suggests a compatible copper conductor in both the American Wire Gauge (AWG) and the metric (mm²) systems. As a quick reference for copper conductors:
- 15 A → 14 AWG (1.5 mm²)
- 20 A → 12 AWG (2.5 mm²)
- 30 A → 10 AWG (6 mm²)
- 40 A → 8 AWG (10 mm²)
- 50 A → 6 AWG (16 mm²)
How many watts can a 15 amp breaker handle?
A 15 A breaker on a 120 V circuit can handle a maximum of 15 A × 120 V = 1800 W. However, because a breaker should not carry a continuous load above 80% of its rating, the safe continuous limit is 1800 W × 0.8 = 1440 W. On a 230 V circuit, the same 15 A breaker handles up to 3450 W (2760 W continuous).
Microwave breaker size and other examples
- Microwave (1200 W at 120 V): draws 10 A; with the 125% factor it needs 12.5 A, so a dedicated 20 A breaker is recommended.
- Dishwasher (1500 W at 120 V): draws 12.5 A; with the factor it needs 15.6 A, so a 20 A breaker is appropriate.
- Electric oven (5000 W at 240 V): draws about 20.8 A; with the factor it needs 26 A, so a 30 A breaker fits.
FAQs
Can I put a bigger breaker on a circuit?
No. The breaker protects the wire, not just the appliance. Installing a breaker larger than the wire can safely carry defeats the protection and creates a fire risk.
Should appliances be on a dedicated circuit?
High-power and fixed appliances such as microwaves, dishwashers, ovens, and air conditioners usually require their own dedicated circuit and breaker according to electrical codes.
Is this calculator a substitute for an electrician?
No. This calculator is an educational starting point. Local electrical codes, wire insulation ratings, ambient temperature, and circuit length all affect the final selection. Always consult a licensed electrician.