What Size Breaker Do I Need? Sizing Rules and Calculator

The question sounds like it should have a lookup-table answer, and for common cases it does. But the reasoning matters, because the intuitive approach — pick the breaker that stops the tripping — is exactly backwards and is how conductors end up overheating inside finished walls.

What determines breaker size?

A breaker protects the conductor, not the appliance.

Every wire size has a safe current-carrying capacity, its ampacity. Exceed it for long enough and the insulation degrades and the conductor heats. The breaker's entire job is to disconnect before that happens.

So the sequence is:

1. Determine the load the circuit must carry.
2. Choose a conductor whose ampacity covers that load, accounting for derating.
3. Choose a breaker that protects that conductor and covers the load.

Never: choose a breaker, then wonder what wire goes with it.

What is the 80% rule for breakers?

Start with amps. If you have watts, divide by volts:

• 1,500 W ÷ 120 V = 12.5 A
• 4,800 W ÷ 240 V = 20 A

Then ask whether it's continuous — expected to run at its maximum for three hours or more. EV charging, electric heating, and most commercial lighting are continuous. A microwave is not.

For continuous loads, the NEC requires the overcurrent device to be rated at 125% of the load. Practically, that means loading a breaker to no more than 80% of its rating:

This is why a 48-amp EV charger requires a 60-amp circuit, and why a 40-amp charger requires 50 amps. Our level 2 charger breaker size calculator applies this automatically.

How does wire size limit breaker size?

Ampacity comes from NEC Table 310.16 and depends on the conductor material, its insulation temperature rating, ambient temperature, and how many current-carrying conductors share a raceway or cable.

Then NEC 240.4(D) caps the small sizes regardless of what the table allows:

Above 10 AWG, the table governs, with derating. Aluminum conductors have lower ampacity than copper at the same size and need larger conductors for the same breaker — plus terminations listed for aluminum.

Our wire and breaker size chart and the wire size by amperage pages lay out the common residential combinations.

Step 3: What the equipment requires

For a listed appliance, the manufacturer's instructions are enforceable under the NEC. Look on the nameplate or in the manual for:

• Minimum circuit ampacity (MCA) — the smallest conductor ampacity permitted
• Maximum overcurrent protection (MOCP) — the largest breaker permitted

Air conditioners and heat pumps almost always specify both, and the MOCP is frequently larger than you'd calculate from the running current, because motor inrush needs headroom. This is one of the few cases where a breaker larger than the running load is correct — and it's specified by the manufacturer, not chosen.

What breaker size do common appliances need?

Treat this as orientation, not as a substitute for a calculation with your actual conductor lengths, insulation type, and installation conditions.

What else affects breaker sizing?

• Voltage drop over long runs. The NEC's ampacity tables don't account for it, but a long run to a detached garage or a well may need a larger conductor to keep voltage drop within recommended limits.
• Bundling and ambient temperature. More than three current-carrying conductors in a raceway, or a hot attic, both derate ampacity.
• Termination temperature ratings. Most residential terminations are rated 75°C, which caps the ampacity you may use even if the conductor insulation is rated higher.
• Aluminum. Different ampacity, and terminations must be listed for aluminum.

The one thing to take away

If a circuit trips repeatedly, the answer is never a bigger breaker. It's fewer loads on that circuit, or another circuit. See why does my breaker keep tripping.

Code reference

The requirements behind this guide, for looking up in your adopted edition:

• NEC 240.4(D) — small-conductor overcurrent limits (14 AWG → 15 A, 12 AWG → 20 A, 10 AWG → 30 A)
• NEC 240.6 — standard ampere ratings for fuses and breakers
• NEC 310.16 — allowable ampacity table
• NEC 110.14(C) — temperature limitations at terminations

Code editions and local amendments vary by jurisdiction. Confirm the adopted edition with your AHJ before relying on any specific article.

Where to go next

• Wire and breaker size chart
• Single-pole vs double-pole breakers
• Home electrical load calculator
• 20 amp outlet vs 15 amp
• 50 amp wire size

More in our electrical panel guides.