What size breaker do you need?

What size breaker do I need?

Find the load current and, if it runs 3 hours or more, multiply it by 1.25; then pick the next standard size. A 1,500 W heater on 120 V draws 12.5 A; as a continuous load it needs 12.5 × 1.25 = 15.6 A, so a 20 A breaker.

What size wire goes with a 20 amp breaker?

12 AWG copper. The National Electrical Code limits 14 AWG copper to 15 A, 12 AWG to 20 A and 10 AWG to 30 A, whatever the table ampacity. Aluminum needs one size larger: 10 AWG aluminum for 20 A.

What wire do I need for a 40 or 50 amp breaker?

With NM cable (Romex), which uses the 60°C column: 8 AWG copper for 40 A and 6 AWG copper for 50 A. With THHN in conduit and 75°C terminals, 8 AWG copper carries 50 A. Long runs may need a bigger wire for voltage drop — check it with the wire size calculator.

What counts as a continuous load?

A load that runs at full current for 3 hours or more. Water heaters, baseboard heaters, pool pumps and electric vehicle chargers are treated as continuous; a toaster or a hair dryer is not.

What are the standard breaker sizes?

NEC 240.6(A) lists 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175 and 200 A, and larger. Homes mostly use 15 and 20 A for general circuits and 30 to 60 A for dryers, ranges, water heaters and EV chargers.

Why 125% for a continuous load?

NEC 210.20(A) requires the branch-circuit breaker to be rated for the noncontinuous load plus 125% of the continuous load. Put the other way, a continuous load should stay at or below 80% of the breaker rating, because breakers and their terminals heat up when they carry full current for hours.

Can I put a bigger breaker on a circuit that keeps tripping?

No. The breaker protects the wire, so it may never be larger than the wire's allowed current. A 20 A breaker on 14 AWG copper is a fire hazard. A tripping breaker means the load is too large for the circuit, so reduce the load or have an electrician run a new circuit with larger wire.

Does this work for motors and air conditioners?

No. Motor circuits, compressors and similar equipment follow special NEC rules (Article 430 and the equipment's nameplate), and the breaker may be sized above the running current to allow for starting surge. Use the nameplate and a licensed electrician for those.

How the calculation works

The calculator first finds the load current. If you give watts, it divides by the voltage (and by the power factor and the square root of 3 where applicable for three-phase loads). A continuous load, one that runs for 3 hours or more, is multiplied by 1.25. The result is rounded up to the next standard breaker rating from NEC 240.6(A). The wire shown is the smallest copper or aluminum conductor whose allowed current is at least the breaker rating, using the NEC 310.16 ampacity table and the small-conductor limits in 240.4(D) (14 AWG copper at 15 A, 12 AWG at 20 A, 10 AWG at 30 A).

Worked examples

A 1800 W heater on 120 V draws 1800 ÷ 120 = 15 A. As a continuous load that becomes 15 × 1.25 = 18.75 A, so the next standard size is a 20 A breaker. A 7200 W electric-vehicle charger on 240 V draws 30 A; at 125% that is 37.5 A, so it needs a 40 A breaker. A 3000 W load on 240 V that runs only for short periods draws 12.5 A, with no 125% factor, so a 15 A breaker is enough.

Common mistakes

• Forgetting the voltage. The same 1,800 W is 15 A at 120 V and 7.5 A at 240 V. Check which voltage the circuit really uses.
• Choosing the breaker first and the wire second. The wire must carry the breaker's rating. Going up a breaker size also means checking the wire size.
• Counting several loads on one circuit separately. Add the loads on the circuit before picking the breaker, and apply 125% to the part that is continuous.

Sources and related calculators

Rules: National Electrical Code (NFPA 70), sections 210.20(A) for branch-circuit rating, 240.6(A) for standard ratings, 240.4(D) for small conductors and Table 310.16 for ampacity. Related: wire size calculator, watts to amps calculator and box fill calculator.

Code minimums for planning only. Your local code edition and a licensed electrician decide, and electrical work should be done by a qualified person. How we check our calculators.