RoughInSizing calculators for the trades

Breaker size calculator

Enter the load in watts or amps, or pick a motor. You get the breaker size, the wire that goes with it, and how much of the breaker the load uses, by the NEC rules for continuous loads and motors.

NEC 210.20 continuous loadsMotor tables 430.248 and 430.250Wire for every breaker

Inputs

What is on the circuit?
Load in
W
From the nameplate or label.
Power
Runs for 3 hours or more?
NM cable is sized from the 60 °C column. Long runs may need a bigger wire for voltage drop.

Result

Breaker size
—

Breaker sizes for common loads

Single-phase, by the rules on this page. Continuous loads are sized at 125%.

LoadAmpsContinuousBreakerNM cableTHHN
Space heater, 1,500 W at 120 V12.5 AYes20 A12 AWG12 AWG
Baseboard heater, 2,000 W at 240 V8.3 AYes15 A14 AWG14 AWG
Electric water heater, 4,500 W at 240 V18.8 AYes25 A10 AWG10 AWG
EV charger, 48 A at 240 V48.0 AYes60 A4 AWG6 AWG
EV charger, 32 A at 240 V32.0 AYes40 A8 AWG8 AWG
Electric dryer, 5,600 W at 240 V23.3 ANo25 A10 AWG10 AWG
Microwave, 1,200 W at 120 V10.0 ANo15 A14 AWG14 AWG
Dishwasher, 1,800 W at 120 V15.0 ANo15 A14 AWG14 AWG
Hot tub heater, 5,500 W at 240 V22.9 AYes30 A10 AWG10 AWG
Shop heater, 10 kW at 240 V41.7 AYes60 A6 AWG6 AWG

How breaker size is worked out

amps = watts ÷ volts (single-phase) amps = watts ÷ (volts × 1.732) (three-phase) breaker ≥ amps × 1.25 for a continuous load (3 hours or more) breaker ≥ amps for anything else then round up to the next standard size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100…

The 125% rule (NEC 210.20(A)) is why a 1,500 W heater, which draws 12.5 A, needs a 20 A breaker and not a 15. Standard breakers are only rated to carry 80% of their rating continuously, and 125% is the same thing turned around. Space heaters, baseboard heat, water heaters, EV chargers, pool and hot tub heaters and lighting left on all evening are all continuous loads.

The wire has to match the breaker

The breaker protects the wire, not the appliance. Once the breaker size is set, the wire must be rated for it: 14 AWG copper for 15 A, 12 AWG for 20 A and 10 AWG for 30 A (NEC 240.4(D)), whatever the ampacity table says. Above 30 A the wire follows its ampacity, and NM cable must be sized from the 60 °C column. Where a wire’s ampacity falls between two breaker sizes, NEC 240.4(B) allows the next breaker up, as long as the wire still covers the load: that is how 6 AWG NM, rated 55 A, can go on a 60 A breaker.

Motors follow different rules

A motor draws several times its running current for a moment as it starts, so its breaker is sized to ride through that, and a separate overload device protects it while running. The NEC uses its own full-load current tables, not the nameplate: Table 430.248 for single-phase motors and 430.250 for three-phase. An inverse-time breaker can be up to 250% of that current, rounded up to the next standard size, and the wire is sized at 125%. Because of that, a motor circuit can legally have a 30 A breaker on 14 AWG wire. If the motor still trips the breaker on start, NEC 430.52 allows up to 400% for motors drawing 100 A or less.

Air conditioners and heat pumps use the minimum circuit ampacity and maximum breaker printed on their nameplate instead. Use the minimum circuit ampacity calculator for those.

How many watts can a breaker take?

Continuous loads are limited to 80% of the breaker; anything else can use the full rating.

Breaker120 V, continuous120 V, max240 V, continuous240 V, max
15 A1,440 W1,800 W2,880 W3,600 W
20 A1,920 W2,400 W3,840 W4,800 W
25 A2,400 W3,000 W4,800 W6,000 W
30 A2,880 W3,600 W5,760 W7,200 W
40 A3,840 W4,800 W7,680 W9,600 W
50 A4,800 W6,000 W9,600 W12,000 W
60 A5,760 W7,200 W11,520 W14,400 W
70 A6,720 W8,400 W13,440 W16,800 W
80 A7,680 W9,600 W15,360 W19,200 W
100 A9,600 W12,000 W19,200 W24,000 W

Wire size for each breaker

The smallest wire whose ampacity covers the full breaker rating, by NEC Table 310.16 and 240.4(D).

BreakerCopper NM, 60 °CCopper THHN, 75 °CAluminum, 75 °C
15 A14 AWG14 AWG12 AWG
20 A12 AWG12 AWG10 AWG
25 A10 AWG10 AWG10 AWG
30 A10 AWG10 AWG8 AWG
40 A8 AWG8 AWG8 AWG
50 A6 AWG8 AWG6 AWG
60 A4 AWG6 AWG4 AWG
70 A4 AWG4 AWG3 AWG
80 A3 AWG4 AWG2 AWG
90 A2 AWG3 AWG2 AWG
100 A1 AWG3 AWG1 AWG
125 A1/0 AWG1 AWG2/0 AWG
150 A3/0 AWG1/0 AWG3/0 AWG
200 A250 kcmil3/0 AWG250 kcmil

Motor breaker chart

Single-phase motors, NEC Table 430.248 full-load current, inverse-time breaker at 250% rounded up, copper THHN at 125%.

Motor115 V FLCBreakerWire230 V FLCBreakerWire
1/6 hp4.4 A15 A14 AWG2.2 A15 A14 AWG
1/4 hp5.8 A15 A14 AWG2.9 A15 A14 AWG
1/3 hp7.2 A20 A14 AWG3.6 A15 A14 AWG
1/2 hp9.8 A25 A14 AWG4.9 A15 A14 AWG
3/4 hp13.8 A35 A14 AWG6.9 A20 A14 AWG
1 hp16 A40 A14 AWG8 A20 A14 AWG
1-1/2 hp20 A50 A12 AWG10 A25 A14 AWG
2 hp24 A60 A10 AWG12 A30 A14 AWG
3 hp34 A90 A8 AWG17 A45 A12 AWG
5 hp56 A150 A4 AWG28 A70 A10 AWG
7-1/2 hp80 A200 A3 AWG40 A100 A8 AWG
10 hp100 A250 A1 AWG50 A125 A6 AWG

Three-phase motors, NEC Table 430.250.

Motor208 V FLCBreakerWire460 V FLCBreakerWire
1/2 hp2.4 A15 A14 AWG1.1 A15 A14 AWG
3/4 hp3.5 A15 A14 AWG1.6 A15 A14 AWG
1 hp4.6 A15 A14 AWG2.1 A15 A14 AWG
1-1/2 hp6.6 A20 A14 AWG3 A15 A14 AWG
2 hp7.5 A20 A14 AWG3.4 A15 A14 AWG
3 hp10.6 A30 A14 AWG4.8 A15 A14 AWG
5 hp16.7 A45 A12 AWG7.6 A20 A14 AWG
7-1/2 hp24.2 A70 A10 AWG11 A30 A14 AWG
10 hp30.8 A80 A8 AWG14 A35 A14 AWG
15 hp46.2 A125 A6 AWG21 A60 A10 AWG
20 hp59.4 A150 A4 AWG27 A70 A10 AWG
25 hp74.8 A200 A3 AWG34 A90 A8 AWG
Check before you change a breaker. Never put a bigger breaker on an existing circuit to stop it tripping; the wire in the wall sets the limit. Breakers must also be the type listed for your panel, and AFCI or GFCI protection may be required depending on the room. When in doubt, use a licensed electrician and the code your area has adopted.

Questions

How do I calculate what size breaker I need?

Work out the amps (watts divided by volts), multiply by 1.25 if the load runs for three hours or more, and round up to the next standard breaker size. A 1,500 W heater at 120 V draws 12.5 A; times 1.25 is 15.6 A, so it needs a 20 A breaker on 12 AWG copper wire.

What is the 80% rule for breakers?

A standard breaker can carry only 80% of its rating continuously, meaning for three hours or more. That is the same as sizing the breaker at 125% of a continuous load. A 20 A breaker can carry 16 A continuously, which is 1,920 W at 120 V.

What size breaker do I need for a 48 amp EV charger?

A 60 A breaker. EV chargers are continuous loads, so 48 A × 1.25 = 60 A. With copper THHN that needs 6 AWG; with NM cable, 4 AWG.

What size breaker for a 4,500 watt water heater?

4,500 W at 240 V is 18.75 A. Water heaters count as continuous loads, so × 1.25 = 23.4 A, which rounds up to a 25 A breaker on 10 AWG copper. Many installers use 30 A on 10 AWG, which is also allowed.

How many amps can a 20 amp breaker handle?

20 A for short loads, and 16 A for loads that run three hours or more. At 120 V that is 2,400 W and 1,920 W.

What size breaker for a 2 hp motor?

For a 2 hp single-phase motor at 230 V, NEC Table 430.248 gives 12 A full-load current. 250% of that is 30 A, so a 30 A inverse-time breaker, with 14 AWG copper sized at 125% (15 A). At 115 V it is 24 A and a 60 A breaker on 10 AWG. The motor also needs its own overload protection.

Can I put a 30 amp breaker on 12 gauge wire?

Not for a normal circuit: 12 AWG copper is limited to a 20 A breaker by NEC 240.4(D). The exceptions are motor and air-conditioner circuits, where the code sizes the breaker to let the motor start and protects the wire with a separate overload, so the nameplate’s maximum breaker can exceed the usual limit.

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