Electrical load and breaker size calculator

What size breaker do I need?

Convert each load to amps, count continuous loads (running 3 hours or more) at 125%, and pick the next standard rating at or above the total (NEC 2023 210.20(A) and 240.6(A)). A continuous 1,500 W heater plus a 150 W TV and two 60 W lamps on 120 V needs at least 17.875 A, so a 20 A breaker.

This sizes the protection for one branch circuit. Feeders, whole-house service loads and motor circuits follow other NEC articles, and the wire is then sized to the breaker.

How to convert watts to amps

On a single-phase supply, I = P ÷ (V × pf). On a balanced three-phase supply, I = P ÷ (√3 × V × pf), with V the line-to-line voltage. A 2,000 W kettle on 230 V draws 8.70 A; 11 kW on 400 V three-phase draws 15.88 A per line.

The power factor (pf) is 1 for resistive loads such as heaters; motors and some electronics run about 0.8 to 0.95. A lower factor means more current for the same power: 1,000 W at 120 V draws 8.33 A at pf 1 and 10.42 A at pf 0.8.

Worked example: heater, TV and lamps on 120 V

1. Continuous part. 1,500 W ÷ 120 V = 12.5 A, counted at 125%: 15.625 A.
2. Noncontinuous part. 150 W + 2 × 60 W = 270 W, or 2.25 A.
3. Minimum rating. 15.625 + 2.25 = 17.875 A.
4. Breaker. 15 A is too small, so the next standard rating is 20 A.

The circuit actually draws 14.75 A, which is 73.75% of the 20 A breaker. The 125% factor raises the minimum rating, not the current, so 17.875 ÷ 20 = 89.375% is not the breaker's loading.

The 125% rule and the 80% rule

They are the same rule: 1 ÷ 1.25 = 0.8. A continuous load may use at most 80% of a standard breaker, so a 20 A breaker can carry 16 A continuously, or 1,920 W at 120 V.

A minimum that lands exactly on a rating uses that rating: 16 A × 1.25 = 20 A, so the 1,920 W heater fits a 20 A breaker.

Apply 125% only to the continuous part. Multiplying every load by 1.25 overstates the minimum: in the example above it gives 18.4375 A instead of 17.875 A.

The exception is a breaker in an assembly listed for continuous operation at 100% of its rating, as Schneider Electric explains. Without that listing, use the 125% factor.

Standard breaker sizes

NEC 240.6(A) standard ratings run 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 and 100 A, then continue in larger steps. IEC 60898-1 miniature circuit breakers (MCBs) come in 6, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A.

The 125% factor is an NEC rule, and IEC-based wiring rules may not require it. Unmarked, the 11 kW three-phase load draws 15.88 A and the next IEC rating is 16 A, loaded to 99%; marked continuous, it needs 19.85 A and a 20 A MCB.

Circuit examples

The kitchen row is a 2,000 W kettle, 1,200 W microwave and 800 W toaster on 230 V, none continuous, sized to IEC ratings. The exact twenty row is a 1,920 W continuous heater on 120 V.

The EV row is a 32 A charger on 240 V, charging continuously. The three-phase row is 11 kW on 400 V at pf 1, marked continuous and sized to IEC ratings.

NEC 625.41 sizes EV charger protection for continuous duty, so the 32 A charger needs 32 × 1.25 = 40 A. The default row is the worked example above.

Size the wire to the breaker

Under NEC 2023 240.4(D), copper conductors may be protected at no more than 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG. A 20 A circuit therefore needs at least 12 AWG copper.

Hot locations and bundled cables lower a conductor's ampacity, and long runs add voltage drop; either can call for a larger size. Check a long run with the wire gauge and voltage-drop calculator.

Load list format

• One load per line: a name, then the power in W, kW or A, as in Kettle, 2000 W. A number with no unit is read as watts.
• Repeat identical loads with Lamp, 60 W x 2 or 2 x Lamp, 60 W.
• Add continuous (or cont) to anything that runs 3 hours or more.
• A load in amps keeps its current and is converted to watts at the chosen voltage: 32 A at 240 V is 7,680 W.
• One power factor covers the whole list, so give a motor with a different factor in amps from its nameplate.

Mistakes to avoid

• Sizing to the running current. 14.75 A seems to fit a 15 A breaker, but that loads it to 98% with a heater running for hours. The 125% rule moves the circuit to 20 A.
• Using output instead of input. Use the electrical input on the nameplate, not an air conditioner's cooling capacity or a motor's shaft power. The horsepower calculator converts units, but efficiency separates output from input.
• Raising the breaker without the wire. A 20 A breaker on 14 AWG copper exceeds the 240.4(D) limit.
• Mixing standards. NEC sizes on a 230 V or 400 V supply, or the 125% factor on an IEC installation, may not match local rules.

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