Find the standard breaker size for a load in watts or amps on single- or three-phase circuits. The calculator applies the 125% rule for continuous loads, rounds up to a standard NEC breaker size and suggests a typical copper or aluminum wire gauge to match.
Last updated:
Load given in
W
A
V
1 for heaters and resistive loads; about 0.8–0.9 for motors
Adds 125%: EV chargers, water heaters, space heaters, lighting
Enter your values to see the result.
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How to use the Circuit Breaker Size Calculator
Enter the load in watts (with power factor) or in amps.
Choose the voltage and phase of the circuit.
Tick continuous load if it runs for 3 hours or more.
Choose the wire material and temperature rating for the wire suggestion.
Read the breaker size, wire gauge and 80% loading, then check the table.
Breaker sizing formulas
Step
Formula
Example
Single-phase current
I = P ÷ (V × PF)
3,600 W ÷ 240 V = 15 A
Three-phase current
I = P ÷ (√3 × V × PF)
10 kW ÷ (1.732 × 480 × 0.85) = 14.2 A
Continuous load
Design current = I × 1.25
15 A × 1.25 = 18.75 A
Breaker
Next standard size ≥ design current
18.75 A → 20 A
Breaker and copper wire size chart
Breaker
Copper wire (60 °C, NM-B)
Copper wire (75 °C, THHN)
Max continuous load at 120 V
at 240 V
15 A
14 AWG
14 AWG
1,440 W
2,880 W
20 A
12 AWG
12 AWG
1,920 W
3,840 W
30 A
10 AWG
10 AWG
2,880 W
5,760 W
40 A
8 AWG
8 AWG
3,840 W
7,680 W
50 A
6 AWG
8 AWG
4,800 W
9,600 W
60 A
4 AWG
6 AWG
5,760 W
11,520 W
100 A
1 AWG
3 AWG
9,600 W
19,200 W
Wire sizes follow NEC Table 310.16 at 30 °C ambient with the small-conductor limits of 240.4(D). Residential service and feeder conductors may use NEC 310.12 instead. This chart is informational; always follow your local electrical code.
Frequently asked questions
How do I calculate breaker size?
Find the load current (amps = watts ÷ volts for single-phase, or watts ÷ (1.732 × volts × PF) for three-phase). For continuous loads multiply by 1.25. Then round up to the next standard breaker size. A 4,500 W water heater at 240 V draws 18.75 A; × 1.25 = 23.4 A, so a 25 A breaker (30 A is also common).
What is the 80% rule for breakers?
A standard breaker should carry no more than 80% of its rating continuously (3 hours or more). That is the same as sizing the breaker at 125% of the continuous load. A 20 A breaker can carry 16 A continuously, about 1,920 W at 120 V.
What size wire do I need for a 20, 30 or 50 amp breaker?
With copper wire typical minimums are 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A, 8 AWG for 40 A and 6 AWG for 50 A (60 °C NM cable needs 6 AWG for 50 A; with 75 °C THHN 8 AWG reaches 50 A). Long runs may need a larger size for voltage drop.
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, 200, 225, 250, 300, 350, 400, 450, 500 and 600 A, among larger sizes.
Can I put a bigger breaker on the same wire?
No. The breaker protects the wire, so its rating must not exceed the wire's allowed ampacity. Putting a 30 A breaker on 12 AWG wire is a fire hazard.
Is this calculator a substitute for an electrician?
No. It gives an informational estimate. Motor circuits, air conditioners (use the nameplate MCA/MOP), high ambient temperatures, many wires in one conduit and local amendments change the result. Electrical work should follow local code and be done by a qualified person.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.