Circuit Breaker Size Calculator

How the Breaker Size Is Calculated

Under the NEC, a continuous load is one whose maximum current is expected to last 3 hours or more. Two rules then set the minimum:

1. Breaker rating (210.20(A)): not less than the noncontinuous load plus 125% of the continuous load.
2. Round up to the next standard rating in Table 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and up.
3. Conductor (210.19(A)(1)): ampacity not less than the noncontinuous load plus 125% of the continuous load. This calculator also requires the wire to carry the full breaker rating, and it applies the 240.4(D) caps on 14, 12 and 10 AWG.
4. Column (110.14(C)(1)): on circuits of 100 A or less, ampacity is read from the 60°C column unless the equipment is listed for higher-rated conductors; above 100 A, the 75°C column.

Worked example: a 16 A load that runs for hours. 1.25 × 16 = 20 A, which is itself a standard rating, so the breaker is 20 A. In the 60°C copper column, 14 AWG carries 15 A (too small) and 12 AWG carries 20 A; 240.4(D) allows 12 AWG copper on a breaker up to 20 A. Result: 20 A breaker, 12 AWG copper. The load uses 80% of the breaker, which is the same rule seen from the other side.

The NEC Rules Behind the Result

All section numbers below are from the 2023 edition of NFPA 70.

• 210.20(A), breaker rating: not less than the noncontinuous load plus 125% of the continuous load. Exception: where the assembly, including its overcurrent devices, is listed for operation at 100% of its rating, the sum of the loads is enough.
• 210.19(A)(1), conductor ampacity: not less than the noncontinuous load plus 125% of the continuous load, and the conductors must comply with 110.14(C) at the equipment terminations.
• 240.4(D), small conductors: unless another section specifically permits it, overcurrent protection may not exceed 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, 15 A for 12 AWG aluminum or copper-clad aluminum, and 25 A for 10 AWG aluminum or copper-clad aluminum.
• Table 240.6(A): standard breaker ratings.
• Table 310.16: ampacities for not more than three current-carrying conductors in a raceway, cable or earth at a 30°C (86°F) ambient. The rows this page uses most:

A 14 AWG copper conductor is listed at 20 A in the 75°C column, but 240.4(D) still limits its breaker to 15 A. That cap, not the table, is why 14 AWG goes on 15 A circuits.

Published Example: Eaton's Feeder Table

Eaton's application paper on 100%-rated breakers (AP01200008E, January 2026) works the same 210.20(A) formula on larger circuits. Feeder #1 carries 200 A noncontinuous and 400 A continuous: 200 + 1.25 × 400 = 700 A, met by a 700 A trip. Feeder #2 carries 800 A continuous: 1.25 × 800 = 1,000 A. With an assembly listed for 100% operation, the same feeders need only 200 + 400 = 600 A and 800 A. The calculator reproduces all four ratings; at those currents it reports that a single conductor per phase is not enough, since feeders that size use parallel sets designed by an engineer.

Breaker and Wire Size for Common Loads

Whether a load counts as continuous is decided by the code article for that equipment and by the inspector. Electric vehicle charging is treated as a continuous load (U.S. Department of Energy). The 48 A charger rows show the effect of the termination rule: 6 AWG copper is enough only where the breaker, charger and wire are all rated for 75°C. Equipment instructions often name a larger breaker than the minimum; a larger breaker is fine as long as the wire is sized for it.

Common Mistakes When Sizing a Breaker

• Sizing for the load without the 125%. A 16 A continuous load on a 15 A breaker breaks the rule even before it trips; 16 A continuous needs 20 A.
• Upsizing the breaker to stop nuisance trips. Putting a 20 A breaker on 14 AWG copper exceeds 240.4(D). The breaker protects the wire, so it can only be as large as the wire allows.
• Reading the 90°C column for a 15 to 100 A circuit. THHN is 90°C wire, but the terminals on most breakers and devices limit the usable ampacity to the 60°C or 75°C value.
• Using copper sizes for aluminum. Aluminum needs about one size larger: 16 A continuous needs 10 AWG aluminum, not 12.
• Assuming household loads are always continuous, or never are. Water heaters, EV chargers and fixed heating are often treated as continuous; a toaster is not.

What This Calculator Does Not Check

• Voltage drop. A long run can need larger wire than the ampacity table. Check it separately with the wire gauge calculator.
• Derating. More than three current-carrying conductors together, or a hot ambient such as an attic, reduces ampacity (310.15). The table values here assume neither.
• Terminal and equipment ratings beyond the default column. If any termination is rated lower than the column you pick, the lower rating governs.
• Motors, air conditioners and heat pumps. These follow their own articles and nameplate maximum breaker sizes, which can exceed the wire ampacity on purpose.
• Exceptions that allow a smaller conductor (such as the next-size-up rule in 240.4(B)), three-phase loads entered in watts, feeders, services, and local amendments.

Frequently Asked Questions

How to Use This Calculator

1. Enter the continuous load — the current (or watts) of anything that runs 3 hours or more at a time.
2. Enter any noncontinuous load — other loads on the same circuit that run for shorter periods.
3. Choose amps or watts, and the voltage — watts are divided by the voltage for single-phase circuits.
4. Pick the material, column and breaker type — leave the column on the 110.14(C)(1) setting unless every termination is rated 75°C.
5. Read the breaker and wire — then check voltage drop and derating, and have a licensed electrician confirm the design.