Wire Size Calculator (NEC Ampacity)
Enter your load and conditions to find the smallest wire and the standard breaker size that satisfy NEC Table 310.16 ampacities. The calculator applies the 125% continuous-load rule, the 60/75°C termination limits, ambient-temperature correction and the adjustment for more than three current-carrying conductors in a raceway.
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Result
Enter your numbers to see the result.
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How it picks the wire
- Required ampacity = load × 1.25 for continuous loads, otherwise the load itself (NEC 210.19, 215.2).
- Each size is checked two ways: the derated ampacity (the insulation column, times the ambient correction factor in 310.15(B), times the conductor-count adjustment) and the terminal ampacity (the 60°C or 75°C column the equipment is listed for, 110.14(C)). The lower of the two governs.
- The first size that carries the required current is selected and the overcurrent device is rounded up to the next standard size (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and so on).
Small-conductor limits (240.4(D): 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A copper) are applied. Always confirm against the adopted code edition and the local amendments.
Worked example
A 40 A continuous load needs 50 A of ampacity. On copper THHN (90°C) into 75°C terminals, 8 AWG is only 50 A in the 75°C column, so 8 AWG copper on a 50 A breaker works.
Frequently asked questions
Why is the 75°C column used for a 90°C wire?
You may use the 90°C column for derating calculations, but the final ampacity cannot exceed the column that matches the terminals, which are usually rated 60°C or 75°C.
What counts as continuous?
A load expected to run three hours or more. Lighting in a store and EV chargers are common examples.
Does this cover motors or special cases?
No. Motors, welders, air conditioners and other special loads follow their own NEC articles. Use this for general feeders and branch circuits.
Is this a substitute for the code book?
No. It follows NEC Table 310.16 (2023 values, which are unchanged from earlier editions for these columns) but your inspector and local amendments govern.
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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.