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Find the minimum copper or aluminum wire size for a given load using the NEC Table 310.16 ampacity values, with the 240.4(D) small-conductor rule applied.
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The minimum conductor size for a circuit is the smallest wire whose allowable ampacity meets or exceeds the load. This calculator looks up the ampacity from NEC Table 310.16 for the chosen material and insulation temperature column, then returns the smallest size that carries your load.
The 60, 75, and 90 degree columns reflect the insulation rating. Most equipment terminations are listed for 75 degrees, so even with 90 degree wire you often size to the 75 degree column.
NEC 240.4(D) caps overcurrent protection for small conductors regardless of the ampacity table: 15 amps for 14 AWG copper, 20 amps for 12 AWG copper, and 30 amps for 10 AWG copper, with lower values for aluminum. This calculator applies those caps so it never suggests a size the code would not allow on a standard breaker.
Table 310.16 values assume not more than three current-carrying conductors and a 30 degree ambient. More conductors in a raceway or a hotter location reduces the usable ampacity under NEC 310.15. Size up when derating applies.
For copper at the 75 degree column, 8 AWG carries 50 amps. For aluminum you would step up to 6 AWG. Always confirm the termination temperature rating and any derating.
NEC 240.4(D) limits 12 AWG copper to a 20 amp overcurrent device even though the 90 degree column shows 30 amps. The calculator applies that rule automatically.
Use the column that matches the lowest-rated termination or device in the circuit. Most breakers and lugs are rated 75 degrees, so the 75 degree column is the safe default.
No. This sizes for ampacity only. On long runs, check voltage drop separately and increase the wire size if the drop exceeds 3 percent on a branch circuit.
Copper is smaller for the same load and common on branch circuits. Aluminum is cheaper for large feeders and services but needs a larger size and listed terminations.