Wire Size Calculator (AWG & Required Ampacity)
Look up the smallest copper or aluminum gauge whose NEC Table 310.16 ampacity covers your load, then run derated picks by a licensed electrician.
smallest copper at 60°C ≥ 20 A = 12 AWG (20 A)
How it works
Every conductor has an ampacity — the current it can carry continuously without overheating its insulation. Sizing a wire means picking the smallest gauge whose ampacity is at least as large as the load it has to feed. This calculator reads the ampacity straight from NEC Table 310.16, the column-by-column table for copper and aluminum conductors at insulation ratings of 60°C, 75°C, and 90°C, and returns the first gauge that covers your amperage. It is a planning aid — the final gauge on any circuit should be confirmed by a licensed electrician against your local code.
Which temperature column applies is not a free choice. NEC 110.14(C) ties you to the lowest temperature rating in the circuit, which for most breakers, lugs, and receptacles rated 100 A or less is the 60°C column — so this tool defaults there. Larger equipment is commonly rated 75°C. The 90°C column is generally used only as a starting point for derating, not for the final ampacity. Copper carries more current than aluminum of the same size, which is why an aluminum feeder is usually one or two gauges larger.
Derating matters when a run is hot or crowded. High ambient temperature (NEC 310.15(B)) and bundling more than three current-carrying conductors (NEC 310.15(C)) both cut ampacity, so the calculator lets you enter a single combined derating factor. It divides your load by that factor to find the ampacity the conductor must actually meet, then sizes up accordingly.
required ampacity = 50 A ÷ 0.8 = 62.5 A
NEC 310.16 copper 75°C: 8 AWG = 50 A (too small), 6 AWG = 65 A
result = 6 AWG copper (65 A ampacity, about 13.3 mm²)