Wire Size Calculator (AWG)

Find the minimum copper or aluminum wire gauge for a load from NEC Table 310.16 ampacity, with a note for derating — a planning estimate to confirm with a licensed electrician.

Your load
A
Results update as you type — no submit
ResultsLIVE
Minimum wire size
12 AWG copper
≈ 3.31 mm² · 20 A at the 60°C column
Required ampacity20 A
Conductor ampacity20 A
Cross-section≈ 3.31 mm²
Formula
required = 20 A ÷ 1 = 20 A
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.

Worked example
Load `50 A` · copper · `75°C` terminations · derating factor `0.8`
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`)
Questions about wire sizing
For a 20 A load on copper at the 60°C column, the answer is 12 AWG, which is rated exactly 20 A in NEC Table 310.16. This also lines up with NEC 240.4(D), the small-conductor rule that caps 12 AWG copper at a 20 A breaker. Always confirm the final size with an electrician, because bundling, temperature, and the breaker all factor in.
SOURCES — Ampacity: NEC Table 310.16 (60/75/90°C, Cu & Al) · terminations NEC 110.14(C) · derating NEC 310.15(B)/(C) · small-conductor cap NEC 240.4(D)
NEC-referenced estimate for planning only — electrical work must meet local code and be done or inspected by a licensed electrician; local code governs.
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