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Wire Gauge to Amperage Calculator

Look up the NEC allowable ampacity of a copper conductor from its AWG size, at 60, 75 and 90 °C ratings.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-27 · v1.1.0

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Allowable ampacity at 75 °C

25 A

High confidence

NEC Table 310.16, copper, 75 °C column headline. Table value only — apply the adjustment and correction factors for conduit fill and ambient temperature, and never exceed the 240.4(D) cap on 14, 12 and 10 AWG.

60 °C column
20 A
75 °C column
25 A
90 °C column
30 A
Maximum overcurrent protection — NEC 240.4(D)
20 A

The reverse lookup an electrician actually performs on an existing installation: the cable is already in the wall and the question is what it may safely be protected at. That is a different task from sizing a new circuit, and it is the one where the small-conductor rule in 240.4(D) most often surprises people.

Add the equipment this sizes

This result is a specification — 25 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • NFPA 70 (NEC) Table 310.16 — Allowable Ampacities of Insulated Conductors, copper, 60/75/90 °C columns, ambient 30 °C, not more than three current-carrying conductors
  • NEC 240.4(D) — small-conductor overcurrent limits: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG copper

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Frequently asked questions

Why is 12 AWG protected at 20 A when the table says 25 A?
NEC 240.4(D) caps overcurrent protection for the small conductors regardless of the table: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG. The table ampacity is what the conductor can carry; the cap is what you may protect it at. The lower of the two governs, and for these three sizes it is almost always the cap.
Which temperature column applies to my circuit?
The lowest rating in the whole path — conductor insulation, terminations and equipment. Most breakers and devices are listed at 75 °C, and much residential equipment at 60 °C, so a 90 °C cable is usually still limited to the 75 °C or 60 °C column. The 90 °C figure is generally used only as the starting point for derating.
Does this figure change with conduit fill or ambient temperature?
Yes, and often substantially. More than three current-carrying conductors in a raceway triggers an adjustment factor, and ambient above 30 °C triggers a correction factor. Both reduce the allowable ampacity, and both are applied to the table value rather than replacing it — see the derating calculator.
Can I use this for aluminium?
No. Aluminium carries materially less current for the same gauge — 12 AWG aluminium is 20 A at 75 °C against copper's 25 A — and it has its own column in Table 310.16. Using the copper figure on an aluminium conductor oversizes the protection, which is the dangerous direction.
Why can I not enter 7 AWG or 9 AWG?
Because Table 310.16 does not publish rows for the odd sizes between 8 and 1 — they exist as manufactured wire but carry no tabulated ampacity in this table. An earlier version of this page accepted any number and had no answer for those sizes; offering only the published rows is the honest shape of the tool.