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Wire Gauge to Amperage Calculator

Look up the NEC allowable ampacity of a copper conductor from its AWG size, at the 60, 75 and 90 °C (140, 167 and 194 °F) insulation ratings.

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The AWG size of the copper conductor, as printed on the jacket.

AWG runs backwards: a smaller number is a bigger conductor. 14 AWG is a lighting circuit, 12 AWG a general power circuit, 10 AWG a small cooker or shower feed. The list holds exactly the sizes Table 310.16 publishes. The table does not run through every number: it steps 8, 6, 4, 3, 2, 1, so 3 and 1 have rows while 7, 5 and 9 have none, and only the sizes with a published ampacity are offered. Aluminium is not covered; its ampacity is materially lower for the same size.

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
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • 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

Inputs used

Conductor size
12 AWG

Intermediate steps

60 °C column
20 A
75 °C column
25 A
90 °C column
30 A
Maximum overcurrent protection — NEC 240.4(D)
20 A
Final result25 A

Confidence note: 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.

What this calculation does not cover

  • Copper only. Aluminium has its own column with materially lower values — using the copper figure on aluminium oversizes the protection, which is the dangerous direction.
  • Assumes ambient 30 °C and not more than three current-carrying conductors in the raceway. Outside either condition, the table value must be derated — see the derating calculator.
  • The temperature column that governs is the lowest rating in the whole path: conductor, terminations and equipment. Most breakers are listed at 75 °C and much residential equipment at 60 °C.

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.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-01 · in the site-wide review of 2026-09-06 · v1.1.1

Regulatory standards & verification citations2
  1. 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
  2. NEC 240.4(D) — small-conductor overcurrent limits: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG copper

Which documents these citations point at

  • National Electrical Code (NFPA 70) — 240.4(D) (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

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.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

  • Running Power to a Detached Garageuses this calculator

    A garage feeder is decided by distance, not by load: terminal temperature, a shared volt budget, and whether a fault can still get home.

Called something else where you work? Gauge — the term in each market, how close the equivalence really is, and the standard that governs it.

The data behind it: Copper conductor sizes and ampacity

How to calculate wire gauge to amperage in 2 steps

  1. Conductor sizeThe AWG size of the copper conductor, as printed on the jacket.
  2. Allowable ampacity at 75 °CThe tool computes the allowable ampacity at 75 °C from those figures and shows the formula, its sources, and a confidence rating alongside it.

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 has no row for them. The table steps 14, 12, 10, 8, 6, 4, 3, 2, 1 and then into the 1/0 sizes, so it is not that odd numbers are skipped — 3 and 1 are both published — it is that 5, 7, 9, 11 and 13 are not. Those exist as manufactured wire but carry no tabulated ampacity here. 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.
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 — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.