Workshop and Metalwork Calculators
Wire gauge amp rating.
Enter the current in amps, choose copper or aluminum and the temperature rating of the wire. You get the smallest listed AWG or kcmil size, the next size up, and the whole ampacity column. Runs in your browser; nothing is sent or stored.
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Result
Fill the form and press Calculate.
Answer first. On the chart used here, 30 amps on copper rated 75 C needs 10 AWG (35 A). 14 AWG copper is rated 20 A at 75 C, 12 AWG is 25 A, and 10 AWG is 35 A. These are chart ampacities only, not breaker limits: electrical code caps the breaker or fuse size for small conductors such as 14, 12 and 10 AWG below these figures, so do not size a home circuit breaker from this chart. Breaker size, derating and code rules are separate checks.
Worked example
A 30 A load on copper wire rated at 75 C: the table has 8 AWG at 50 A, 10 AWG at 35 A and 12 AWG at 25 A, so the smallest size that meets 30 A is 10 AWG (35 A), leaving 5 A of headroom. The next size up is 8 AWG at 50 A.
How it works
Source and limits. Values are from Cerrowire's ampacity chart, which says the ampacities are based on the 2017 NEC and do not reflect any temperature correction or ampacity adjustments that may be required. Copper is listed at 60, 75 and 90 C; aluminum at 75 and 90 C. A dash in the chart means no rating is listed, so copper at 60 C stops at 2 AWG and aluminum starts at 12 AWG.
Which column? The chart pairs insulation types with temperature columns: NM-B and UF-B in the 60 C column; THW, THWN, SE, USE and XHHW at 75 C; THWN-2, THHN, XHHW-2 and USE-2 at 90 C. Check the rating marked on the cable, and on the breaker and terminals it connects to; the lowest rating in the chain is the cautious choice.
What is not included. Conductors bundled in conduit, high ambient temperatures, long runs (voltage drop), continuous loads and the code limits on overcurrent protection for small conductors are not applied. The chart itself tells you to consult a qualified electrician or professional engineer. For voltage drop use the voltage drop tool linked below.
Safety. Electrical work can cause shock, fire and death. Undersized or wrongly protected wire overheats. Have a licensed electrician size and install circuits and check your local code. This tool is a look-up of a published chart, not professional electrical advice.
Sources
- Ampacity Charts | Wire Gauge Chart | Cerrowire (opened 2026-10-08)
All ampacities are copied from the chart above (Wire Size & Amp Ratings, copper 60/75/90 C and aluminum 75/90 C). Cross-checked against the ampacity columns in the American wire gauge article on Wikipedia for 8, 10 and 12 AWG copper.
FAQ
What gauge wire for 30 amps?
On this chart, copper at 75 C needs 10 AWG (35 A). At 60 C copper needs 10 AWG as well (30 A). Check the breaker rules and the wire's insulation rating before you rely on it.
What is the amp rating of 12 gauge wire?
Copper 12 AWG is listed at 20 A (60 C), 25 A (75 C) and 30 A (90 C). Aluminum 12 AWG is 20 A (75 C) and 25 A (90 C).
Why are there three temperature columns?
Ampacity depends on how much heat the insulation and terminals can take. 90 C insulation can carry more current than 60 C, but the connected equipment may limit you to a lower column.
Is this a breaker size calculator?
No. It looks up the conductor ampacity from the chart. Overcurrent protection rules, such as the limits on small conductors, are separate.
Does this include voltage drop?
No. For long runs, check voltage drop too, which can require a larger wire than the ampacity alone.