Wire Ampacity Calculator
A wire ampacity calculator finds the minimum conductor size (AWG) needed to safely carry a specified electrical current without overheating. Undersized wiring is a leading cause of electrical fires. The NEC specifies ampacity ratings for conductors based on material (copper or aluminum), insulation temperature rating (60C, 75C, or 90C), and installation method. This calculator looks up the NEC Table 310.12 ampacity for common AWG sizes, applies an optional derating factor for conduit bundling or high temperature, and identifies the minimum wire gauge that meets or exceeds your required current. Always have a licensed electrician verify wire sizing before installation.
Wire sizing procedure
Required ampacity = Load (A) / (Derating Factor / 100)
Select smallest AWG where Ampacity (from NEC Table 310.12) >= Required ampacity
Common derating factors: 3-6 conductors in conduit = 80%, 7-9 conductors = 70%, 10-20 conductors = 50%. Ambient temperature above 30C also requires correction per NEC Table 310.15(B)(1).
Frequently asked questions
What is wire ampacity?
Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. NEC Table 310.12 provides ampacity ratings for common conductor sizes at 60C, 75C, and 90C temperature ratings. Ampacity depends on conductor material (copper or aluminum), insulation temperature rating, and installation conditions.
What wire gauge do I need for a 20-amp circuit?
A 20-amp circuit requires a minimum of 12 AWG copper wire at 60C or 75C (ampacity = 20A for 12 AWG at 60C per NEC Table 310.12). For a 30-amp circuit, use 10 AWG copper. For a 40-amp circuit, use 8 AWG copper. Always confirm with the applicable NEC table and apply any required derating.
What is conductor derating?
Derating reduces the allowable ampacity when conductors are bundled together in conduit (more than 3 current-carrying conductors) or installed in high ambient temperatures. NEC Table 310.15(B)(1) provides temperature correction factors and Table 310.15(C)(1) provides conduit fill adjustment factors.
What is the difference between copper and aluminum wire ampacity?
Aluminum conductors have lower ampacity than copper for the same AWG size. A 2 AWG aluminum wire has similar ampacity to a 4 AWG copper wire. Aluminum is lighter and less expensive but requires special terminations to prevent oxidation and loosening. NEC requires anti-oxidant compound on all aluminum connections.
What is the 80% rule for circuit breakers?
NEC 210.20 and 220.10 require that continuous loads (loads energized for 3 hours or more) be limited to 80% of the circuit breaker rating. A 20A breaker can carry only 16A of continuous load. Size the breaker at 125% of the continuous load: breaker size = continuous load / 0.80.
Official sources
- NFPA: NFPA 70 NEC - Article 310: Conductors for General Wiring (Table 310.12).
- NFPA: NEC 310.15 Ampacity Tables and Derating Corrections.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.