Wire size calculator
Pick your wire gauge, breaker size and conductor — see instantly whether the wire's ampacity is enough for the circuit, per NEC Table 310.16.
For a 20 A circuit, use at least 12 AWG copper (recommended: 10 AWG).
Ampacity from NEC Table 310.16 (60°C column, copper; 75°C column, aluminium) — the wire's heat limit, separate from voltage drop. This is a planning reference: your local wiring rules, installation method, conductor bundling and ambient temperature can change the allowed ampacity — confirm with the code edition in force where you are.
Common questions
- Can I run 14 gauge wire on a 20 amp circuit?
- No. 14 AWG copper is rated 15 amps under NEC Table 310.16 — putting it on a 20-amp breaker lets it carry more current than it can safely dissipate as heat, and the breaker won't trip until well past the wire's limit. Use 12 AWG or larger on a 20-amp circuit.
- What size wire do I need for a 20 amp circuit?
- 12 AWG copper, minimum — it's rated 20 amps under NEC Table 310.16. For aluminium, the minimum is 10 AWG, since 12 AWG aluminium is only rated for a 15/20A range depending on termination; check the table above.
- What size wire do I need for a 30 amp circuit?
- 10 AWG copper (rated 30 amps) is the minimum for a 30-amp circuit — common for dryers and some water heaters. Confirm the breaker and wire ampacity match before wiring it in.
- What size breaker do I need for my circuit?
- Size the breaker to the wire's ampacity, and — if the load runs 3 hours or more (a continuous load) — keep the load itself at no more than 80% of the breaker's rating (NEC 210.20(A)). See the full breaker-sizing guide for worked examples.
- Is a bigger wire gauge always safer?
- A larger wire (lower gauge number) can safely carry more current, so yes, oversizing is safe from an ampacity standpoint. It's just more expensive and harder to bend into small boxes — the goal is to meet the minimum for the breaker, not necessarily maximize it.
Want the why behind the answer? Read the full wire gauge guide → or check the run length with the voltage drop calculator →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-07.