Wire Size Calculator

This calculator uses NEC 2023 (NFPA 70) Table 310.16, specifically the 75°C column for insulated conductors in raceway or cable with not more than three current-carrying conductors at 30°C (86°F) ambient. Under NEC 110.14(C), the conductor ampacity used for a circuit cannot exceed the lowest temperature rating of any connected termination, conductor, or device. For circuits rated 100 A or less (or 14 AWG through 1 AWG), 110.14(C)(1)(a) defaults to the 60°C column unless the equipment is listed for 75°C (or higher) conductors — most modern equipment is, so the 75°C column is the one most commonly used in practice.

Per NEC 210.20(A) and 215.3, a continuous load (energized 3+ hours continuously) must be limited to 80% of the breaker/wire rating. Enabling this toggle applies the NEC 125% inverse factor (210.20(A)) to your load amps before finding the minimum wire size — the same effect as the 80% rule.

Voltage drop is the loss of voltage along a wire due to its resistance. Per NEC 210.19(A) informational notes, at 3% drop on a 120 V branch circuit the device at the end sees 116.4 V. The NEC recommends (but does not mandate) a maximum of 3% voltage drop on branch circuits and 5% total from service to outlet. This calculator defaults to 3% for branch circuits.

Wire must satisfy two independent requirements: it must be large enough to carry the current safely (ampacity), and large enough to keep voltage drop within your target. Either constraint may be the binding one depending on run length. The recommended size is the larger of the two — the conservative, safe choice.

Aluminum is commonly used for service entrance conductors, feeders, and larger circuits. The NEC permits aluminum down to the smaller branch-circuit sizes (NEC 310.3(B) requires 8, 10, and 12 AWG aluminum to be AA-8000-series alloy), but in common practice many electricians and jurisdictions avoid small aluminum and use it only at about 6 AWG and larger. Aluminum requires rated connectors and anti-oxidant compound. Always verify with your local AHJ and a licensed electrician before specifying aluminum.

No. This tool gives the base ampacity from NEC Table 310.16 under standard conditions (30°C ambient, ≤3 conductors in raceway). Real installations may require derating for high ambient temperature (NEC Table 310.15(B)(1)), bundled conductors (NEC 310.15(C)), or conduit fill. A licensed electrician must apply all applicable derating factors.

Yes — completely free, no account required. All calculations happen in your browser. Your inputs never leave your device.