Wire Size Chart: What Gauge Wire for Every Amperage

NEC Table 310.16 lists three ampacities for every wire size, one per insulation temperature rating. You may only use the column matching the lowest temperature rating in the circuit— and that is almost always the terminations, not the wire.

• 60°C:NM-B cable (Romex) is locked to this column by NEC 334.80, regardless of its 90°C-rated conductors. This is why a 60A circuit in Romex needs #4 copper while the same circuit in conduit needs only #6.
• 75°C:The working column for most installations — modern breakers, panel lugs, and devices are rated 60/75°C, and THWN/XHHW conductors in conduit qualify. Every recommendation on this page uses the 75°C column.
• 90°C:THHN's dry-location rating. Used as the starting point for conduit-fill and ambient-temperature derating calculations, but you cannot land a 90°C ampacity on a 75°C lug.

See the full three-column table on the wire ampacity chart.

Aluminum needs roughly two wire sizes largerthan copper for the same ampacity, but still costs about half as much in feeder sizes — which is why virtually every residential service drop and most large feeders are aluminum, while branch circuits stay copper.

Modern AA-8000 alloy aluminum (#8 AWG and larger, typically as SER cable or XHHW-2 in conduit) is safe and code-compliant. The rules that matter: use terminations marked AL/CU, apply antioxidant paste where the manufacturer requires it, and torque lugs to the listed specification. The bad reputation belongs to 1960s small-gauge branch-circuit aluminum, which is a different product entirely.

Ampacity tables assume distance doesn't exist. On long runs, resistance steals voltage: the NEC recommends keeping drop under 3% on a branch circuit or feeder (informational notes to 210.19 and 215.2). As a rule of thumb, a 240V circuit loaded to its breaker rating hits 3% around 100 feetat the minimum wire size — and a 120V circuit at only about half that distance.

Past those distances, upsize one gauge per rough doubling of the shortfall — or skip the rules of thumb and use the voltage drop calculator with your actual load and length. Detached garages, well pumps, and RV pads are the classic cases where voltage drop, not ampacity, picks the wire.