What Size Aluminum Wire for 125 Amps?
A user asked the app what size aluminum wire is needed for 125 amps, and a related question came in the same week about 1/0 aluminum feeding a 125 amp disconnect enclosure. These are really the same question, and the answer hinges on which rule you are allowed to use. The short version: 1/0 aluminum is the common and usually correct answer, and here is why.
The table numbers
NEC 2023 Table 310.16 rates 1/0 aluminum at 120 amps in the 75 degree C column, which is the column that matches modern breaker and lug terminations per 110.14(C). That is 5 amps shy of 125. The next size, 2/0 aluminum, is rated 135 amps. So a strict ampacity match says 2/0. But two other rules change the outcome in most real installs.
The next size up rule
Section 240.4(B) permits protecting a conductor with the next higher standard overcurrent device rating when the conductor ampacity does not correspond to a standard breaker size, the device is 800 amps or less, and the circuit does not supply multiple receptacles. 120 amps is not a standard rating in 240.6, and the next standard size above it is 125. So 1/0 aluminum protected by a 125 amp breaker is compliant, provided the calculated load does not exceed 120 amps. That last condition is the one to check honestly: if your load calculation lands above 120, you need 2/0.
The dwelling service shortcut
If the 125 amp conductors supply the entire load of a single family dwelling as service or main feeder conductors, Section 310.12 applies its 83 percent allowance: 125 times 0.83 is 103.75 amps, and 1/0 aluminum at 120 amps clears that easily. In fact Table 310.12(A) lists 1 AWG aluminum for a 125 amp dwelling service. For a disconnect enclosure serving a whole house, 1/0 aluminum is comfortably legal. For a feeder to a barn, shop, or subpanel that does not carry the whole dwelling load, use the 240.4(B) path instead, or upsize.
Practical notes: use conductors rated for the environment, typically XHHW-2 or SER, apply antioxidant where the terminal manufacturer requires it, and torque lugs to spec per 110.14(D). On long runs, aluminum at this size often gets upsized to 2/0 anyway for voltage drop, which the Code recommends keeping around 3 percent for feeders.