What Is the Ampacity of #3 AWG Aluminum Wire?

A user asked the app for the ampacity of number 3 AWG aluminum. From NEC 2023 Table 310.16, #3 AWG aluminum is rated 65 amps at 60 degrees C, 75 amps at 75 degrees C, and 85 amps at 90 degrees C, based on not more than three current carrying conductors in a raceway or cable at 30 degrees C ambient.

Which column you actually get to use

Terminations decide the answer. Per 110.14(C), circuits over 100 amps or conductors larger than 1 AWG generally use the 75 degree column, and smaller circuits use 60 degrees unless every termination is listed for 75. Modern breakers and lugs are almost always marked AL9CU and rated 75 degrees C, so in practice #3 aluminum is treated as a 75 amp conductor. The 90 degree column is not a sales number; XHHW-2 at 85 amps is only a starting point for derating, never the final circuit rating.

Where #3 aluminum shows up

The most common use is a 70 amp feeder to a subpanel, where 75 amp ampacity covers the load and 240.4(B) would even allow rounding up to the next standard device if the calculation supported it. It also appears in SER cable for ranges and in mobile home feeder assemblies. Remember that SE style cables installed in thermal insulation are limited to the 60 degree column by 338.10(B)(4), which knocks #3 aluminum back to 65 amps in that situation.

Derating and connection practice

Bundle more than three current carrying conductors or run through a 45 degree attic and the adjustment factors of 310.15 apply to the 90 degree value first, which is the real job of that column. On the workmanship side, aluminum terminations want a torque screwdriver per 110.14(D) and the torque values from the equipment label. An oxide inhibitor is good practice where the lug maker calls for it. Most aluminum failures trace to loose or under torqued connections, not to the conductor metal itself.

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