For a 100-amp circuit, the wire size is usually #3 AWG copper or #1 AWG aluminum for 100-amp service, though some guides list #4 AWG copper or #2 AWG aluminum in certain service/subpanel setups depending on code assumptions and terminal ratings. The exact size depends on whether this is a service entrance, subpanel, feeder, or branch circuit , plus distance, insulation rating, and local code.

Quick scoop

For a typical 100-amp installation, these are the common choices:

Use case Copper Aluminum
Typical 100A service #3 AWG or #4 AWG, depending on the guide and code context #1 AWG or #2 AWG, depending on the guide and code context
Longer runs / voltage-drop concerns May need larger than #3 AWG May need larger than #1 AWG
Ground wire commonly noted #8 AWG copper #6 AWG aluminum

What to use

A practical rule from the sources is: #2 AWG copper or #1 AWG aluminum is a common 100-amp choice, while some service-only references also accept #4 AWG copper or #2 AWG aluminum in specific setups. If the run is long, the wire may need to be upsized to reduce voltage drop.

Important note

Because ampacity changes with wire type, insulation temperature rating, and whether the installation is a main service or subpanel, the safest answer is to size it from the exact panel, breaker, and distance requirements rather than using one universal gauge. For anything permanent, a licensed electrician should confirm the final size against local code.

Bottom line

If you want the shortest answer: 100 amps is commonly wired with #3 AWG copper or #1 AWG aluminum, but many real-world installs may call for #4 copper / #2 aluminum or larger depending on the application.