The Correct Wire for a 100-Amp Subpanel
For a typical 100-amp subpanel feeding a garage, workshop, or addition, the wire size is 3 AWG copper or 1 AWG aluminum. These values come from the 75 degrees Celsius column of NEC Table 310.16, which is the standard for modern panels and breaker lugs. Aluminum is the popular choice for feeders at this size because it costs far less than copper for the same ampacity.
The 83 Percent Rule, and Why It Usually Does Not Apply
You will often see 4 AWG copper or 2 AWG aluminum listed for 100 amps. That smaller size comes from the dwelling allowance in NEC 310.12, which lets service and feeder conductors be sized at 83 percent of the rating, but only when that feeder carries the entire load of the dwelling unit. A subpanel that feeds just a garage or shop does not carry the entire dwelling load, so it does not qualify, and the full 3 AWG copper or 1 AWG aluminum applies. Mixing these up is one of the most common sizing mistakes.
Run a 4-Wire Feeder
A modern subpanel feeder uses four conductors: two hots, a neutral, and an equipment ground. At the subpanel the neutral bar must be isolated from the ground bar and from the enclosure, because the neutral and ground are bonded only at the main service. The equipment ground for a 100-amp feeder is 8 AWG copper or 6 AWG aluminum per NEC Table 250.122.
Detached Buildings Have Extra Rules
If the subpanel is in a detached building such as a garage or shop, NEC 250.32 requires its own grounding electrode, usually two ground rods, in addition to the equipment ground run with the feeder. This is a step that is easy to miss and that inspectors check closely.
Account for Distance
3 AWG copper is good for a 100-amp feeder up to roughly 100 feet. For longer runs, voltage drop can push you to 2 AWG copper or 1/0 aluminum to stay within the recommended 3 percent. Many feeders to detached buildings are long enough that this matters.
Code and Safety Note
Feeder and subpanel work involves the main panel and carries real hazard. Aluminum conductors require AL-rated lugs and antioxidant compound. Follow the National Electrical Code and local amendments, confirm with your authority having jurisdiction, and use a licensed electrician for all feeder and panel work.
IB Lighting carries SER feeder cable and THWN-2 building wire for 100-amp subpanels in our electrical wire and cable collection, with free freight plus free freight insurance on every order. Call (800) 674-9019 and we will size the feeder for your run and distance.