What Size Wire for a 100 Amp Service? Service vs Subpanel (4 AWG vs 3 AWG)

The quick answer

If it's the main service feeding a house — the wire from the meter to the main panel — use 4 AWG copper or 2 AWG aluminum. If it's a subpanel or feeder — say, a 100 amp panel out in a garage fed from a bigger main — use 3 AWG copper or 1 AWG aluminum.

Why a 100 amp service is 4 AWG copper (the 310.12 rule)

Homes get a break in the code. NEC 310.12, the 83% rule, lets the main service conductors of a dwelling use wire rated for just 83% of the service rating, because a house never actually draws its full rating continuously.

For a 100 amp service:

1. 100 A × 83% = 83 A — the ampacity your wire needs.
2. At the 75°C column, 4 AWG copper carries 85 A and 2 AWG aluminum carries 90 A — both clear 83 A.
3. So a 100 amp main service is legal on 4 copper / 2 aluminum.

This is the value you'll see called the "minimum for a 100 amp service," and it's correct — for the service entrance. The catch is that the same rule does not extend to an ordinary subpanel.

Why a 100 amp subpanel is 3 AWG copper (not 4)

Here's the twist that makes 100 amps trickier than any other service size. The 83% reduction in 310.12 applies to the main service, and to a main feeder that carries the entire dwelling load — not to a run-of-the-mill subpanel fed from a larger service. A subpanel falls back to the full ampacity table, NEC 310.16, where 100 amps needs 3 AWG copper (100 A) or 1 AWG aluminum (100 A) at 75°C.

That single distinction is why "3 AWG copper for a 100 amp service" is actually the most-searched wire question at this amperage — a huge share of 100 amp runs are subpanels to garages, shops, and additions, and those correctly land on 3 copper. When in doubt about whether your run counts as the main feeder, ask your inspector before you buy.

100 amp wire size chart

Every conductor for both cases, in one place:

Many electricians deliberately size up — to 2 AWG copper or 1/0 aluminum — for margin and to keep voltage drop low on longer runs. That's a fine choice; the sizes above are the code minimums, not a ceiling.

What size ground wire for a 100 amp service?

The grounding electrode conductor (GEC) — the wire from the panel to your ground rods, water pipe, or Ufer — is set by NEC Table 250.66. For a 100 amp service, that's 8 AWG copper.

Don't confuse the GEC with the neutral in the service cable — that grounded conductor is sized under 220.61, not the grounding table. Mixing them up is the single most common ground-wire mistake.

Direct burial: wire for a 100 amp underground run

A big share of 100 amp runs go underground — a subpanel out to a detached garage, a shop, or a pedestal. For that you need direct-burial-rated conductors, not ordinary indoor cable.

The common choice is 2-2-2-4 aluminum quadruplex URD — three #2 aluminum conductors plus a #4 ground, rated for direct burial without conduit. The other route is individual USE or RHW conductors pulled through conduit. Both are valid; the pick depends on your path and local rules.

Copper vs aluminum for a 100 amp service

At this size aluminum is the budget default — a 100 amp service or subpanel in aluminum (2 AWG service, 1 AWG subpanel, or 2-2-2 URD) costs far less than copper (4 AWG / 3 AWG). Copper shows up where an installer prefers it, where terminations demand it, or on shorter runs where the price gap is small.

If you go aluminum, treat the terminations right: aluminum-rated lugs and breakers, an antioxidant compound, and torque to spec. Done properly, aluminum service conductors have a long, reliable record; done carelessly, loose aluminum connections are a real hazard. For the ampacity behind all of these numbers, see the full AWG ampacity chart.

Long runs and voltage drop

100 amp feeders to detached buildings are a classic long run, and that's where voltage drop starts to matter. The NEC recommends keeping feeder drop under 3% (an Informational Note under 210.19(A) — advisory, not a hard limit).

This is exactly why installers so often jump to #2 copper or #1/0 aluminum on a long garage run — the extra size buys distance. The Voltage Drop Calculator checks your exact length and load in one step.

100, 200 and 400 amp service (quick reference)

The same 310.12 dwelling rule sizes every residential service, so here's the quick reference across the common ratings (the values a normal house actually uses):

Stepping up from 100 to 200 amps is a big jump in copper — 4 AWG all the way to 2/0. See the full breakdown on our 200 amp service wire guide.

Common 100 amp wiring mistakes

The signature 100 amp mistake is using the 4 AWG service value on a subpanel that actually needs 3 AWG — the 83% rule doesn't follow the wire out to the garage. The reverse also happens: oversizing a main service to 3 AWG copper when 4 is legal, and overpaying. After that it's grounding — undersizing the GEC or confusing it with the neutral — and the underground classics: burying non-direct-burial-rated wire, digging a trench that's too shallow, or forgetting voltage drop on a long feeder. And as always, the local AHJ can amend any of this, so confirm before a service job.

Frequently asked questions

Dr. Artie Vance

Technical reviewer · Ph.D. in Physics, MIT · 14 years' experience · Cross-checked against NEC 2026 & 2026

This guide is for general information based on the National Electrical Code. Local codes and your authority having jurisdiction (AHJ) may differ, and service work is often permit-and-inspection territory. Electrical work can be dangerous, so when in doubt, hire a licensed electrician.