Wire Size for 100 Amp Service

Standard wire size for 100 amp service

NEC Table 310.16 provides ampacity ratings that determine the minimum conductor size. For 100 amp service at 75°C insulation:

• Copper: 1 AWG is rated for 130 amps — comfortably above 100A.
• Aluminum: 1/0 AWG is rated for 120 amps — adequate margin for 100A.

In practice, many electricians and inspectors prefer 1/0 copper or 2/0 aluminum for service entrance conductors. The extra size provides headroom for voltage drop, future load growth, and meets the more conservative requirements of some local jurisdictions and utility companies.

Two different sizing contexts

"100 amp service" can mean two different things, and the wiring requirements differ:

• Service entrance conductors — from the utility meter base to the main breaker panel. These are typically short (10–20 ft) and sized primarily by ampacity. Voltage drop is rarely an issue over such short distances.
• Feeder conductors — from the main panel to a remote sub-panel (detached garage, workshop, barn, guest house). These runs can be 50–200+ feet and voltage drop becomes the controlling factor. The NEC recommends that total voltage drop (feeder + longest branch circuit) stay under 5%, with the feeder portion ideally under 3%.

If you are running a 100 amp feeder to a detached structure, do not just size by ampacity — always check voltage drop for your specific run length.

Copper vs. aluminum for 100 amp service

At the 100 amp level, aluminum becomes a practical and widely used choice. The cost difference is significant — aluminum service entrance cable (SER) costs roughly 40–50% less than equivalent copper per foot.

• Copper (1 AWG or 1/0): smaller diameter, easier to work in tight panels, lower resistance per foot. Preferred for short runs and when conduit space is limited.
• Aluminum (1/0 or 2/0): lighter, significantly cheaper, widely accepted for service entrance and feeder applications. Requires AL-rated terminals and anti-oxidant compound. Standard practice for residential service entrance.

Most residential 100 amp services use aluminum SER cable (e.g., 2-2-2-4 aluminum for a 100A service). This is industry standard and universally accepted when properly terminated.

Long feeder runs: when to upsize

For service entrance conductors (meter to main panel), runs are typically short enough that voltage drop is negligible. For feeders to sub-panels, voltage drop is the real sizing constraint.

At 100 amps, 240V, with 1 AWG copper:

• 3% drop threshold: reached at approximately 45 feet one-way.
• Stepping up to 1/0 copper extends this to about 55 feet.
• For runs of 100+ feet, you may need 2/0 or 3/0 copper (or 3/0 to 4/0 aluminum).

Every 100 amp feeder run should be checked with a voltage drop calculator. The cost of upsizing conductor one step is far less than the cost of undersized wiring causing equipment problems, nuisance tripping, or failed inspections.

Limitations of this guide

• Ampacity ratings based on NEC Table 310.16, 75°C column, standard conditions (3 conductors, 30°C ambient).
• Voltage drop estimates assume a two-conductor loop (single-phase, 240V) and nominal resistance values.
• Service entrance requirements vary by utility company and local jurisdiction — always confirm with your utility and AHJ.
• Grounding electrode conductor and equipment grounding conductor sizing are separate requirements not covered here (see NEC Table 250.66 and 250.122).
• Three-phase service, commercial installations, and services above 200 amps involve additional considerations beyond this guide.

Run the numbers for your installation