To correctly size wire for 100 amp service or a subpanel feeder, use 3 AWG copper or 1 AWG aluminum with a 100A breaker. This baseline assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the conduit.

Baseline Assumptions for This Guide:
  • Material: Copper (Cu) or Aluminum (Al) — never mix or interchange without specific transition lugs.
  • Insulation: THHN/THWN-2 (90°C rated wire, but terminated at 75°C per NEC 110.14(C)).
  • Ambient Temperature: 30°C (86°F) or lower.
  • Conduit Fill: Maximum of 3 current-carrying conductors in a single raceway (PVC or EMT).

Note: NEC-style guidance provided here is for educational purposes; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.

The Ampacity Table: Sizing Wire for 100 Amp Service

The most common mistake DIYers make when pulling wire is looking at the 90°C column on the wire spool and assuming that is the safe operating limit. According to National Fire Protection Association (NFPA) guidelines in NEC 110.14(C), unless your breaker and panel lugs are explicitly marked otherwise, you must use the 75°C column of NEC Table 310.16 to determine ampacity.

Here is the exact breakdown for wire sizes hovering around the 100A threshold. Find the 75°C column to see why 3 AWG Copper and 1 AWG Aluminum are the correct choices for a standard 100A breaker.

AWG Size Material 60°C Column (Amps) 75°C Column (Amps) 90°C Column (Amps) Max Standard Breaker
4 AWG Copper 70A 85A 95A 90A
3 AWG Copper 85A 100A 110A 100A
2 AWG Copper 95A 115A 130A 125A
2 AWG Aluminum 75A 90A 100A 90A
1 AWG Aluminum 85A 100A 115A 100A

Notice that 4 AWG Copper only provides 85A in the 75°C column, which is insufficient for a 100A breaker under standard rules. You must step up to 3 AWG to hit exactly 100A. Similarly, 2 AWG Aluminum falls short at 90A, requiring an upgrade to 1 AWG Aluminum.

The 83% Rule: Why This Size and Not One Smaller?

If you have been reading forums, you might have seen people claim they used 4 AWG copper or 2 AWG aluminum for a 100A service. Are they breaking the law? Not necessarily, but context is everything.

NEC Article 310.12 outlines a special exception for Residential Service Conductors. If you are wiring the main service entrance that feeds the entire single-family dwelling, the NEC allows you to apply an 83% demand factor. Because a whole house rarely draws its absolute maximum rated current simultaneously across every circuit, the code permits smaller wire for the main service drop or lateral.

  • The Math: 100A × 0.83 = 83A.
  • The Result: 4 AWG Copper (rated 85A at 75°C) and 2 AWG Aluminum (rated 90A at 75°C) are legally permitted for a 100A residential main service.
Critical Distinction: Subpanels vs. Main Services
The 83% rule (NEC 310.12) does not apply to a 100A subpanel feeder. If you are running a 100A feeder to a detached garage, workshop, or barn, you are not wiring a "residential service entrance"—you are wiring a feeder. For subpanels, you must strictly use the Table 310.16 baseline: 3 AWG Copper or 1 AWG Aluminum. Using 4 AWG for a 100A subpanel is a code violation and a fire hazard.

Derating Factors: What Changes the Required Wire Size

The baseline sizes (3 AWG Cu / 1 AWG Al) assume perfect conditions. On a real jobsite, three main factors will force you to upsize your wire: voltage drop, conduit bundling, and continuous loads.

1. Voltage Drop Over Distance

The NEC recommends a maximum voltage drop of 3% on feeders to ensure equipment operates efficiently. Let us run a voltage drop check for a 100A copper feeder at a distance of 150 feet using the standard formula: VD = (2 × K × I × L) / Circular Mills.

Assuming a realistic continuous draw of 80A on a 240V circuit:

  • 3 AWG Copper (52,620 CM): Yields a 2.45% drop (5.88V). This passes the 3% rule.
  • However, if you actually pull the full 100A at 150 feet, the drop jumps to 3.06%, failing the recommendation.

If your run exceeds 150 feet, or if you anticipate running heavy continuous loads like a large welder or EV charger at the end of the line, you must upsize to 2 AWG Copper to keep the voltage drop under 3%. You can verify your specific run parameters using the Southwire Voltage Drop Calculator before purchasing wire.

2. Conduit Bundling and Temperature Derating

If you pull more than three current-carrying conductors through a single conduit (for example, running two separate 100A multi-wire feeders in one 2-inch PVC pipe), the wires heat each other up. NEC Table 310.15(C)(1) requires you to apply a derating factor.

With 4 to 6 current-carrying conductors, you must derate the ampacity to 80%. Here is the trick: you are allowed to use the 90°C column for the derating math. For 3 AWG Copper, the 90°C rating is 110A. Multiply 110A by 0.80, and you get 88A. Because 88A is less than your 100A breaker, you must upsize to 2 AWG Copper (90°C rating of 130A × 0.80 = 104A) to maintain your 100A capacity.

3. Aluminum vs. Copper Installation Realities

Never treat aluminum and copper as interchangeable. Aluminum wire is significantly cheaper and lighter, making 1 AWG Aluminum highly attractive for long runs. However, aluminum expands and contracts more than copper under thermal cycling, which can loosen terminal lugs over time and cause arcing.

If you choose aluminum, you must:

  • Apply an anti-oxidant compound (like Noalox) to the stripped conductor ends to prevent galvanic corrosion.
  • Use a calibrated torque screwdriver or wrench to tighten the lugs exactly to the manufacturer's specification (typically printed on the panel label, often around 40-45 in-lbs for 100A lugs, per NEC 110.14(D)).
  • Ensure your panel lugs are rated for aluminum (most modern 75°C lugs are marked AL/CU, but always verify).

When an Engineer or the AHJ Must Confirm

While the rules above cover 95% of residential and light commercial 100A installations, certain edge cases require a licensed professional engineer or direct consultation with your local electrical inspector (AHJ).

  1. Continuous Commercial Loads: If your 100A service is dedicated to a commercial load that will run for 3 hours or more continuously (like a massive server room cooling system or industrial air compressors), NEC 210.20 requires the breaker and wire to be sized at 125% of the load. A 100A continuous load requires a 125A breaker and 1 AWG Copper wire.
  2. High Ambient Temperatures: If your conduit is routed across a rooftop, through a heavily insulated attic in a hot climate, or near a boiler, the 30°C ambient assumption fails. You must apply temperature correction factors from NEC Table 310.15(B)(1), which will almost certainly force you to upsize the wire.
  3. Local Municipal Amendments: Some municipalities have strict local amendments that ban aluminum wire entirely in residential branch circuits and feeders, or mandate rigid metal conduit (RMC) over PVC for exterior service masts. Always pull the permit and check local amendments before ordering materials.

Sizing wire correctly is about matching the physics of the conductor to the reality of the installation environment. Stick to 3 AWG Copper or 1 AWG Aluminum for your standard 100A feeders, respect the 83% exception only where the code explicitly allows it, and always torque your lugs to spec.