For a standard 100-amp subpanel feed, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100A two-pole breaker. This assumes THHN/THWN-2 conductors in a raceway, rated at the 75°C column per NEC 310.16, with a run under 50 feet.

Safety & Code Caveat: Working inside a main electrical panel involves lethal mains voltage. Always de-energize the main breaker, verify dead with a tested multimeter, and wear PPE. NEC-style guidance provided here is for educational purposes; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.

The 100 Amp Sub Panel Wire Size Chart

Before pulling wire, you must establish your baseline assumptions. The chart below is built on the following strict parameters:

  • Material: Copper (primary) and Aluminum (secondary)
  • Insulation: THHN/THWN-2 (standard modern building wire)
  • Temperature Column: 75°C (per NEC 110.14(C), as standard residential breaker lugs are rated for 75°C)
  • Ambient Temperature: 30°C (86°F) or lower
  • Conduit Type: Single raceway (PVC or EMT) containing no more than three current-carrying conductors
NEC 310.16 Ampacity & Sizing for 100A Subpanel Feeders
Conductor Material Wire Size (AWG/kcmil) Ampacity at 75°C Breaker Size Equipment Ground (NEC 250.122)
Copper 3 AWG 100 Amps 100A 2-Pole 8 AWG Copper
Aluminum 1 AWG 100 Amps 100A 2-Pole 6 AWG Aluminum

Note: A 100-amp subpanel requires a 4-wire feed (two hots, one neutral, one ground). The neutral and ground must remain strictly separated at the subpanel bus bars.

Why 3 AWG Copper (And Why Not 4 AWG?)

A frequent mistake among DIYers is sizing down to 4 AWG copper. Looking at the 90°C column of the NEC 310.16 ampacity tables, 4 AWG copper is rated for 95 amps, which feels close enough to 100A. However, this is a code violation.

Per NEC 110.14(C), you must use the 75°C column because the termination lugs on standard residential breakers and load centers are only tested and rated for 75°C. In the 75°C column, 4 AWG copper is rated for just 85 amps. Therefore, 3 AWG (rated at exactly 100 amps in the 75°C column) is the minimum legal size.

The 83% Rule Does Not Apply Here: You may have heard that you can multiply a 100A service by 83% (allowing smaller wire). Per NEC 310.15(B)(7), this derating applies only to service-entrance conductors supplying an entire single-family dwelling. It does not apply to feeders supplying a subpanel (like a detached garage or workshop). A 100A subpanel feeder must be sized for the full 100A.

Decision Path: Adjusting for Distance, Bundling, and Material

Wire sizing is not static; it changes based on your physical installation environment. Use this decision tree to determine your exact wire pick.

Installation Condition Required Action Final Concrete Pick
Run is under 50 ft, standard 30°C ambient Use base 75°C ampacity 3 AWG Copper THHN
Run is 50–120 ft, expecting 80A continuous load Calculate voltage drop; bump one size 2 AWG Copper THHN
Run is over 120 ft, or heavy motor starting loads Calculate voltage drop; bump two sizes 1 AWG Copper THHN
Budget constrained, run under 50 ft Switch to Aluminum; use anti-oxidant paste 1 AWG Aluminum XHHW
Pulling more than 3 current-carrying wires in one conduit Apply NEC 310.15(C) bundling derating (80%) 2 AWG Copper THHN

Default Recommendation: If you are running a standard feeder to a detached garage or workshop under 50 feet away, buy Southwire 3 AWG THHN Copper (Black, Red, White) plus an 8 AWG bare copper ground.

Voltage Drop Calculations for Long Runs

Ampacity dictates the minimum wire size to prevent a fire; voltage drop dictates the wire size needed to ensure your equipment actually runs properly. The NEC recommends a maximum 3% voltage drop on feeders (NEC 310.15(B) Informational Note).

Let's run the math for a realistic scenario: feeding a 100A subpanel located 100 feet from the main panel, with a realistic maximum continuous load of 80 amps.

  • Formula: VD = (2 × K × I × L) / CM
  • K (Copper resistance constant): 12.9
  • I (Current): 80A
  • L (One-way length): 100 ft
  • CM (Circular Mils for 3 AWG): 52,620

Calculation: VD = (2 × 12.9 × 80 × 100) / 52,620 = 3.92 Volts.
Percentage: 3.92V / 240V = 1.63%.

At 100 feet, 3 AWG copper is perfectly adequate. But what if your subpanel is at the back of a large property, 200 feet away?

At 200 feet: VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84 Volts (3.26%).
This exceeds the 3% recommendation. To fix this, you must step up to 2 AWG copper (CM = 66,360), which drops the voltage loss to 2.59%, safely back under the 3% threshold. Always run voltage drop math for any subpanel run exceeding 50 feet.

When to Call an Engineer or the AHJ

While the chart and decision tree above cover 95% of residential subpanel installations, certain edge cases require professional engineering or explicit AHJ approval:

  1. High Ambient Temperatures: If your conduit runs through an attic that routinely exceeds 30°C (86°F) in the summer, you must apply temperature correction factors from NEC 310.15(B)(1). An attic hitting 110°F requires derating the ampacity by 82%, meaning 3 AWG copper drops to 82A—forcing you to upsize to 2 AWG.
  2. Parallel Conductors: If you are attempting to parallel smaller wires to achieve 100A (e.g., two sets of 8 AWG), stop. NEC 310.10(H) generally prohibits paralleling conductors smaller than 1/0 AWG. You must use a single 3 AWG or larger.
  3. Utility Service vs. Subpanel: If this 100A feed is actually a new service entrance from the utility pole rather than a subpanel feed from your main breaker, different rules (and utility company specs) apply regarding meter base clearance and grounding electrode systems.

For standard interior or short-run exterior subpanel feeds, stick to the 3 AWG copper baseline. Torque your breaker and busbar lugs to the exact inch-pound specification printed on the equipment label using a calibrated torque screwdriver—a critical step often ignored, but one that prevents thermal failure at the termination point years down the line.