Sizing 100 Amp Sub Panel Wire: AWG, Breaker, and Voltage Drop Rules

For a standard 100 amp sub panel wire run, use #3 AWG copper or #1 AWG aluminum THHN/THWN-2 conductors, protected by a 100A 2-pole breaker at the main panel. This assumes individual conductors pulled through conduit. If using NM-B (Romex) cable, you must step up to #2 AWG copper because NM-B is restricted to the 60°C ampacity column.

Ampacity Data and the "Why Not Smaller" Rule

To understand why #3 AWG copper is the minimum, we have to look at NEC Table 310.16, which dictates the allowable ampacities for insulated conductors. The most common mistake DIYers make is looking at the 90°C column because THHN wire is printed with a 90°C rating. However, NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected component.

Virtually all modern 100A breakers and subpanel lugs are rated for 75°C. Therefore, you must use the 75°C column to determine your baseline wire size, regardless of the wire's 90°C insulation capability.

Why not use #4 AWG copper? Looking at the table, #4 AWG copper in the 75°C column is only rated for 85A. While NEC 240.4(B) allows you to round up to the next standard breaker size (which would be 90A), it does not allow you to round up to 100A. To carry a full 100A load safely without violating the termination temperature limits of your breaker, #3 AWG copper (rated exactly 100A at 75°C) is the absolute minimum. If you opt for NM-B cable, you are forced into the 60°C column, where #3 AWG is only 85A, forcing an upsize to #2 AWG (95A, rounded up to 100A via 240.4(B)).

Voltage Drop: When Distance Forces an Upsize

Ampacity tables assume your run is relatively short. When wire gets long, resistance increases, causing voltage to drop before it reaches the subpanel. The NEC recommends a maximum voltage drop of 3% for feeders to ensure efficient operation and prevent motor burnout or dimming lights.

For a 240V system, a 3% drop equals 7.2 volts. Let us run the math on #3 AWG copper carrying a full 100A load, using the standard voltage drop formula: VD = (2 × K × I × D) / CM.

• K (Copper constant) = 12.9
• I (Current) = 100A
• CM (Circular Mils for #3 AWG) = 52,620
• D = One-way distance in feet

Solving for Distance (D) where VD = 7.2V:

7.2 = (2 × 12.9 × 100 × D) / 52,620
7.2 = 2580D / 52,620
7.2 = 0.049D
D = 146.9 feet

Material Swaps, Bundling, and Derating Factors

The baseline answer changes the moment you alter the installation environment or material. Here is how real-world conditions force you to adjust your wire sizing.

Switching to Aluminum

Aluminum is significantly cheaper than copper, but it has lower conductivity and expands/contracts more under heat. For a 100A feeder, you must use #1 AWG aluminum (rated 100A at 75°C). Never use #2 AWG aluminum, as it is only rated for 90A. Additionally, you must use an anti-oxidant compound (like Noalox) on aluminum terminations and ensure your breaker lugs are explicitly rated for aluminum (marked AL/CU). Torque is especially critical with aluminum; under-torquing leads to arcing, while over-torquing cold-flows the metal and loosens over time.

Ambient Temperature and Attic Runs

Table 310.16 assumes an ambient temperature of 30°C (86°F). If your conduit runs through a hot attic where temperatures routinely hit 40°C (104°F) or higher, you must apply temperature correction factors from the bottom of Table 310.16. At 40°C, the 90°C column derating factor is 0.91. While this rarely forces an upsize for a single 100A feeder (115A × 0.91 = 104A, which still covers the 100A requirement), it becomes a major factor if the attic hits 50°C (122°F), where the derating factor drops to 0.82, forcing an upsize to #2 AWG copper.

Conduit Fill and Bundling

If you pull multiple circuits through the same conduit, you must apply bundling derating factors (NEC 310.15(C)(1)). A standard subpanel feeder consists of two hot wires, one neutral, and one ground. Only the current-carrying conductors (the two hots and the neutral) count. Three conductors require no derating. However, if you add a second circuit to that same conduit, bringing the total current-carrying conductors to 4, 5, or 6, you must derate the ampacity to 80%. In that scenario, #3 AWG copper (115A at 90°C × 0.80 = 92A) fails the 100A requirement, and you must upsize to #2 AWG.

The Equipment Grounding Conductor (EGC)

Do not use #3 AWG wire for your ground. It is a massive waste of money. Per NEC Table 250.122, the minimum equipment grounding conductor for a 100A breaker is #8 AWG copper or #6 AWG aluminum. Always pull a separate EGC in conduit rather than relying on the conduit itself as the ground path, as conduit joints can loosen over time and compromise the fault-current path.

When the AHJ or an Engineer Must Confirm

While the rules above cover 95% of residential and light commercial subpanel installations, certain edge cases require professional engineering or explicit approval from your local Authority Having Jurisdiction (AHJ).

• Parallel Feeds: If you are trying to achieve higher amperage by running multiple sets of wires in parallel, NEC 310.10(G) strictly prohibits paralleling conductors smaller than 1/0 AWG. You cannot parallel two sets of #3 AWG to get 200A. An engineer must design parallel feeds.
• High Fault Current Scenarios: If your main service is a large 400A or 600A commercial service, the available fault current at the main bus may exceed the 10,000 AIC (Ampere Interrupting Capacity) rating of standard residential 100A breakers. You may need to specify breakers with 22kAIC or 65kAIC ratings, which an engineer must calculate.
• Continuous Loads Exceeding 80A: If the subpanel will supply continuous loads (running for 3 hours or more, like EV chargers or heavy server racks) that exceed 80A, NEC 210.20(A) requires the breaker to be sized at 125% of the continuous load. A 100A breaker is only rated for 80A of continuous draw. If your calculated continuous load is 85A, you must upsize the breaker to 110A or 125A, which consequently forces an upsize in your wire gauge.