For a 100-amp subpanel, the minimum code-compliant wire is 3 AWG copper or 1 AWG aluminum. However, the practical standard for continuous loads and voltage drop margins is 1 AWG copper or 1/0 AWG aluminum, protected by a 100-amp double-pole breaker.
The Baseline Sizing Decision and Core Assumptions
Wire sizing is never a single universal number; it is a calculation bound by physics and the National Electrical Code (NEC). Before pulling any wire, you must lock in your baseline assumptions. The direct answers provided above rely on the following strict parameters:
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2/THWN-2).
- Temperature Column: 75°C column per NEC 110.14(C), as standard subpanel lugs are rated for 75°C maximum.
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: Maximum of three current-carrying conductors in the raceway (no neutral derating required for standard 240V split-phase loads, but a neutral is present for 120/240V panels).
- Configuration: 120/240V single-phase, 4-wire feed (two hots, one neutral, one equipment grounding conductor).
Ampacity Breakdown: Why Not One Size Smaller?
To understand why we don't just use the smallest wire that won't melt, we have to look at NEC Table 310.16 and the rules governing terminal temperatures. Even though modern THHN wire insulation is rated for 90°C, NEC 110.14(C) dictates that we must size the wire based on the lowest temperature rating of any connected terminal. Since almost all residential subpanel lugs and breaker lugs are rated at 75°C, we are legally bound to the 75°C ampacity column.
| Wire Size (AWG/kcmil) | Copper Ampacity (75°C) | Aluminum Ampacity (75°C) | Suitable for 100A Breaker? |
|---|---|---|---|
| 3 AWG | 100A | 75A | Yes (Copper only, strict minimum) |
| 2 AWG | 115A | 90A | Yes (Copper), No (Aluminum) |
| 1 AWG | 130A | 100A | Yes (Both - Practical Standard) |
| 1/0 AWG | 150A | 120A | Yes (Both - Recommended for distance) |
Why not use 3 AWG Copper? While 3 AWG copper hits exactly 100A in the 75°C column and is technically legal for a 100A non-continuous breaker, it leaves zero margin for error. If your subpanel will serve continuous loads (defined by the NEC as loads expected to run for 3 hours or more, like EV chargers, baseboard heaters, or server racks), NEC 210.20(A) requires the conductors to be sized at 125% of the continuous load. If you plan to pull 80A continuously on a 100A breaker, your wire must be rated for 100A continuously, which pushes you safely into the 1 AWG copper tier. Furthermore, 3 AWG is highly susceptible to voltage drop over distance, which brings us to the next critical variable.
Voltage Drop: When Distance Forces an Upsize
Ampacity tells you what size wire will prevent a fire; voltage drop tells you what size wire will actually let your equipment run properly. The NEC recommends a maximum 3% voltage drop on feeders. For a 240V subpanel, a 3% drop is 7.2 volts.
Let's run the math using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is 100 amps, D is one-way distance, and CM is the circular mil area of the wire.
- At 100 feet using 3 AWG Copper (52,620 CM): The drop is 4.9V (2.0%). This passes the 3% rule, making 3 AWG acceptable for short runs.
- At 150 feet using 3 AWG Copper: The drop is 7.35V (3.06%). This fails the 3% recommendation. Your 240V tools and appliances at the subpanel will only see 232V, causing motors to run hot and draw excess current.
- At 150 feet using 1 AWG Copper (83,690 CM): The drop is 4.6V (1.9%). This easily passes.
As verified by standard calculation tools like the Southwire Voltage Drop Calculator, distance is the primary reason electricians routinely upsize a 100A feeder from 3 AWG to 1 AWG or even 1/0 AWG. Aluminum requires even more aggressive upsizing due to its higher resistance (K=21.2).
Decision Tree: Finalizing Your Wire and Breaker Pick
Stop guessing and use this decision matrix to lock in your exact materials list. Find the row that matches your installation parameters and buy the concrete pick listed.
| Scenario Condition | Load Profile | One-Way Distance | Concrete Copper Pick | Concrete Aluminum Pick | Breaker Size |
|---|---|---|---|---|---|
| Short Run, Light Duty | Non-continuous (< 3 hrs) | Under 75 ft | 3 AWG THHN | 1 AWG XHHW-2 | 100A 2-Pole |
| Standard Run, Mixed Duty | Mixed / Standard Residential | 75 ft to 125 ft | 2 AWG THHN | 1/0 AWG XHHW-2 | 100A 2-Pole |
| Long Run or Heavy Duty | Continuous (EV, HVAC, Shop) | 125 ft to 180 ft | 1 AWG THHN | 2/0 AWG XHHW-2 | 100A 2-Pole |
| Extreme Distance | Any | Over 180 ft | 1/0 AWG THHN | 3/0 AWG XHHW-2 | 100A 2-Pole |
Note: The equipment grounding conductor (EGC) for a 100A breaker must be a minimum of 8 AWG copper or 6 AWG aluminum per NEC Table 250.122, regardless of how large you upsize the hot conductors for voltage drop.
Derating, Bundling, and When to Call the AHJ
The baseline assumptions hold true for 90% of residential garage, shed, and basement subpanel feeds. However, physical environment changes the math. If your conduit runs through a hot attic, or if you are pulling multiple circuits through the same pipe, you must apply NEC derating factors.
1. Ambient Temperature Derating: If your PVC conduit runs through an attic that reaches 45°C (113°F) in the summer, you cannot use the 75°C column for derating; you must use the 90°C column to apply a correction factor, then verify the final derated ampacity still meets your 100A requirement. At 45°C, the correction factor is 0.82. A 1 AWG copper wire (150A at 90°C) derates to 123A, which is still safe for a 100A breaker. A 3 AWG copper wire (115A at 90°C) derates to 94.3A, which will cause your 100A breaker to nuisance trip under full load.
2. Conduit Fill (Bundling): If you pull more than three current-carrying conductors in a single conduit (for example, feeding two separate subpanels from one main breaker using a shared neutral or multiple hots), Table 310.15(C)(1) requires an 80% derating factor for 4-6 conductors. This almost always forces an upsize to 1 AWG or 1/0 AWG copper.
3. When an Engineer or AHJ Must Confirm: You must step back and consult your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer if your project involves any of the following:
- The subpanel is being fed from a service entrance rather than a downstream breaker (service entrance conductors fall under NEC Article 230, not Article 215).
- You are paralleling multiple sets of smaller wires to achieve 100A (NEC 310.10(G) strictly regulates paralleling, usually requiring a minimum of 1/0 AWG).
- The ambient temperature along the conduit run consistently exceeds 50°C (122°F).
- Your local municipality has amended the NEC to require a 100% continuous load rating for all detached structure feeders regardless of actual load profiles.
For the vast majority of standard 100-amp subpanel installations under 150 feet, purchasing 1 AWG copper THHN/THWN-2 (or 1/0 AWG aluminum) alongside an 8 AWG copper ground and a 100-amp double-pole breaker is the definitive, code-compliant, and future-proof choice.
