The correct 100 amp wire size for a sub panel is 3 AWG copper or 1 AWG aluminum when using THHN/THWN-2 conductors in conduit, protected by a 100A breaker. If you are running standard NM-B (Romex) cable, you must use 2 AWG copper or 1/0 AWG aluminum due to lower temperature ratings.
- Material: Copper (primary), Aluminum (secondary)
- Temperature Column: 75°C (standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F)
- Conduit: Standard EMT or PVC, maximum 3 current-carrying conductors
- Load Type: Non-continuous (under 3 hours at full draw)
The Baseline: NEC Ampacity and Conductor Sizing
Wire sizing is not a guessing game; it is dictated by the National Electrical Code (NEC), specifically NFPA 70, Table 310.16. The most common mistake DIYers make is looking at the 90°C column for THHN wire. While THHN insulation can withstand 90°C, the lugs inside your breaker and sub panel are almost universally rated for 75°C. Per NEC 110.14(C), you must size your wire based on the lowest temperature rating in the circuit.
| AWG Size | Material | Insulation Type | 75°C Ampacity | Suitable for 100A Breaker? |
|---|---|---|---|---|
| 4 AWG | Copper | THHN / XHHW | 85A | No (Undersized) |
| 3 AWG | Copper | THHN / XHHW | 100A | Yes (Exact Match) |
| 2 AWG | Copper | NM-B (Romex) | 95A* | No (Requires 60°C column limit) |
| 2 AWG | Copper | NM-B (Romex) | 115A** | Yes (If 90°C wire used, but NM-B is limited to 60°C) |
| 1 AWG | Aluminum | THHN / XHHW | 100A | Yes (Exact Match) |
*Note: NM-B cable is strictly limited to the 60°C column per NEC 334.80, meaning 2 AWG NM-B copper is rated for 95A. To legally run 100A via NM-B, you must step up to 1 AWG copper or use a 90A breaker. For a true 100A NM-B feed, individual THHN conductors in conduit are vastly superior and cheaper.
Voltage Drop: When 100 Feet Changes the Math
Ampacity tells you what size wire will prevent a fire. Voltage drop tells you what size wire will actually let your appliances run correctly. The NEC recommends (but does not strictly mandate for feeders) a maximum 3% voltage drop on branch circuits and feeders.
Let us run a voltage drop check for our baseline 3 AWG copper wire at a distance of 100 feet, pulling a full 100A at 240V:
- Formula: VD = (2 × K × I × L) / Circular Mils
- K (Copper): 12.9
- I (Current): 100A
- L (Length): 100 ft
- Circular Mils (3 AWG): 52,620
- Calculation: (2 × 12.9 × 100 × 100) / 52,620 = 4.9 Volts
A 4.9V drop on a 240V system is a 2.04% drop. This easily passes the 3% recommendation. However, if your sub panel is located 150 feet away, that same 3 AWG wire will experience a 3.06% drop. At 150 feet, you must upgrade to 2 AWG copper to maintain power quality, especially if you are running sensitive electronics or motor loads like a well pump or HVAC compressor.
Decision Tree: Picking Your Exact Wire and Breaker
Use this decision matrix to lock in your exact materials list before heading to the electrical supply house. This assumes a standard 120/240V split-phase residential system.
| Installation Scenario | Distance from Main | Wire Material | Required AWG | Breaker Size |
|---|---|---|---|---|
| Individual wires in PVC/EMT conduit | Under 100 ft | Copper (THHN) | 3 AWG | 100A |
| Individual wires in PVC/EMT conduit | 100 ft to 150 ft | Copper (THHN) | 2 AWG | 100A |
| Individual wires in PVC/EMT conduit | Under 100 ft | Aluminum (XHHW) | 1 AWG | 100A |
| NM-B (Romex) Cable in framing | Any distance | Copper | 1 AWG (or use 90A brk w/ 2 AWG) | 100A (or 90A) |
| Underground Direct Burial (UF-B) | Any distance | Copper | 1 AWG | 100A |
Why Not One Size Smaller? (The 83% Rule Trap)
A frequent question on forums is: "Why can't I use 4 AWG copper or 2 AWG aluminum? I heard you can size feeders at 83%."
This is a dangerous misunderstanding of NEC Article 310.12 (formerly 310.15(B)(7)). The 83% rule allows you to use smaller wire (like 2 AWG copper for a 100A service) only for the main service entrance conductors supplying a single-family dwelling. The logic is that a whole house rarely draws its full calculated load simultaneously.
This rule does not apply to sub panels. A sub panel feeder is treated as a standard feeder circuit. If you are feeding a detached garage, a workshop, or an addition, you must size the wire for 100% of the breaker rating. Therefore, 4 AWG copper (rated 85A at 75°C) is strictly illegal and unsafe on a 100A breaker. The breaker will not trip until the wire is pulling over 100A, meaning the 4 AWG wire will be operating 15% beyond its safe thermal limit, slowly degrading the insulation and creating a fire hazard.
When to Call an Engineer or the AHJ
The sizes listed above are the baseline for standard residential conditions. You must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ / electrical inspector) if your installation hits any of these edge cases:
- High Ambient Temperatures: If your conduit runs through an attic in a hot climate where ambient temperatures regularly exceed 30°C (86°F), you must apply NEC Table 310.15(B)(1) correction factors. At 40°C (104°F), a 3 AWG copper wire's ampacity derates to 88A, forcing you to upsize to 2 AWG or 1 AWG.
- Conductor Bundling: If you pull more than three current-carrying conductors through the same conduit (for example, running two separate 240V sub panel feeds in one pipe), you must apply NEC Table 310.15(C)(1) adjustment factors. Four to six conductors require an 80% derating, instantly dropping 3 AWG copper below the 100A threshold.
- Continuous Loads: If the sub panel will supply loads that run for 3 hours or more continuously (like commercial lighting, server racks, or heavy HVAC), NEC 215.2 requires the feeder to be sized at 125% of the continuous load. If your continuous load is 80A, your wire and breaker must be sized for 100A. If your continuous load is 85A, you need a 110A+ setup (requiring 1 AWG copper and a 125A breaker).
For standard residential garage or workshop sub panels under 100 feet, stick to 3 AWG copper THHN in conduit. It is the most cost-effective, code-compliant, and thermally safe choice for a 100A feed.
