For a 100-amp dwelling service, NEC 310.12 allows 4 AWG copper. For a 100-amp non-dwelling feeder or subpanel, you must use 3 AWG copper. Pair either with a 100A breaker. This assumes THHN/THWN-2 insulation, 75°C terminations, and 30°C ambient conditions.
Baseline Assumptions for This Guide
- Material: Copper (Do not use aluminum sizing charts for this wire)
- Insulation: THHN/THWN-2 (90°C rated wire, but terminated at 75°C)
- Termination Temp: 75°C (Standard for modern 100A breakers and lugs)
- Ambient Temp: 30°C (86°F) or lower
- Conduit Fill: Single circuit in a raceway (3 or fewer current-carrying conductors)
The Core Sizing Rules: NEC 310.16 vs. 310.12
The most common mistake DIYers and junior apprentices make is applying the wrong National Electrical Code (NEC) article to their project. Wire sizing for a 100-amp copper service depends entirely on whether you are wiring the main service entrance for a house, or running a feeder to a subpanel.
According to NFPA 70 (NEC), the baseline ampacity table is 310.16. However, Article 310.12 provides a specific exception for dwelling services.
| Wire Size (AWG) | 60°C Column | 75°C Column (Terminations) | 90°C Column (Wire Insulation) |
|---|---|---|---|
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
Scenario A: Main Dwelling Service (NEC 310.12)
If you are connecting the main service panel for a single-family home, NEC 310.12(A) states that service conductors must have an ampacity of at least 83% of the service rating. For a 100A service, 83% is 83 amps. Looking at the 75°C column above, 4 AWG copper is rated for 85A. Therefore, 4 AWG copper is legally sufficient for a 100A residential main service.
Scenario B: Subpanel or Non-Dwelling Feeder (NEC 310.16)
The 83% rule does not apply to subpanels, detached garages, or commercial buildings. The conductors must carry the full 100 amps. Looking at the 75°C column, 3 AWG copper is rated for exactly 100A. You must use 3 AWG copper for these applications.
Why not use the 90°C column? While THHN/THWN-2 wire is insulated for 90°C, NEC 110.14(C) requires you to size the wire based on the temperature rating of the terminations. Almost all standard 100A breakers and panel lugs are rated for 75°C maximum. If you push 115A (the 90°C rating of 2 AWG) through a 75°C lug, you will degrade the lug, melt the breaker housing, and create a fire hazard.
Variables That Force a Wire Size Upgrade
The baseline sizes (4 AWG or 3 AWG) assume perfect conditions. On a real jobsite, environmental and physical factors will force you to derate the wire and bump up to a larger size. Use this decision tree to check your installation conditions.
| Variable | Condition | Impact on 100A Copper Sizing |
|---|---|---|
| Conduit Bundling | 4 to 6 current-carrying conductors in one conduit | Derate to 80%. 3 AWG (100A * 0.8 = 80A) is no longer sufficient. Upgrade to 1 AWG. |
| Ambient Temperature | Attic or rooftop run exceeding 30°C (86°F) | At 40°C (104°F), multiply by 0.88. 3 AWG drops to 88A. Upgrade to 2 AWG or 1 AWG. |
| Continuous Loads | Load operates at 80A+ for 3 hours or more | NEC 210.20 requires 125% sizing for continuous loads. 80A * 1.25 = 100A minimum breaker, requiring 1 AWG wire to handle the continuous thermal mass safely without nuisance tripping. |
| Material Swap | Switching from Copper to Aluminum (SER cable) | Aluminum has higher resistance. For a 100A feeder, you must use 1/0 AWG Aluminum. For a dwelling service (83A), 2 AWG Aluminum is required. |
Voltage Drop Calculation for a 100-Foot Run
The NEC strongly recommends (via Informational Notes) keeping voltage drop under 3% for feeders. Ampacity tables only tell you what the wire can handle thermally; they do not account for the resistance of long wire runs. If your panel is far from the meter or main service, you must calculate voltage drop.
Let's calculate the voltage drop for a 3 AWG copper feeder running to a 100A subpanel located 150 feet away. We will assume a continuous load of 80A (which is the realistic maximum continuous draw on a 100A breaker).
The Formula:
VD = (2 × K × I × L) / CM
- K (Copper constant) = 12.9
- I (Current) = 80 Amps
- L (One-way length) = 150 Feet
- CM (Circular Mils for 3 AWG) = 52,620
The Math:
VD = (2 × 12.9 × 80 × 150) / 52,620
VD = 309,600 / 52,620 = 5.88 Volts
The Percentage:
5.88V / 240V = 2.45%
At 150 feet, 3 AWG copper is perfectly fine (under the 3% limit). However, if that subpanel is 250 feet away, the drop jumps to 9.8V (4.08%). At that distance, you must upgrade to 1 AWG copper to bring the drop back down to an acceptable 2.5%. Always check voltage drop for runs over 100 feet.
When to Defer to an Engineer or the AHJ
While the NEC provides the baseline, local Authority Having Jurisdiction (AHJ) inspectors and licensed engineers have the final say. You must pull a permit and have an engineer review your plans if:
- Parallel Runs: If you are trying to parallel two sets of smaller wires to achieve 100A (Note: NEC 310.10(G) generally prohibits paralleling conductors smaller than 1/0 AWG, so you cannot parallel two 3 AWG wires to make a 100A service).
- Extreme Environments: If the conduit runs through a kiln room, a commercial bakery, or an unventilated Arizona attic where ambient temperatures routinely exceed 50°C (122°F).
- Mixed Lug Ratings: If you are terminating into an older, legacy disconnect switch where the manufacturer's lugs are only rated for 60°C. In a 60°C scenario, 3 AWG copper is only rated for 85A, forcing an upgrade to 1 AWG even for a standard feeder.
Always use a calibrated torque screwdriver to tighten the breaker lugs. For most 100A main breakers, the manufacturer specifies around 40 to 50 inch-pounds of torque. Under-torquing causes arcing and fires; over-torquing strips the aluminum lug threads.
Frequently Asked Questions
Can I use 2 AWG copper for a 100 amp subpanel?
Yes, absolutely. 2 AWG copper is rated for 115A at 75°C. Using a larger wire than the minimum required is always legally permissible and electrically superior (lower voltage drop, less heat). The only drawbacks are physical: 2 AWG is stiffer, harder to pull through conduit bends, and more expensive. Ensure your 100A breaker lugs are physically rated to accept the larger 2 AWG strand diameter.
Does the 83% dwelling rule apply to my detached garage subpanel?
No. NEC 310.12(A) applies strictly to the service conductors feeding the main dwelling unit from the utility. A feeder running from your main house panel to a detached garage subpanel is governed by Article 215 and 310.16. Therefore, you cannot use the 83% rule to downsize the garage feeder; you must use 3 AWG copper (or 1/0 AWG aluminum) for a full 100A feeder.
What size equipment grounding conductor (EGC) do I need for 100A?
According to NEC Table 250.122, the minimum equipment grounding conductor for a 100-amp overcurrent device is 8 AWG copper (or 6 AWG aluminum). If you had to upsize your current-carrying conductors to 1 AWG due to voltage drop over a long distance, NEC 250.122(B) requires you to proportionally upsize the ground wire as well, which would bump your copper ground to 6 AWG.
Why do people say I need 1 AWG for a 100 amp service?
The confusion stems from two places. First, older breakers or specific commercial lugs might only be rated for 60°C, where 1 AWG is required to hit 100A (actually 1 AWG is 110A at 60°C, 2 AWG is 95A). Second, many electricians automatically size up to 1 AWG or 1/0 AWG to account for voltage drop on long runs or to handle future expansion, as pulling 3 AWG through 2-inch PVC is difficult and upgrading later is expensive. However, strictly by the NEC book for a short, standard 75°C run, 3 AWG is the exact minimum for a feeder.
