For a 100-amp service or subpanel feeder, you must use 3 AWG copper wire paired with a 100-amp breaker. This assumes standard 75°C terminations, copper conductors, and a 30°C ambient environment. Using 4 AWG will violate code, while aluminum requires upsizing to 1 AWG.
- Material: Copper (Aluminum requires different sizing)
- Insulation: THHN/THWN-2 or XHHW-2
- Termination Rating: 75°C (Standard for most residential breakers and lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Raceway Condition: Installed in conduit with no more than 3 current-carrying conductors
The Ampacity Data: Why 3 AWG Copper is the Minimum
The National Electrical Code (NEC) dictates conductor sizing through NFPA 70 (NEC) Table 310.16. However, the most common mistake DIYers and junior apprentices make is looking at the 90°C column because modern THHN wire is rated for 90°C. You cannot use the 90°C column for final sizing unless you are applying specific derating factors. The breaker lugs and panelboard terminations are almost universally rated for 75°C. Under NEC 110.14(C), you must size the conductor based on the lowest temperature rating of any connected component.
| AWG Size (Copper) | 60°C Column | 75°C Column (Use This) | 90°C Column (Derating Only) |
|---|---|---|---|
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 110A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
Looking at the 75°C column, 3 AWG copper provides exactly 100 amps of ampacity. This perfectly matches a standard 100-amp breaker.
Why not use 4 AWG? A 4 AWG copper wire maxes out at 85 amps in the 75°C column. NEC 240.4 requires the conductor ampacity to be equal to or greater than the breaker rating. While there is a 'next size up' rule for odd calculated loads (like a 92A load on a 100A breaker), you cannot use an 85A-rated wire on a 100A breaker under any standard feeder circumstance. The breaker would allow 100A to flow, overheating the 4 AWG wire and creating a severe fire hazard before the thermal trip mechanism engages.
Variables That Force an Upsize (Decision Matrix)
The 3 AWG specification holds true only under ideal baseline conditions. Real-world jobsites introduce voltage drop, conductor bundling, and material changes that will force you to pull thicker wire. Use the decision matrix below to verify if 3 AWG is actually sufficient for your specific run.
| Site Condition | Impact on 3 AWG Copper | Required Action |
|---|---|---|
| Run exceeds 145 feet (240V) | Voltage drop exceeds 3% (NEC recommendation) | Upsize to 2 AWG or 1 AWG to compensate for resistance. |
| 4 to 6 current-carrying conductors in one conduit | 80% derating applied to 90°C column (110A x 0.8 = 88A) | Upsize to 2 AWG (130A x 0.8 = 104A) to maintain 100A capacity. |
| Using Aluminum or Copper-Clad Aluminum | 2 AWG Aluminum is only rated 90A at 75°C | Must use 1 AWG Aluminum (rated 100A at 75°C). |
| Ambient temp exceeds 30°C (e.g., hot attic) | Temperature correction factors reduce ampacity | Upsize to 2 AWG or route conduit through conditioned space. |
The Voltage Drop Trap
The NEC does not strictly mandate voltage drop limits for feeders, but it strongly recommends a maximum 3% drop for branch circuits and feeders to ensure equipment operates efficiently. Using the Southwire Voltage Drop Calculator or standard engineering formulas, a 100-amp load on 3 AWG copper at 240V will hit the 3% drop threshold at approximately 145 feet. If your subpanel is 200 feet away from the main service, 3 AWG will result in a ~4.1% voltage drop. Motors will run hot, and sensitive electronics may brown out. For a 200-foot run, you must upsize to 1 AWG copper.
The Bundling Derating Secret
This is where many inspectors fail DIYers. If you pull two hots, a neutral, and a ground for a standard 120/240V split-phase subpanel, you have 3 current-carrying conductors (the ground does not count). You are fine. But if you pull a second circuit in that same conduit, or if you are running a 3-phase feeder with a neutral, you now have 4 or more current-carrying conductors. Per NEC 310.15(C)(1), you must apply an 80% adjustment factor. You apply this factor to the 90°C column (110A for 3 AWG). 110A x 0.80 = 88A. Your wire is now only legally good for 88 amps, meaning it cannot be protected by a 100-amp breaker. You must jump to 2 AWG.
Installation Realities and AHJ Boundaries
Sizing the wire correctly on paper is only half the job. The physical termination of 3 AWG wire into a 100-amp breaker introduces mechanical and legal variables that dictate the success of the installation.
Torque Specifications are Non-Negotiable
A 100-amp lug requires significant mechanical pressure to maintain a low-resistance connection. Most residential 100-amp breakers (such as Square D Homeline/QO or Siemens EQ series) require between 40 and 50 inch-pounds of torque. You cannot guess this by 'cranking it until it stops.' Under-torqued lugs create microscopic air gaps that increase resistance, leading to thermal expansion, arcing, and eventually a melted breaker bus stab or a panel fire. Use a calibrated inch-pound torque screwdriver or a digital torque adapter. Always verify the exact torque value printed on the breaker label or the panel schematic.
Line-Side vs. Load-Side (When to Call the AHJ)
It is vital to distinguish between a subpanel feeder and a main service entrance. If you are installing a 100-amp subpanel in a detached garage, 3 AWG copper in PVC conduit is perfectly standard. However, if you are running the service drop or service lateral from the utility transformer to the main meter/disconnect, different rules apply.
Utility companies often have their own 'Green Books' or service requirements that supersede standard NEC feeder rules. Many utilities mandate a minimum of 1/0 or 2/0 aluminum (or 2 AWG copper) for any service entrance conductors, regardless of the calculated load, to ensure mechanical strength and fault-current survivability. Furthermore, service entrance conductors on the line-side of the main disconnect do not have overcurrent protection at their source (the utility transformer can deliver thousands of amps). If your project involves the service mast, meter base, or line-side connections, you must consult your local Authority Having Jurisdiction (AHJ) and the local utility provider before pulling any wire.
