For a standard 100-amp circuit, you need 3 AWG copper wire or 1 AWG aluminum wire, paired with a 100-amp breaker. This applies to THHN/THWN-2 conductors in conduit with 75°C-rated terminations. If using NM-B (Romex) cable, you must upsize to 1 AWG copper due to the 60°C ampacity restriction.
- Material: Copper (primary) and Aluminum (secondary).
- Temperature Column: 75°C for THHN/THWN-2 in conduit; 60°C for NM-B cable.
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Standard raceway/conduit (not direct burial or bundled with more than 3 current-carrying conductors).
- Code Reference: NEC-style guidance (2020/2023 cycles); your local AHJ has final authority.
The Core Ampacity Rules (NEC Table 310.16)
Sizing wire for a 100-amp breaker requires matching the conductor's ampacity to the overcurrent protective device (OCPD). The National Electrical Code (NEC) dictates these limits based on insulation type and termination ratings.
| Conductor Material | Insulation / Cable Type | NEC Temp Column | Required AWG Size | Ampacity |
|---|---|---|---|---|
| Copper | THHN / THWN-2 | 75°C | 3 AWG | 100A |
| Copper | NM-B (Romex) | 60°C | 1 AWG | 110A |
| Aluminum | THHN / THWN-2 | 75°C | 1 AWG | 100A |
| Aluminum | NM-B (Romex) | 60°C | 1/0 AWG | 100A |
Why This Size and Not One Smaller?
A common mistake is attempting to use 4 AWG copper wire on a 100-amp breaker. In the 75°C column, 4 AWG copper is rated for exactly 85 amps. While NEC 240.4(B) allows you to round up to the next standard breaker size when a wire's ampacity falls between standard sizes, this only applies if the wire ampacity doesn't match a standard breaker. Because 100A is a standard breaker size (NEC 240.6), the wire must possess a minimum ampacity of 100A. Placing 85A-rated wire on a 100A breaker creates a fire hazard, as the wire will melt before the breaker trips during a sustained overload.
The NM-B (Romex) 60°C Trap
Even though modern breakers and panel lugs are rated for 75°C, NEC 334.80 strictly limits Nonmetallic-Sheathed Cable (NM-B/Romex) to the 60°C ampacity column. In the 60°C column, 3 AWG copper is only rated for 85A, and 2 AWG is rated for 95A. Therefore, if you are pulling Romex for a 100-amp subpanel feeder, you are legally required to use 1 AWG copper (rated 110A at 60°C). For this reason, most electricians prefer pulling individual THHN/THWN-2 conductors in PVC or EMT conduit, allowing them to use the 75°C column and save money with 3 AWG copper.
Voltage Drop: The Hidden Distance Trap
Ampacity tables only tell you what size wire prevents a fire. They do not account for voltage drop over distance. NEC Informational Note 210.19(A)(1) recommends a maximum voltage drop of 3% for branch circuits and feeders to ensure equipment operates efficiently. On a 240V feeder, a 3% drop equals 7.2 volts.
Let's run the math on 3 AWG copper (resistance ≈ 0.245 Ω/kft) carrying a full 100A load:
- At 100 feet (200 ft total loop): Voltage drop = 100A × (0.245 × 0.2) = 4.9V. This is a 2.04% drop, which passes the 3% recommendation.
- At 150 feet (300 ft total loop): Voltage drop = 100A × (0.245 × 0.3) = 7.35V. This is a 3.06% drop, which fails the recommendation.
If your run exceeds 145 feet, you must upsize your conductors to compensate for resistance, even though the breaker remains 100 amps. Use the Cerrowire voltage drop calculator to verify exact distances for your specific load profile.
| One-Way Distance | Copper Wire Size (75°C) | Aluminum Wire Size (75°C) | Expected Voltage Drop (at 100A / 240V) |
|---|---|---|---|
| 0 - 145 ft | 3 AWG | 1 AWG | < 3.0% |
| 146 - 185 ft | 2 AWG | 1/0 AWG | < 3.0% |
| 186 - 235 ft | 1 AWG | 2/0 AWG | < 3.0% |
| 236 - 300 ft | 1/0 AWG | 3/0 AWG | < 3.0% |
Derating: What Changes Your Required Wire Size?
The baseline sizes above assume perfect conditions. Real-world jobsite factors require you to apply NEC derating multipliers, which effectively reduce the ampacity of the wire and force you to upsize.
Ambient Temperature Corrections
NEC Table 310.16 assumes an ambient temperature of 30°C (86°F). If you route conduit through an unventilated attic in the summer, ambient temperatures can easily exceed 104°F (40°C) or 122°F (50°C). At 41-45°C, you must multiply the wire's ampacity by 0.82. A 3 AWG copper wire (100A) derates to 82A, meaning it can no longer legally sit on a 100A breaker. You would need to upsize to 2 AWG or 1 AWG.
Conductor Bundling
If you pull more than three current-carrying conductors (CCCs) in a single raceway, the trapped heat requires derating per NEC 310.15(C)(1). For 4-6 CCCs, you multiply the ampacity by 80%. For a 100A circuit, 3 AWG copper derated to 80% yields 80A—again, requiring an upsize to 2 AWG or 1 AWG. Note that the equipment grounding conductor does not count as a CCC, but the neutral in a multi-wire branch circuit or 3-phase system usually does.
Aluminum vs. Copper Terminations
Aluminum and copper are not interchangeable at the termination point. If you use 1 AWG aluminum wire, the breaker and panel lugs must be explicitly rated for aluminum (marked AL or CU/AL). You must also apply an anti-oxidant compound (like Noalox) to the stripped aluminum conductor before torquing it to the manufacturer's exact inch-pound specification to prevent cold creep and high-resistance arcing over time.
When an Engineer or AHJ Must Confirm
Step back and consult a licensed electrical engineer or your local Authority Having Jurisdiction (AHJ) if your project involves:
- Runs exceeding 300 feet (parallel conductors may be more cost-effective than massive single runs).
- Continuous loads (running at 100A for 3 hours or more requires sizing the wire and breaker at 125%, meaning you need a 125A breaker and 1 AWG copper wire).
- High-heat environments where ambient temperature corrections push your required wire size beyond 1/0 AWG.
Frequently Asked Questions
Can I use 2 AWG copper wire for a 100 amp breaker?
Yes, absolutely. 2 AWG copper is rated for 115A in the 75°C column. Using a larger wire than the minimum required is always permitted by the NEC, provided the wire physically fits into the breaker lugs. Many electricians default to 2 AWG for 100-amp subpanel feeders to mitigate voltage drop on runs between 100 and 150 feet, or simply because 2 AWG is often easier to source at local big-box stores than 3 AWG.
What size ground wire do I need for a 100 amp subpanel feeder?
Per NEC Table 250.122, the minimum size equipment grounding conductor (EGC) for a 100-amp overcurrent device is 8 AWG copper or 6 AWG aluminum. If you upsized your hot conductors to mitigate voltage drop (e.g., pulling 1 AWG copper instead of 3 AWG), NEC 250.122(B) requires you to proportionally upsize the ground wire as well to maintain the same ratio of ground-to-hot conductor cross-sectional area.
Will 4 AWG wire handle 100 amps in a pinch?
No. 4 AWG copper is rated for 85A at 75°C and 70A at 60°C. Placing it on a 100A breaker is a direct code violation and a severe fire hazard. The breaker will not trip until the current exceeds 100A (or trips thermally on a massive overload), meaning the 4 AWG wire will carry 15% to 42% more current than its insulation is designed to dissipate, leading to insulation meltdown, short circuits, and structural fires. Always match or exceed the breaker rating with your wire ampacity.
