For a standard 60-amp 240V subpanel, use 6 AWG copper THHN wire in conduit protected by a 60A double-pole breaker. For a 100-amp feed, use 3 AWG copper or 1 AWG aluminum. These baseline sizes assume 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (unless aluminum is explicitly specified).
- Temperature Column: 75°C (based on standard breaker and panel lug ratings).
- Ambient Temperature: 30°C (86°F) or lower.
- Raceway: EMT, PVC, or flexible metal conduit with a maximum of 3 current-carrying conductors (2 hots, 1 neutral).
- Code Basis: NEC 2023/2026 style guidance; your local AHJ has final authority.
Baseline Sizing for 240V Sub Panel Wiring
When pulling a feeder to a detached garage, workshop, or addition, the breaker size dictates your minimum wire gauge. The table below maps the most common subpanel feed sizes to their required conductor gauges. Notice that aluminum requires a larger physical wire diameter than copper to carry the exact same current safely.
| Feed Breaker | Copper AWG | Aluminum AWG | 90°C THHN Ampacity (Cu) | Max Continuous Load (80%) |
|---|---|---|---|---|
| 60A | 6 AWG | 4 AWG | 75A | 48A |
| 100A | 3 AWG | 1 AWG | 110A | 80A |
| 125A | 1 AWG | 1/0 AWG | 145A | 100A |
| 200A | 2/0 AWG | 4/0 AWG | 195A | 160A |
Note: The 90°C ampacity column is shown for reference during derating calculations, but the final allowable ampacity for breaker sizing is locked to the 75°C column due to termination limits.
Why These Specific Sizes? The 75°C Termination Rule
A common mistake on the jobsite is sizing wire based on the 90°C column of NFPA 70 (NEC) Table 310.16 because THHN wire is rated for 90°C. Why, then, do we use 3 AWG copper for a 100A breaker when 4 AWG THHN has a 90°C ampacity of 95A?
The answer lies in NEC 110.14(C), which governs termination temperatures. The lugs inside your main breaker, the subpanel main lugs, and the neutral/ground bars are almost universally rated for 75°C. Even if your wire insulation can handle 90°C, the mechanical connection point cannot. As detailed in industry standard references like EC&M's guide on the 75°C rule, you must use the 75°C column to determine the base ampacity of the conductor before applying any adjustments.
For a 60A breaker, 6 AWG copper has a 75°C ampacity of 65A. For a 100A breaker, 3 AWG copper hits exactly 100A at 75°C. You cannot use a smaller wire and rely on the breaker to protect it, because the heat generated at the termination point under continuous load would degrade the lug over time, leading to arcing or thermal failure.
Voltage Drop: When Distance Forces an Upsize
Ampacity tables tell you what size wire will prevent a fire. They do not tell you what size wire will actually deliver 240V to your subpanel. The NEC recommends a maximum voltage drop of 3% for feeders (and 5% total from service to the furthest outlet).
Let’s run the math on a 60A 240V subpanel feeder using 6 AWG copper over a 100-foot run. The formula for single-phase voltage drop is:
VD = (2 × K × I × D) / CM
Where K = 12.9 (copper), I = 60A, D = 100 ft, CM = 26,240 (circular mils for 6 AWG)
VD = (2 × 12.9 × 60 × 100) / 26,240 = 5.9V
Percentage = (5.9 / 240) × 100 = 2.45%
At 100 feet, 6 AWG copper is perfectly acceptable. But what if your detached workshop is 150 feet away from the main panel?
At 150 feet, the voltage drop jumps to 8.85V, which is 3.68%. You have now exceeded the 3% feeder recommendation. To fix this, you must upsize to 4 AWG copper (CM = 41,740), which drops the loss to 2.32% over 150 feet. Always calculate voltage drop based on the actual maximum continuous load you expect to draw, not just the breaker size, but sizing for the full breaker rating is the safest baseline for future expansion.
Derating and Material Variables
The baseline table assumes ideal conditions. Real-world installations rarely stay ideal. Here is what forces you to change your wire size.
Conduit Bundling (Derating)
If you pull two separate 240V subpanel feeds through the same 1.5-inch PVC conduit, you now have four current-carrying conductors (the equipment grounding conductor does not count). According to NEC Table 310.15(C)(1), 4 to 6 current-carrying conductors require an 80% derating factor.
You must apply this derating to the 90°C column. For 6 AWG THHN (90°C ampacity = 75A), 75A × 0.80 = 60A. Because the derated ampacity (60A) is still equal to or greater than the 75°C column ampacity (65A) and the load, 6 AWG is still legally permissible for a 60A breaker. However, if you bundle 7 to 9 conductors (70% derating), 75A × 0.70 = 52.5A. You must now upsize to 4 AWG THHN to maintain a 60A feed.
Aluminum vs. Copper
Aluminum and copper are not interchangeable. Aluminum is lighter and significantly cheaper—often saving hundreds of dollars on a 200A feeder run—but it has a lower ampacity per cross-sectional area and a higher coefficient of thermal expansion.
If you use aluminum (like XHHW-2 or THWN-2), you must upsize the gauge as shown in Table 1. Furthermore, you must apply an anti-oxidant compound (like Noalox) to the stripped conductor ends before termination to prevent galvanic corrosion. Finally, aluminum requires precise lug torque using a calibrated inch-pound torque screwdriver; under-torqued aluminum lugs will loosen over thermal cycles and cause catastrophic arcing.
When an Engineer or AHJ Must Confirm
While the guidelines above cover 95% of residential and light-commercial 240V sub panel wiring scenarios, certain conditions require formal engineering review or explicit approval from your local Authority Having Jurisdiction (AHJ).
- Feeds Over 400A: Once you move into parallel conductor territory (required for feeds over 400A or when single conductors become too stiff to bend), NEC Article 310.10(H) mandates strict matching of length, material, and routing. AHJs often require stamped diagrams for parallel runs.
- Extreme Ambient Temperatures: If your conduit runs through an attic in a desert climate where ambient temperatures exceed 40°C (104°F), you must apply ambient temperature correction factors from the bottom of Table 310.16. This frequently forces an upsize even on short runs.
- Utility Intertie / Solar Subpanels: If the subpanel will host backfed solar breakers or battery inverters, NEC Article 705 (Interconnected Electric Power Production Sources) introduces the "120% rule" for busbar loading. An engineer must verify the busbar rating and breaker placement to prevent overloading the panel's main bus.
Always pull a permit for subpanel installations. A licensed inspector will verify your grounding electrode system at the detached structure, ensure your neutral and ground bars are isolated in the subpanel, and confirm your wire sizing matches the specific conditions of your jobsite.
