For a standard 100-amp residential subpanel, use 3 AWG copper THHN wire on a 100A breaker. For a 60-amp subpanel, use 6 AWG copper on a 60A breaker. Always run four wires (two hots, one neutral, one ground) and keep the neutral and ground buses isolated in the subpanel.
- Material: Copper conductors (THHN/THWN-2 insulation).
- Temperature Column: 75°C termination rating (per NEC 110.14(C)).
- Ambient Temperature: 30°C (86°F) or lower.
- Installation: Up to 3 current-carrying conductors in EMT, PVC, or NM-B cable.
The Core Sub Panel Wire Size Chart (Copper)
When sizing feeders for a subpanel, you are balancing the breaker's overcurrent protection limit with the physical ampacity of the wire. The table below provides the exact minimums based on the 75°C column of NEC Table 310.16. Remember that the neutral (grounded conductor) must be the same size as the hots for a standard feeder, while the equipment grounding conductor (EGC) can be smaller.
| Breaker Size | Copper AWG (Hots & Neutral) | Min. Copper EGC (Ground) | Max Continuous Load (80%) | Typical Use Case |
|---|---|---|---|---|
| 60A | 6 AWG | 10 AWG | 48A (11,520W @ 240V) | Detached garage, small workshop |
| 100A | 3 AWG | 8 AWG | 80A (19,200W @ 240V) | Standard garage, basement finish |
| 125A | 1 AWG | 6 AWG | 100A (24,000W @ 240V) | Large addition, single EV charger |
| 150A | 1/0 AWG | 6 AWG | 120A (28,800W @ 240V) | Heavy shop, multiple EV chargers |
| 200A | 2/0 AWG | 4 AWG | 160A (38,400W @ 240V) | Secondary dwelling unit (ADU) |
Why You Can't Downsize: The 75°C Terminal Rule
A common mistake on the workbench is looking at the 90°C column of the ampacity table and assuming you can use a smaller wire. For instance, 4 AWG THHN is rated for 95A in the 90°C column. It seems logical to put it on a 100A breaker, assuming the 5A buffer is close enough, or relying on the NEC 240.4(B) next-size-up rule. Don't do this.
NEC 110.14(C) dictates that the ampacity of a conductor must be based on the lowest temperature rating of any connected termination, conductor, or device. Standard residential breakers, panel lugs, and wire nuts are rated for 75°C. Therefore, you must use the 75°C column to determine your maximum ampacity. In the 75°C column, 4 AWG copper is only rated for 85A. Because 85A is less than your 100A breaker, the wire is under-protected. You must step up to 3 AWG, which is rated exactly 100A at 75°C.
The 90°C column is only useful for one specific purpose: derating. If you have to apply ambient temperature or conduit fill correction factors, you start your math at the 90°C column, apply the multiplier, and then verify that the final derated number still meets or exceeds the 75°C column requirement for your breaker size.
Voltage Drop: The Hidden Distance Trap
The chart above assumes a relatively short run. The NEC does not strictly mandate voltage drop limits for feeders in standard residential builds, but Informational Note 310.15(B) strongly recommends a maximum 3% drop for feeders to maintain system efficiency and prevent motor burnout or flickering lights.
Let's run the math on a 100A subpanel feeder using 3 AWG copper. We will calculate for an 80A continuous load (the realistic 80% maximum of the breaker) at 240V.
- Formula: VD = (2 × K × I × L) / Circular Mils
- K (Copper): 12.9
- I (Current): 80A
- Circular Mils (3 AWG): 52,620
Scenario A: 150-foot run
VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.88V.
5.88V / 240V = 2.45% drop. This is well under the 3% threshold. 3 AWG is perfectly fine.
Scenario B: 200-foot run
VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84V.
7.84V / 240V = 3.26% drop. You have exceeded the recommended 3% limit. You must upsize to 2 AWG copper (CM = 66,360) to bring the drop back down to 2.59%.
Derating, Aluminum, and When the AHJ Steps In
Wire sizing is not a static lookup table; it reacts to your physical environment and material choices. Here is what changes the baseline answers provided in the chart.
Aluminum vs. Copper
Aluminum is significantly cheaper but has a lower ampacity per gauge and expands/contracts more under load, requiring anti-oxidant paste (like Noalox) and specific torque settings on lugs. If you switch to aluminum (XHHW-2 or USE-2), you generally need to go up two AWG sizes. For a 100A subpanel, 3 AWG copper becomes 1/0 AWG aluminum (rated 120A at 75°C, providing a buffer for voltage drop). Never mix aluminum and copper directly without rated bimetallic lugs or connectors.
Ambient Temperature and Bundling Derating
If your conduit runs through a hot attic in the summer, the ambient temperature might hit 113°F (45°C). According to NEC Table 310.15(B)(2)(a), you must multiply the 90°C ampacity by 0.87. If you pull four current-carrying conductors through the same conduit (rare for a standard single-phase subpanel, but common if you are pulling a second circuit alongside it), NEC Table 310.15(C)(1) requires an 80% derating factor. These multipliers stack, and if the final calculated ampacity drops below your breaker size, you must upsize the wire.
When to Defer to an Engineer or the AHJ
While the rules above cover 95% of residential subpanel installations, you must pull a permit and have your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer review your plans if:
- Parallel Conductors: You are feeding a massive 400A+ shop and want to parallel multiple sets of 3/0 or 4/0 wires. NEC 310.10(H) has strict rules about identical lengths, routing, and sizing for parallel feeds.
- Service Entrance vs. Feeder: If the subpanel is actually a service disconnect for a separate building (meaning it is on the line side of the main utility meter), the grounding electrode system and bonding rules change drastically compared to a standard downstream feeder.
- High Fault Current Available: If your utility transformer can deliver massive fault current, standard 10kAIC (Ampere Interrupting Capacity) breakers might be insufficient, requiring specialized series-rated panels or higher AIC breakers.
Always verify your final wire and breaker selections against the specific labeling inside your panelboard. If the manufacturer sticker specifies a different torque value or wire stripping length, the manufacturer's instructions override general code tables per NEC 110.3(B).
