For a standard 100-amp main panel to sub panel wiring run, use 3 AWG copper or 1 AWG aluminum THHN/THWN-2 conductors protected by a 100A 2-pole breaker. This assumes a 75°C temperature rating, 30°C ambient temperature, and no more than three current-carrying conductors in a single raceway.
The Baseline Sizing Assumptions (And Why They Matter)
Wire sizing is never a one-size-fits-all calculation. The 3 AWG copper / 1 AWG aluminum recommendation for a 100A feeder relies on specific baseline conditions. If your installation deviates from these, the required wire size changes.
- Conductor Material: Copper or Aluminum (rated separately, never interchangeably)
- Insulation Type: THHN/THWN-2 (rated for 90°C, but terminations limit us to the 75°C column)
- Temperature Column: 75°C (per NEC 110.14(C), as most residential breakers and lugs are rated for 75°C)
- Ambient Temperature: 30°C (86°F) or lower
- Raceway: PVC or EMT conduit with no more than 3 current-carrying conductors (2 hots, 1 neutral; the ground does not count)
Why this size and not one smaller? A common mistake is using 4 AWG copper for a 100A subpanel. According to NEC Table 310.16, 4 AWG copper in the 75°C column is rated for 85A. While NEC 240.4(B) allows you to round up to the next standard breaker size (90A) for overcurrent protection, a 100A subpanel has a 100A busbar rating. The feeder must be capable of carrying the full 100A to match the panel's capacity. Therefore, you must step up to 3 AWG copper, which is rated exactly 100A at 75°C.
Ampacity and Voltage Drop Decision Matrix
Use this spec-sheet table to select your conductors based on your target subpanel size. Note that voltage drop is calculated assuming an 80% continuous load diversity (e.g., 80A actual draw on a 100A panel) on a 240V circuit.
| Subpanel Size | Breaker Size | Copper (THHN 75°C) | Aluminum (THHN 75°C) | Voltage Drop Check (at 100 ft) |
|---|---|---|---|---|
| 60 Amp | 60A 2-Pole | 6 AWG (65A) | 4 AWG (65A) | 1.8% (Pass) |
| 100 Amp | 100A 2-Pole | 3 AWG (100A) | 1 AWG (100A) | 2.1% (Pass) |
| 125 Amp | 125A 2-Pole | 1 AWG (130A) | 1/0 AWG (120A)* | 1.9% (Pass) |
*Note: 1/0 Aluminum is rated 120A. For a strict 125A continuous load, you would need 2/0 Aluminum. However, for a 125A panel busbar with diverse loads, 1/0 is standard practice under NEC 240.4(B) next-size-up rules if the calculated load does not exceed 120A.
The Distance Factor: The NEC recommends a maximum of 3% voltage drop on feeders. At 100 feet, 3 AWG copper carrying 80A yields a ~2.1% drop. If your conduit run extends to 150 feet, that drop climbs to 3.1%, violating the recommendation. For runs exceeding 125 feet, you must bump your wire size to 1 AWG copper or 1/0 AWG aluminum solely to mitigate voltage drop, even though the ampacity is already sufficient.
What Changes the Answer: Derating and Environment
The baseline numbers above assume ideal conditions. Real-world jobsites rarely comply. Here is what forces you to upsize your wire:
- Bundling (Conduit Fill): If you pull more than three current-carrying conductors in a single raceway (for example, running two separate 240V subpanel feeders in one 2-inch PVC pipe), NEC 310.15(C)(1) requires derating. With 4-6 conductors, you must derate to 80% of the 90°C column. This frequently forces a one-size jump in wire gauge.
- Ambient Temperature: If your conduit runs through an unventilated attic in a southern climate where ambient temperatures reach 50°C (122°F), you must apply the temperature correction factors in Table 310.15(B)(1). A 3 AWG copper wire drops from 100A to roughly 75A of usable ampacity, requiring you to upsize to 1 AWG copper.
- Aluminum vs. Copper Terminations: Aluminum is significantly cheaper (roughly $2.50/ft for 1 AWG SER vs $3.50/ft per conductor for 3 AWG THHN copper). However, aluminum expands and contracts more than copper. You must use an anti-oxidant compound (like Noalox) on aluminum strands and strictly adhere to the manufacturer's torque specs using a calibrated torque screwdriver to prevent high-resistance connections and melted lugs.
When an Engineer or the AHJ Must Confirm
While the NEC provides the framework, local Authority Having Jurisdiction (AHJ) interpretations and engineering limits override general guides. You must pull a permit and consult a licensed electrical engineer or your local inspector when:
- Continuous Loads Exceed 80%: If your subpanel will supply a continuous load (defined as operating for 3 hours or more, like a commercial kiln or heavy EV charging array) that exceeds 80% of the breaker rating, the breaker and wire must be upsized by 125%.
- Service Entrance Proximity: If your subpanel feeder taps directly into the service entrance conductors before the main disconnect (a feeder tap rule scenario under NEC 240.21), specialized engineering calculations are required.
- Unusual Soil or Routing Conditions: Direct burial cables require specific depth and thermal resistivity calculations. If you are routing under a concrete slab or through high-thermal-resistivity clay, the AHJ may require a site-specific derating calculation.
Main Panel to Sub Panel Wiring FAQ
Do I need a ground wire for main panel to sub panel wiring?
Yes. Under NEC 250.32, a detached structure requires a 4-wire feeder system: two hot wires, one neutral, and one Equipment Grounding Conductor (EGC). In the subpanel, the neutral busbar and the ground busbar must remain isolated (separated). The neutral carries return current; the ground carries fault current. Bonding them together at the subpanel creates a parallel path for neutral current to flow through the grounding system, which is a severe shock and fire hazard. For a 100A subpanel, use a minimum of 8 AWG copper or 6 AWG aluminum for the EGC.
Can I use NM-B Romex for main panel to sub panel wiring?
You can use NM-B (Romex) only if the entire run is indoors, inside finished walls, and not subject to physical damage or wet locations. For a 60A indoor subpanel, 6/3 NM-B with an integrated ground is common. However, NM-B is strictly forbidden for outdoor runs, underground conduit, or detached garages where the conduit transitions outside. For any run that exits the building or enters a damp garage, you must use individual THHN/THWN-2 wires in conduit or outdoor-rated SER/USE-2 cable. Furthermore, NM-B ampacity is limited to the 60°C column, meaning 6/3 NM-B is only good for 55A, requiring you to upsize to 4/3 NM-B for a full 60A breaker.
What size breaker do I need in the main panel for a 60 amp subpanel?
You need a 60A 2-pole breaker. A common misconception is that if you run oversized wire (e.g., 4 AWG copper, rated for 85A) to a 60A subpanel to prevent voltage drop over a long distance, you should upsize the breaker to 70A or 80A. This is incorrect and dangerous. The breaker protects the weakest link in the circuit, which in this case is the 60A busbar rating of the subpanel itself. Even if the wire can handle 85A, the subpanel lugs and busbars cannot. Always match the main panel breaker to the subpanel's maximum busbar rating.
