Main panel to subpanel wire refers to the specific gauge, insulation type, and conductor configuration of the feeder cable used to route electrical power from a primary service disconnect to a secondary distribution panel. Getting this right dictates your maximum safe load capacity, limits voltage drop over long runs, and ensures the neutral and ground remain strictly separated at the destination. Homeowners and novice DIYers commonly confuse this feeder wiring with standard branch circuit wiring, mistakenly trying to use standard NM-B (Romex) for high-amperage outdoor runs or failing to pull a dedicated equipment grounding conductor for detached structures.
Sizing the Feeder: Ampacity and Voltage Drop
When selecting your main panel to subpanel wire, you are balancing two distinct physics problems: thermal limits (ampacity) and resistance over distance (voltage drop). The National Electrical Code (NEC) governs ampacity primarily through Article 310.16. A critical rule that trips up many beginners is the temperature rating of the termination points. Even if you buy 90°C rated THHN/THWN-2 wire, standard breaker lugs and panel bus bars are typically rated for 75°C. Therefore, you must size your wire using the 75°C column of the ampacity table, though you can use the 90°C column for derating factors like ambient temperature or conduit fill.
Below is a reference table for the most common subpanel feeder sizes. Distances are calculated for a 240V split-phase system, assuming a maximum 3% voltage drop (7.2V) at the wire's maximum rated amperage.
| Wire Size (AWG/kcmil) | Material | Ampacity (75°C Col) | Max Distance for 3% Drop | Typical Cost per Foot (2026) |
|---|---|---|---|---|
| 4 AWG | Copper (THHN) | 85A | 85 ft @ 85A | $0.95 |
| 2 AWG | Copper (THHN) | 115A* | 170 ft @ 100A | $1.55 |
| 1/0 AWG | Aluminum (XHHW) | 100A | 155 ft @ 100A | $0.75 |
| 2/0 AWG | Aluminum (XHHW) | 115A | 185 ft @ 115A | $0.95 |
| 4/0 AWG | Aluminum (XHHW) | 180A | 160 ft @ 150A | $1.60 |
*Note: 2 AWG Copper is rated 115A in the 75°C column. If your specific breaker lugs are strictly rated 60°C, you must drop to the 60°C column (2 AWG = 95A). Always verify lug ratings.
Worked Example: 100A Subpanel at 150 Feet
Let’s run the math for a highly common scenario: powering a detached garage workshop with a 100-amp subpanel located 150 feet from the main house panel. We need to supply 240V split-phase power while keeping voltage drop under the NEC-recommended 3% for feeders.
Step 1: Determine Minimum Ampacity
We need a wire that can safely carry 100A. Looking at the 75°C column, 1/0 AWG Aluminum (100A) or 2 AWG Copper (115A) both meet the baseline thermal requirement.
Step 2: Calculate Voltage Drop
The formula for single-phase voltage drop is: VD = (2 × K × I × D) / CM
- K = Direct current constant (12.9 for Copper, 21.2 for Aluminum)
- I = Current (100A)
- D = One-way distance (150 ft)
- CM = Circular mils of the wire
Testing 1/0 AWG Aluminum:
1/0 Al has 105,600 CM.
VD = (2 × 21.2 × 100 × 150) / 105,600 = 6.02 Volts.
6.02V is exactly 2.5% of 240V. This passes the 3% rule perfectly.
Step 3: Factor in Conduit and Cost
Because this is a detached structure, the wire must be run in a protective raceway like Schedule 80 PVC or rigid metal conduit. You will need four individual conductors (two hots, one neutral, one ground).
Using standard voltage drop calculators and current 2026 material pricing, pulling four 150-foot strands of 1/0 Aluminum XHHW-2 will cost roughly $450 in wire. If you chose 2 AWG Copper THHN instead, the material cost jumps to roughly $930. For runs over 50 feet, aluminum feeder wire is the industry standard for cost savings, provided you use anti-oxidant paste (like Noalox) on the terminations and torque the lugs to the manufacturer's exact inch-pound specifications.
Where You Meet This in Practice
You will encounter main panel to subpanel wire requirements in three primary residential scenarios:
- Detached Garages and Workshops: This requires a 4-wire feeder (two hots, neutral, ground) per NEC Article 250.32. The neutral and ground must remain isolated in the subpanel, and a separate grounding electrode system (ground rods) is often required at the detached building.
- EV Charger Additions: Modern Level 2 chargers pull 40A to 48A continuous. If your main panel lacks space or capacity, running a 60A or 100A subpanel specifically for EVSE (Electric Vehicle Supply Equipment) load management is common. Here, voltage drop is critical because EV chargers will throttle their charging speed if they detect low line voltage.
- Basement Finishes and Additions: When finishing a basement, adding a local subpanel saves you from running a dozen individual home-runs back to the main panel. Because the subpanel is in the same building, you do not need a separate grounding electrode, but you still must pull a 4-wire feeder and keep the neutral bus isolated from the ground bus.
FAQ: Grounding, Bonding, and Conductor Counts
Do I need a 3-wire or 4-wire feeder for a subpanel?
You almost always need a 4-wire feeder (two hots, one neutral, one equipment grounding conductor). The NEC eliminated the 3-wire feeder exception for new installations in detached buildings in the 2008 code cycle. The neutral carries return current; the ground carries fault current. They must never be bonded together at the subpanel.
Can I use standard Romex (NM-B) to feed a subpanel?
You can use NM-B cable for an indoor subpanel in the same building if it is protected from physical damage and routed through framing. However, you cannot use NM-B for outdoor runs, underground conduit, or detached structures. For those, you must use individual THHN/THWN-2 wires in conduit, or outdoor-rated SER/MHU cable.
What size ground wire do I need for a 100A subpanel?
According to NEC Table 250.122, a 100A overcurrent device requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor. If you upsize your current-carrying conductors to mitigate voltage drop (e.g., using 1/0 Al instead of 2 Al), you must proportionally increase the size of your ground wire as well.
Do I need to drive ground rods for an attached subpanel?
No. If the subpanel is in the same building or an attached structure, it relies on the main panel's grounding electrode system. You only need to drive supplementary ground rods (a grounding electrode system) if the subpanel is in a completely detached structure supplied by an underground or overhead feeder.
For comprehensive code requirements, always consult the latest edition of the NFPA 70 National Electrical Code and verify your specific installation plan with your local Authority Having Jurisdiction (AHJ) or a licensed electrical inspector.
