For a standard 100-amp subpanel, use 3 AWG copper or 1 AWG aluminum wire with a 100A breaker. For a 60-amp subpanel, use 6 AWG copper or 4 AWG aluminum with a 60A breaker. For a 200-amp subpanel, use 2/0 AWG copper or 4/0 AWG aluminum. These baseline sizes assume 75°C terminations and a maximum 3% voltage drop.
- Material: Copper (Aluminum requires specific upsizing and anti-oxidant treatment)
- Temperature Column: 75°C (per NEC 110.14(C) termination limits)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit: EMT or PVC Schedule 80, containing no more than 3 current-carrying conductors
- Insulation: THHN/THWN-2 or XHHW-2
The Core Sizing Matrix: Copper vs. Aluminum Feeders
Sizing a subpanel feeder is not just about matching the breaker to the wire; it is about matching the wire to the termination temperature rating of the lugs. The table below provides the exact wire sizes for common subpanel ratings based on the NFPA 70 (National Electrical Code) Table 310.16.
| Subpanel Rating | Feeder Breaker | Copper AWG (75°C) | Aluminum AWG (75°C) | Min. EMT Conduit |
|---|---|---|---|---|
| 60 Amp | 60A | 6 AWG | 4 AWG | 1 inch |
| 100 Amp | 100A | 3 AWG | 1 AWG | 1-1/4 inch |
| 125 Amp | 125A | 1 AWG | 1/0 AWG | 1-1/4 inch |
| 150 Amp | 150A | 1/0 AWG | 2/0 AWG | 1-1/2 inch |
| 200 Amp | 200A | 2/0 AWG | 4/0 AWG | 2 inch |
Note: Grounding conductor sizes are not listed here. A 100A subpanel typically requires an 8 AWG copper or 6 AWG aluminum equipment grounding conductor (EGC) per NEC Table 250.122, which must be routed with the feeder wires.
Why This Size? Ampacity, Temperature Columns, and the Next-Breaker Rule
A frequent mistake on the jobsite is using the 90°C column for THHN wire to size the breaker. While THHN/THWN-2 insulation is indeed rated for 90°C, the breakers and panel lugs you are terminating into are almost universally rated for 75°C. Per NEC 110.14(C), you must size the conductor based on the lowest temperature rating of any connected component. Therefore, the 75°C column dictates your baseline ampacity.
Why not use a smaller wire if the calculated load is slightly under the breaker size? The NEC allows the 'next standard size up' rule (NEC 240.4(B)). If your calculated load is 95 amps, there is no 95A breaker. You step up to the next standard size (100A) and use the wire rated for 100A (3 AWG copper). You cannot use 4 AWG copper (rated 85A at 75°C) on a 100A breaker, even if your actual continuous load is only 80 amps, because the breaker must protect the wire's physical ampacity limit, not just the anticipated load.
Never swap copper for aluminum without changing the wire size, torque specs, and preparation. Aluminum expands and contracts more than copper under thermal loading, which can cause lugs to loosen and arc over time. If using aluminum, you must apply an anti-oxidant compound (like Noalox) to the stripped conductor before termination and torque the lugs to the specific inch-pound rating printed on the breaker label using a calibrated torque screwdriver.
Voltage Drop: When Distance Forces a Larger Wire
Ampacity tables only tell you what size wire will prevent a fire. They do not tell you if the wire will deliver usable voltage to the subpanel. The NEC recommends a maximum 3% voltage drop on feeders (and 5% total for feeder plus branch circuits). When your subpanel is located far from the main panel, voltage drop becomes the governing factor, forcing you to upsize the wire.
Let us run a voltage drop check for a 100-amp subpanel located 150 feet away from the main panel, using 240V single-phase power and 3 AWG copper wire.
- Formula: VD = (2 × K × I × D) / CM
- K (Copper Resistivity): 12.9 ohms per mil-foot
- I (Current): 100 Amps
- D (One-way Distance): 150 feet
- CM (Circular Mils for 3 AWG): 52,620
Calculation: VD = (2 × 12.9 × 100 × 150) / 52,620 = 387,000 / 52,620 = 7.35 Volts.
To find the percentage: (7.35V / 240V) × 100 = 3.06%. This slightly exceeds the 3% NEC recommendation. For a 150-foot run, you must bump the wire up to 2 AWG copper (CM = 66,360), which drops the voltage loss to 2.42%, safely within the 3% threshold. You can verify these calculations using the Cerrowire Voltage Drop Calculator or similar manufacturer tools.
Derating and Environmental Factors That Change the Answer
The baseline sizes in the matrix assume ideal conditions. Real-world installations rarely stay ideal. Three primary factors will force you to increase your wire gauge:
1. Ambient Temperature Corrections
If your feeder runs through an unconditioned attic in a hot climate, the ambient temperature can easily exceed 30°C (86°F). If the attic reaches 43°C (110°F), you must apply a temperature correction factor. For THHN (90°C insulation), the correction factor at 41-45°C is 0.87. You multiply the 90°C ampacity (not the 75°C ampacity) by 0.87 to find your derated ampacity, then ensure that derated value still exceeds your breaker size. If it does not, you must upsize the wire.
2. Conduit Fill and Bundling
NEC 310.15(C)(1) requires ampacity adjustment when you have more than three current-carrying conductors in a single raceway. If you pull two separate 240V circuits (4 hot wires) plus a neutral through one conduit, you have 5 current-carrying conductors. This requires an 80% derating factor. Again, you apply this to the 90°C column ampacity to find your new limit.
3. Continuous Loads
If the subpanel will supply loads that run for 3 hours or more continuously (like EV chargers, server racks, or commercial lighting), NEC 210.20(A) requires the branch circuit overcurrent device to be rated at 125% of the continuous load. A 40A continuous load requires a 50A breaker, which in turn dictates the minimum wire size for that specific circuit.
When an Engineer or AHJ Must Confirm
While the guidelines above cover 95% of residential and light-commercial subpanel feeder installations, certain scenarios cross the line from standard DIY/trade practice into engineered design. You must consult a licensed professional engineer (PE) or your local Authority Having Jurisdiction (AHJ) before pulling wire in the following situations:
- Parallel Conductors: If you are feeding a 400A or 800A subpanel and need to run multiple sets of wires per phase (per NEC 310.10(H)), the spacing, exact conduit routing, and magnetic field balancing require strict engineering oversight.
- High Fault Current Available: If the utility transformer can deliver massive fault current (e.g., 42kA or 65kA), the subpanel's busbar must have a sufficient Short Circuit Current Rating (SCCR), and the feeder wires must withstand the magnetic and thermal stresses of a fault before the breaker clears it.
- Grounding Electrode System Nuances: If the subpanel is in a detached structure, you are required to install a separate grounding electrode system (like ground rods) at the detached building, and the neutral and ground must be bonded at the subpanel. The sizing of the grounding electrode conductor (GEC) depends on the specific electrode type and local soil resistivity, which the AHJ will inspect.
Always pull a permit for subpanel installations. The inspector will verify your torque marks on the lugs, your conduit fill ratio, and your separation of neutrals and grounds, ensuring your installation is both safe and code-compliant.
