To wire a 60-amp subpanel, use 6 AWG copper THHN wire with a 60-amp double-pole breaker. For a 100-amp subpanel, use 3 AWG copper THHN with a 100-amp breaker. These sizes assume standard residential conditions and strictly follow the 75°C ampacity column.
The Core Sizing Matrix: Breaker, Wire, and Conduit
When figuring out how to wire a sub panel, you cannot rely on rule-of-thumb internet forums. You must use NEC Table 310.16 (formerly 310.15(B)(16)) to find the allowable ampacities. Because modern breakers and panel lugs are rated for 75°C, we pull our numbers exclusively from the 75°C column, even though THHN wire itself is rated for 90°C. The 90°C column is only used as a starting point for derating calculations, never for final breaker sizing.
| Breaker Size | Copper AWG (75°C) | Aluminum AWG (75°C) | Min EMT Conduit | Max Continuous Load |
|---|---|---|---|---|
| 60 Amp | 6 AWG | 4 AWG | 1 inch | 48 Amps |
| 100 Amp | 3 AWG | 2 AWG | 1.25 inch | 80 Amps |
Why this size and not one smaller? Let's look at the 60-amp circuit. According to the 75°C column, 6 AWG copper is rated for 65 amps. However, NEC 240.6 dictates standard breaker sizes, and 65A is not a standard size, so we round down to the next standard breaker: 60A. If you tried to use 8 AWG copper (rated for 50A) on a 60-amp breaker, you would violate NEC 240.4. The breaker would allow 60 amps to flow indefinitely, but the 8 AWG wire would overheat and melt its insulation long before the thermal trip mechanism inside the breaker engaged.
Note on Aluminum: Aluminum and copper are not interchangeable. Aluminum expands and contracts more than copper under thermal load, requiring specific torque sequences and anti-oxidant compounds (like Noalox). The aluminum sizes listed above are strictly for XHHW-2 or THWN-2 aluminum conductors and require lugs explicitly marked 'AL' or 'CU-AL'.
Variables That Force You to Upsize Your Feeder
The matrix above is your baseline. But in the field, physics and environment frequently force you to buy thicker wire. Here is what changes the answer.
1. Voltage Drop Over Distance
The NEC recommends (via informational notes in recent editions) a maximum 3% voltage drop on feeders. Let's run a voltage drop check at a stated distance of 100 feet for a 60-amp, 240-volt feeder using 6 AWG copper.
- Formula: VD = (2 × K × I × D) / CM
- Constants: K = 12.9 (copper), I = 60A, D = 100 ft, CM (circular mils for 6 AWG) = 26,240.
- Result: (2 × 12.9 × 60 × 100) / 26,240 = 5.9 volts.
- Percentage: 5.9V / 240V = 2.45% drop. This passes the 3% threshold.
However, if that detached garage is 150 feet away, the drop jumps to 3.68%. You must upsize to 4 AWG copper to bring the drop back under 3%. Always measure your actual wire routing distance, not just the straight-line distance on a blueprint.
2. Conduit Bundling and Derating
If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires derating. If you run two separate 240V circuits (four hot wires total) in one PVC pipe, you must multiply the 90°C ampacity by 80%. This thermal penalty frequently forces you to jump up one or two AWG sizes to maintain your target breaker rating.
3. Continuous Loads
If your subpanel will supply a continuous load (defined as operating for 3 hours or more, like a server rack, EV charger, or commercial lighting), NEC 210.20(A) requires the breaker to be sized at 125% of the continuous load. A 48-amp continuous load requires a 60-amp breaker, which in turn requires 6 AWG wire. Never size a breaker exactly to the continuous load.
Installation Realities and Code Checkpoints
Beyond picking the right wire, executing the installation correctly prevents fires and failed inspections. According to the Copper Development Association, proper termination is just as critical as wire sizing.
- Torque Specifications: NEC 110.14(D) mandates that lugs be tightened to the manufacturer's specified torque. Buy a calibrated inch-pound torque screwdriver. Guessing the tightness leads to loose connections, which create high-resistance hot spots that melt lugs and start fires.
- Separate Grounds and Neutrals: In a subpanel, the neutral bus bar must be completely isolated from the panel enclosure. The equipment grounding conductor (EGC) must land on a separate ground bar bonded to the metal enclosure. Never install the green bonding screw in a subpanel.
- Equipotential Bonding: This is the practice of connecting all exposed metal parts (panel enclosures, conduit, water pipes) to a common ground to prevent shock. Ensure your grounding wire is sized per NEC Table 250.122 (for a 100A breaker, you need an 8 AWG copper ground wire, not just the 10 AWG you might use for a 60A circuit).
When an Engineer or AHJ Must Confirm
You must pull a permit and have the local Authority Having Jurisdiction (AHJ) inspect the work. However, you need a licensed electrical engineer to stamp the plans if:
- You are installing a subpanel in an environment where the ambient temperature routinely exceeds 30°C (86°F), such as an unventilated attic in the Southwest US, requiring complex ambient temperature correction factors.
- The feeder routing involves parallel conductors (typically required for feeds over 400 amps).
- You are modifying the service entrance conductors or meter base to add a disconnect for the new subpanel.
Frequently Asked Questions About Subpanel Wiring
How to wire a sub panel for a detached garage?
Wiring a detached structure requires a 4-wire feeder (two hots, one neutral, one equipment ground) per NEC 250.32. The neutral and ground must remain strictly separated in the subpanel. Additionally, you must install a local grounding electrode system (typically two 8-foot copper ground rods driven 6 feet apart and connected with 6 AWG bare copper) at the detached garage to stabilize the voltage reference and provide a path for lightning strikes.
How to wire a sub panel from a 200 amp main panel?
First, perform a residential load calculation (NEC Article 220) to ensure your 200-amp main service can actually handle the new subpanel load alongside existing appliances. If you have headroom, install a double-pole breaker in the main panel sized for the subpanel feeder. If the main panel's bus bars are full, you may need to install a tandem breaker elsewhere to free up space, or upgrade the main panel entirely. Never double-tap wires on a lug not explicitly rated for two conductors.
How to wire a sub panel with a main breaker vs main lug?
A 'main lug' subpanel has no main disconnect breaker; the feeder wires land directly on the bus bar lugs. This is perfectly legal for subpanels inside the same building as the main panel, as the feeder breaker in the main panel acts as the disconnect. However, if the subpanel is in a detached building, NEC 225.32 requires a local disconnecting means. In that case, you must use a subpanel with a 'main breaker' installed, or install a separate disconnect switch ahead of a main-lug panel.
Can I use the same neutral and ground bar in a subpanel?
Absolutely not. This is the most common and dangerous mistake DIYers make. In a main panel, neutral and ground are bonded together. If you bond them in a subpanel, normal neutral return current will split and travel back to the main panel on both the neutral wire and the bare ground wire. This energizes the grounding system (conduit, panel enclosures, appliance chassis) with measurable voltage. If the neutral wire ever breaks or loosens, the full return current will seek a path through the ground wire, potentially electrifying anything metal connected to the subpanel's grounding system. Always remove the green bonding screw or strap in a subpanel.
