For a standard 60-amp subpanel, use 6 AWG copper THHN for the hots and neutral, and a 10 AWG copper equipment grounding conductor (EGC), protected by a 60A double-pole breaker. This assumes copper conductors, 75°C terminations, 30°C ambient temperature, and installation in PVC conduit.
- Conductor Material: Copper (unless explicitly stated as Aluminum)
- Temperature Rating: 75°C column (per NEC Table 310.16)
- Ambient Temperature: 30°C (86°F)
- Raceway: PVC Schedule 40 conduit, standard burial depth
- Voltage: 240V/120V single-phase split
Note: NEC-style guidance provided here is for educational planning. Your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
Feeder and Equipment Grounding Conductor (EGC) Sizing
When planning sub panel grounding and feeder routing, you are actually sizing two distinct systems: the current-carrying feeder wires (hots and neutral) and the safety ground (the Equipment Grounding Conductor, or EGC). The feeder wires are sized based on ampacity (NEC 310.16), while the EGC is sized based on the rating of the upstream overcurrent protective device (NEC 250.122).
| Subpanel Rating | Main Breaker Size | Hot/Neutral AWG (Cu) | EGC AWG (Cu) | Min. PVC Conduit |
|---|---|---|---|---|
| 60 Amp | 60A (2-Pole) | 6 AWG (65A) | 10 AWG | 1 inch |
| 100 Amp | 100A (2-Pole) | 3 AWG (100A) | 8 AWG | 1-1/4 inch |
| 125 Amp | 125A (2-Pole) | 1 AWG (130A) | 6 AWG | 1-1/2 inch |
| 200 Amp | 200A (2-Pole) | 3/0 AWG (200A) | 4 AWG | 2 inch |
Why these specific sizes? The hot and neutral wires must have an ampacity equal to or greater than the non-continuous load plus 125% of the continuous load. For a 60A breaker, 6 AWG THHN (rated 65A at 75°C) is the minimum. You cannot use 8 AWG (rated 50A) because it would violate NEC 240.4 overcurrent protection rules. The EGC, however, does not carry current under normal operation; it only needs to safely carry fault current long enough to trip the breaker, which is why a 60A circuit only requires a 10 AWG ground wire per the National Fire Protection Association (NFPA) NEC 250.122 table.
The Golden Rule: Sub Panel Grounding vs. Bonding
The most common and dangerous mistake DIYers make when wiring a subpanel is confusing grounding (connecting to the earth) with bonding (connecting metal parts together to create a low-impedance fault path).
At the main service panel, the neutral bus bar and the ground bus bar are bonded together via the Main Bonding Jumper. At a subpanel, this bond must be removed. The neutral bar must be completely isolated from the metal panel enclosure, while the ground bar remains bonded to the enclosure.
If you bond the neutral and ground bars at a subpanel, a portion of the normal 120V neutral return current will flow back to the main panel through the Equipment Grounding Conductor (EGC) and any bonded metal structures (like conduit, appliance chassis, or plumbing). This energizes metal surfaces, creates a severe shock hazard, and will cause upstream GFCI breakers to trip randomly due to current imbalance. Always verify the subpanel's neutral bar isolation screw or strap has been removed before energizing.
For detached structures, NEC 250.32 requires a local Grounding Electrode System (typically two 8-foot ground rods spaced 6 feet apart). You run a Grounding Electrode Conductor (GEC) from the subpanel's ground bar to these rods. However, this local earth ground does not replace the EGC. You must still pull a dedicated EGC wire back to the main panel's ground bar to ensure low-impedance fault clearing.
Voltage Drop and Distance: When to Upsize Your Feeder
The ampacity table above assumes a relatively short run. The NEC recommends (in Informational Note to 210.19 and 215.2) that voltage drop not exceed 3% for feeders. If your subpanel is located far from the main panel, you must calculate voltage drop and potentially upsize your feeder wires.
Worked Example: 100-Amp Subpanel at 100 Feet
Let's check a 100A subpanel fed by 3 AWG copper wire over a 100-foot distance. We calculate using an 80A continuous load (the realistic maximum for a 100A breaker).
- Formula: VD = (2 × K × I × D) / CM
- K (Copper): 12.9 ohms
- I (Current): 80 Amps
- D (Distance): 100 feet
- CM (Circular Mils for 3 AWG): 52,620
Calculation: (2 × 12.9 × 80 × 100) / 52,620 = 3.92 Volts.
3.92V / 240V = 1.63% drop. This is well under the 3% recommendation. 3 AWG is perfectly fine here.
What if the run is 200 feet?
Doubling the distance doubles the voltage drop to 7.84V (3.26%). This exceeds the 3% guideline. To fix this, you must upsize the hot and neutral conductors to 2 AWG copper (CM = 66,360), which drops the voltage loss back down to 2.5%. Note that when you upsize the current-carrying conductors for voltage drop, NEC 250.122(B) requires you to proportionally increase the size of the EGC as well.
What Changes the Answer? Derating, Aluminum, and AHJ Limits
Wire sizing is not a static lookup exercise. Several real-world jobsite conditions will force you to alter the baseline table provided above.
1. Conduit Bundling and Derating
NEC Table 310.15(C)(1) requires ampacity derating when you bundle multiple current-carrying conductors (CCCs) in a single raceway. If you pull two separate 60A subpanel feeders through the same 1-inch PVC conduit, you now have 4 hots and potentially 2 neutrals. If the neutrals carry unbalanced current (counting as CCCs), you have 6 CCCs in the pipe. This requires an 80% derating factor. Your 6 AWG wire (65A × 0.80 = 52A) is now under-sized for a 60A breaker. You would need to upsize to 4 AWG to maintain code compliance.
2. Swapping to Aluminum Feeders
Aluminum wire is significantly cheaper and lighter than copper, making it the standard choice for 100A+ feeders in professional construction. However, you cannot swap them 1:1. Aluminum has higher resistance and requires larger gauge wires to carry the same current. Furthermore, aluminum expands and contracts differently than copper, requiring specific termination procedures.
| Criteria | Copper (THHN/THWN-2) | Aluminum (XHHW-2) |
|---|---|---|
| Hot/Neutral AWG | 3 AWG | 1 AWG |
| EGC AWG | 8 AWG (Cu) | 6 AWG (Al or Cu) |
| Termination Prep | Strip and insert | Wire brush + anti-oxidant paste |
| Lug Compatibility | Standard Cu/Al rated lugs | Must be explicitly AL/CU rated |
Never use standard copper lugs for aluminum wire, and never apply anti-oxidant paste to copper-to-copper connections. For detailed material properties, refer to the Copper Development Association engineering data.
3. When an Engineer or AHJ Must Confirm
While the tables and math above cover 95% of residential subpanel installations, you must pull a permit and involve a licensed electrical engineer or your local AHJ inspector under the following conditions:
- Parallel Feeders: If your subpanel requires 400A+ and you are running parallel sets of conductors (multiple wires per phase), the EGC sizing and conduit fill calculations become highly complex and strictly regulated.
- Detached Structures with Multiple Disconnects: If a detached garage or barn requires more than one main disconnect (e.g., a 200A panel and a separate 60A disconnect for a well pump), the grounding electrode system and bonding jumper sizing require strict adherence to NEC 250.32(B)(2).
- Mixed-Temperature Environments: If your conduit runs through an attic space where ambient temperatures routinely exceed 110°F (43°C), you must apply the temperature correction factors in NEC 310.15(B)(1), which will severely derate your wire ampacity and force an upsize.
Proper sub panel grounding and feeder sizing is about creating a predictable, low-impedance path for fault currents while maintaining strict isolation between neutral return paths and safety grounds. Stick to the 75°C column, respect the voltage drop math on long runs, and always verify your neutral bar isolation before throwing the main breaker.
