For a standard 60-amp subpanel, use 6 AWG copper or 4 AWG aluminum feeder wire protected by a 60A double-pole breaker. This baseline assumes THHN/THWN-2 insulation, the 75°C ampacity column, 30°C ambient temperature, and no more than three current-carrying conductors in a single conduit run.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Temperature Column: 75°C (per NEC 110.14(C) terminal limits)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: PVC Schedule 80 or EMT, containing exactly one 120/240V feeder (3 current-carrying conductors + 1 ground)
Decoding the Sub Panel Wiring Schematic & Feeder Sizing
A proper sub panel wiring schematic for a 120/240V split-phase system requires a 4-wire feeder: two ungrounded "hot" conductors (Line 1 and Line 2), one grounded neutral conductor, and one equipment grounding conductor (EGC). The days of 3-wire subpanels are long gone; the NEC mandated 4-wire feeders for separate buildings in 1993 and for all subpanels in the same building in 2008.
When reading your schematic or planning your pull, the breaker size dictates the minimum wire gauge, but the terminal temperature rating dictates which ampacity column you use. Even though THHN wire is rated for 90°C, nearly all standard residential breakers and panel lugs are only rated for 75°C. Therefore, we must size the wire using the 75°C column in NEC Table 310.16.
| Feeder Breaker | Copper THHN (75°C) | Aluminum XHHW (75°C) | Min. EGC (Copper) |
|---|---|---|---|
| 60A | 6 AWG (65A) | 4 AWG (65A) | 10 AWG |
| 100A | 3 AWG (100A) | 1 AWG (100A) | 8 AWG |
| 125A | 1 AWG (130A) | 1/0 AWG (120A)* | 6 AWG |
| 200A | 3/0 AWG (200A) | 250 kcmil (205A) | 4 AWG |
*Note: 1/0 Aluminum is rated 120A. Per NEC 240.4(B), you are permitted to round up to the next standard breaker size (125A) if the load is not continuous.
Why This Size? Voltage Drop and Derating Variables
A common question on the bench is: "Why use 6 AWG for a 60A breaker instead of 8 AWG?" Looking at the 75°C column, 8 AWG copper is only rated for 50A. You cannot protect 50A wire with a 60A breaker. 6 AWG is rated for 65A, making it the smallest legal copper conductor for a 60A overcurrent device.
However, ampacity is only half the battle. The other half is voltage drop. The NEC recommends (via Informational Notes) a maximum 3% voltage drop on feeders. Let's run a voltage drop check for our 60A, 6 AWG copper feeder at a distance of 100 feet, assuming a continuous 48A load (80% of 60A):
- Formula: VD = (2 × K × I × L) / Circular Mils
- Variables: K=12.9 (Copper), I=48A, L=100 ft, CM=26,240 (for 6 AWG)
- Result: VD = (2 × 12.9 × 48 × 100) / 26,240 = 4.71 Volts
- Percentage: 4.71V / 240V = 1.96% (Passes the 3% rule easily).
If you push that same 6 AWG run to 200 feet, the drop doubles to 9.42V (3.9%), which exceeds 3%. At 200 feet, you must upsize to 4 AWG copper to maintain power quality and prevent motor burnout on heavy loads. You can verify your specific runs using tools like the Southwire Voltage Drop Calculator.
| Variable | Impact on Sizing | Code Reference / Action |
|---|---|---|
| Distance (Length) | Increases voltage drop. Upsize wire if VD > 3%. | NEC 310.15(B) Info Note |
| Bundling (Conduit Fill) | 4-6 current-carrying conductors require 80% derating. 6 AWG (65A × 0.8 = 52A) fails for 60A. Must upsize to 4 AWG. | NEC 310.15(C)(1) |
| High Ambient Heat | Conduit on a hot roof (>86°F/30°C) requires temperature correction factors, reducing ampacity. | NEC 310.15(B)(1) |
| Aluminum vs Copper | Aluminum is cheaper but requires a larger gauge and anti-oxidant paste (e.g., Noalox) on terminations to prevent arcing. | NEC 110.14 / 517.104(A)(4) |
Executing the Schematic: Terminations, Bonding, and Grounding
Once your feeder is sized and pulled, executing the sub panel wiring schematic correctly hinges entirely on the separation of neutral and ground. In a main service panel, the neutral and ground bars are bonded together. In a subpanel, they must remain strictly isolated.
Here is the step-by-step execution for terminating your 4-wire feeder:
- Remove the Bonding Strap: Before wiring anything, locate the green bonding screw or metal strap connecting the neutral bar to the panel enclosure. Remove it. If neutral current has a path back to the source via the ground wire, it will energize the panel enclosure and trip GFCI breakers randomly.
- Land the Hots: Connect Line 1 and Line 2 to the main lugs of the subpanel. Torque the lugs to the manufacturer's specification (usually printed on the panel label, often around 40-50 in-lbs for smaller lugs). Use an inch-pound torque screwdriver; hand-tightening causes high-resistance hot spots.
- Land the Neutral: Terminate the white (or gray) grounded neutral conductor on the isolated neutral bus bar. Do not mix ground wires on this bar.
- Land the Ground: Terminate the bare or green Equipment Grounding Conductor (EGC) on the ground bus bar, which is bonded to the metal enclosure. For a 60A feeder, NEC 250.122 mandates a minimum 10 AWG copper EGC. Do not use 14 AWG or 12 AWG for the ground, even if your branch circuits use those sizes.
You must defer to a licensed professional or your local inspector if:
1. The subpanel is being fed directly from a utility transformer (Service Entrance rules under NEC 310.12 apply, which differ from standard feeder rules).
2. Your calculated continuous load exceeds 80% of the feeder breaker rating (requiring a 125% multiplier on the wire size).
3. You are parallel-running multiple sets of conductors (only permitted for 1/0 AWG and larger).
By strictly following the 75°C ampacity column, calculating for voltage drop over distance, and maintaining the critical neutral-to-ground isolation, your sub panel wiring schematic will translate from a piece of paper into a safe, code-compliant, and highly reliable power distribution system.
