To wire a 60A subpanel from a main panel, you must run a 4-wire feeder consisting of 6 AWG copper THHN wires (Black, Red, White, and Green) through a 3/4-inch conduit. The feeder is protected by a 60A 2-pole breaker at the main panel, and the subpanel must have its neutral-ground bonding screw or strap removed to keep the neutral and ground currents isolated per NEC 250.32.

While the theory is straightforward, the physical execution requires strict adherence to torque specifications, conduit fill limits, and terminal mappings. Below is the complete, decision-forward guide to executing this install without a second trip to the electrical supply house.

The Subpanel Decision Path: 60A vs 100A Feeder

Before buying wire, you must size the feeder to the actual calculated load. Oversizing wastes money on copper and conduit; undersizing creates a fire hazard and nuisance tripping. Use this decision matrix to lock in your materials.

Load Profile Breaker Size Copper Wire Gauge (THHN) Min. Conduit Size (EMT/PVC)
Standard Garage (lights, 120V hand tools, fridge) 60A 6 AWG 3/4 inch
Heavy Workshop (240V table saw, dust collector, compressor) 100A 3 AWG 1 inch
EV Level 2 Charger (40A-48A continuous) + Welder 100A or 125A 3 AWG or 2 AWG 1 inch or 1-1/4 inch
The Concrete Pick: For 90% of detached garage and basement workshop builds, the 60A feeder is the correct default. Buy a Square D Homeline 12-space 125A main lug subpanel (Model: HOM1224L125PGC), a 60A 2-pole breaker (Model: HOM260), and four strands of 6 AWG copper THHN. This provides ample headroom for standard 120V/240V branch circuits without the massive price jump to 3 AWG copper.

Critical Mains Safety & Tool List

DANGER: MAINS VOLTAGE LETHALITY
Working inside a main panel exposes you to 240V AC and the unfused service entrance conductors. Before removing the main panel cover:
1. Turn OFF the main service disconnect breaker.
2. Apply a lockout/tagout device to the main breaker if possible.
3. Verify the bus bars are dead using a Category III or IV non-contact voltage tester (NCVT) and a multimeter.
Note: The utility-side lugs at the top of the main breaker remain live even when the main breaker is OFF. Do not touch the top terminals. If your panel lacks a main disconnect, you must call the utility or a licensed electrician to pull the meter.

Do not attempt this with generic hardware store tools. You need insulated, rated equipment to ensure safe terminations and accurate testing.

  • Voltage Tester: Fluke 117 True-RMS Multimeter and Fluke 1AC-II NCVT.
  • Wire Strippers: Klein Tools 1108-6 (for 6 AWG solid/stranded) or a dedicated cable skinning knife for THHN insulation.
  • Torque Tool: Wiha insulated torque screwdriver or a dial torque wrench capable of measuring inch-pounds (NEC 110.14(D) requires calibrated torque for terminals).
  • Conduit Tools: Hacksaw, reamer (for EMT), and a 3/4-inch nylon fish tape.
  • Materials: 6 AWG THHN (Black, Red, White, Green), 3/4" EMT or Schedule 40 PVC, 60A 2-pole breaker, 12-space subpanel, 3/4" conduit fittings, and anti-oxidant paste (Noalox) if terminating to aluminum bus bars.

Pulling the 4-Wire Feeder Through Conduit

NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Four 6 AWG THHN wires easily fit inside a 3/4-inch conduit (roughly 26% fill), but pulling them requires care to avoid scoring the insulation, which compromises the dielectric strength.

  1. Run and secure the conduit: Install your 3/4-inch EMT or PVC from the main panel knockout to the subpanel knockout. Use a conduit reamer on EMT ends to remove sharp burrs that will slice THHN insulation during the pull.
  2. Prepare the fish tape: Feed the nylon fish tape through the conduit from the subpanel to the main panel.
  3. Attach the Black, Red, White, and Green wires: Strip 3 inches of insulation from the lead ends of all four 6 AWG wires. Stagger the stripped ends by an inch, twist them tightly together, and fold them back over themselves to create a smooth, tapered pulling head. Wrap the head tightly with electrical tape so no bare copper is exposed.
  4. Pull the feeder: Have one person feed the wires into the main panel while you pull the fish tape at the subpanel. Pull steadily. If it binds, push back, apply wire pulling lubricant (like Polywater J), and try again. Never yank the tape, or it will snap.
  5. Leave adequate slack: Ensure you have at least 18 inches of slack inside the main panel and 24 inches inside the subpanel to reach the furthest termination points without splicing.

Terminating at the Main and Subpanel

This is where the specific terminal mappings matter. A miswired neutral or ground will create a parallel path for return current, violating NEC grounding and bonding rules and creating a shock hazard.

Main Panel Terminations (Source)

  1. Install the 60A 2-pole breaker: Snap the 60A breaker (e.g., Square D HOM260) onto the hot bus bars. Ensure it is fully seated.
  2. Terminate the Black and Red Hot wires: Strip 5/8 inch of insulation from the Black and Red wires. Insert the Black wire into one brass terminal screw on the breaker and the Red wire into the other. Torque to the manufacturer's specification (typically 35 in-lbs for 6 AWG on Square D Homeline breakers).
  3. Terminate the White Neutral wire: Strip 3/4 inch of insulation. Land the White wire on an open silver screw on the main panel's neutral bar. In a main service panel, the neutral and ground bars are bonded, but you must use the designated neutral bar to leave room for grounds. Torque to 35 in-lbs.
  4. Terminate the Green Ground wire: Strip 3/4 inch of insulation. Land the bare or Green wire on an open green screw on the main panel's equipment ground bar. Torque to 35 in-lbs.

Subpanel Terminations (Load)

  1. Remove the bonding jumper: Before wiring anything, locate the green bonding screw or metal bonding strap inside the subpanel that connects the neutral bar to the metal chassis. Remove and discard it. (See the 'Common Botch' section below for why this is critical).
  2. Terminate the Black and Red Hot wires: Land the Black and Red wires onto the two main hot lugs of the subpanel. Torque to the panel's specified rating (usually 40 in-lbs for main lugs).
  3. Terminate the White Neutral wire: Land the White wire on the isolated silver neutral bar. Because you removed the bonding jumper, this bar is now electrically floating from the metal chassis, which is exactly what NEC 250.32 requires for separate buildings or downstream panels.
  4. Terminate the Green Ground wire: Land the Green/bare wire on the equipment ground bar, which remains physically bolted to the metal subpanel chassis. This provides the fault-current path back to the main panel.

The Most Common Botch: Neutral-Ground Bonding

The single most frequent—and dangerous—mistake DIYers make when wiring a subpanel is leaving the main bonding jumper (the green screw or metal strap) installed in the subpanel.

The Physics of the Botch: In a main panel, neutral and ground are bonded so that a line-to-ground fault creates a massive short circuit, instantly tripping the breaker. In a subpanel, the neutral must carry normal return current, while the ground must only carry current during a fault. If you leave the bonding strap in the subpanel, you create a parallel circuit. Normal 120V return current will split, flowing partially on the White neutral wire and partially on the Green ground wire, the conduit, and the building's metal framing.

The Symptom: You won't necessarily see a spark or fire. The primary symptoms are 'stray voltage' (a tingling sensation when touching a metal tool chassis plugged into the subpanel), unexplained nuisance tripping of upstream GFCI or AFCI breakers, and a failure to pass electrical inspection. According to Schneider Electric's installation bulletins, subpanels must always have isolated neutral bars.

Verify and Test: Expected Meter Readings

Never close the panel covers and energize the system without verifying the voltages. Set your Fluke 117 multimeter to AC Voltage (V~) and use the following test matrix at the subpanel's main lugs and bus bars before installing any branch circuit breakers.

Test Points (Probes) Expected Reading What it Verifies
Black Lug to Red Lug (L1 to L2) 240V AC (±5%) Both hot legs are energized and out of phase.
Black Lug to Silver Neutral Bar (L1 to N) 120V AC (±5%) Leg 1 neutral return path is intact.
Red Lug to Silver Neutral Bar (L2 to N) 120V AC (±5%) Leg 2 neutral return path is intact.
Silver Neutral Bar to Green Ground Bar (N to G) < 1.0V AC (ideally 0.0V - 0.2V) Confirms the bonding jumper was removed and no neutral current is flowing on the ground bar.

If your Neutral-to-Ground (N-G) reading is higher than 1.0V AC under load, you either have a shared neutral upstream, a loose neutral connection at the main panel, or you failed to remove the subpanel bonding strap. De-energize the main breaker immediately and re-inspect your terminations.

Once readings are confirmed, install the subpanel cover, torque all cover screws, and proceed with wiring your 120V and 240V branch circuits. Always consult the National Fire Protection Association's NEC guidelines or your local Authority Having Jurisdiction (AHJ) regarding specific permit and inspection requirements for your municipality.