If you are running power to a detached garage, workshop, or shed to support a welder, EV charger, or heavy power tools, a 60-amp subpanel fed by 6 AWG copper wire is the standard default. This setup provides 240V for heavy equipment and 120V for standard outlets and lighting, giving you ample headroom without the massive cost of a 100-amp or 200-amp service upgrade.

This guide walks you through exactly how to wire a subpanel, from sizing your feeder to making the final terminations and testing the circuit. We will use a 60-amp, 4-wire feeder as our baseline model.

The Subpanel Sizing Decision Matrix

Before pulling wire, you must match your anticipated load to the correct breaker and wire gauge. The National Electrical Code (NEC) requires that feeder conductors be sized to handle the breaker rating, while also accounting for voltage drop on long runs. Use this decision tree to finalize your parts list.

Target Load Profile Breaker Size Copper Wire Gauge Aluminum Wire Gauge Max Run (Keep V-Drop <3%)
Basic lighting, 2-3 standard 120V outlets, phone charging 30-Amp 2-Pole 10 AWG 8 AWG 75 feet
Workshop: Table saw, air compressor, 240V heater, standard outlets 60-Amp 2-Pole 6 AWG 4 AWG 100 feet
Heavy fab: 240V MIG welder, EV Level 2 charger, large dust collection 100-Amp 2-Pole 3 AWG 1/0 AWG 110 feet
The Concrete Default Pick: For 90% of detached garage and workshop builds, choose a Square D QO 12-space 100-Amp main lug load center (Model QO10012CP) fed by a 60-Amp 2-pole breaker in your main panel, using four strands of 6 AWG copper THHN. The panel is rated for 100A, but we protect it at 60A upstream, which is perfectly legal and leaves room for future expansion.

Tools, Materials, and Wire Sizing

Do not start this job until every component is on your workbench. A subpanel installation requires specific 4-wire feeder cable; you cannot use 3-wire cable for a detached structure under current NEC Article 250.32.

Materials List

  • Subpanel: Square D QO 12-space 100A main lug panel (QO10012CP) or Eaton BR equivalent.
  • Main Breaker: 60-Amp 2-pole breaker matching your main panel brand (e.g., Eaton CH260, Square D QO260).
  • Feeder Wire: Four individual conductors of 6 AWG copper THHN/THWN-2. Colors: Black, Red, White, Green. (If running underground in conduit, THWN-2 is mandatory).
  • Conduit: 1-inch PVC Schedule 80 for underground or physical damage areas; Schedule 40 or EMT for above-ground protected runs.
  • Grounding Electrode: Two 5/8-inch x 8-foot copper-clad ground rods, plus a #6 AWG bare copper grounding electrode conductor (GEC) and an acorn ground clamp.

Tool List

  • Voltage Tester: Fluke 1AC-II non-contact voltage tester (CAT IV rated).
  • Multimeter: Fluke 117 True-RMS digital multimeter.
  • Torque Screwdriver: Calibrated inch-pound torque screwdriver (e.g., Klein 325) with 1/4-inch hex bits. NEC 110.14(D) strictly requires calibrated torque tools for terminations.
  • Wire Strippers: Klein 11063 wire strippers (handles 10-6 AWG).
  • Impact Driver & Drill Bits: For mounting the panel and drilling conduit knockouts.

Critical Safety: De-Energize and Verify

DANGER: MAINS VOLTAGE LETHAL HAZARD
Working inside a main service panel exposes you to bare, uninsulated 240V mains lugs that remain hot even when the main breaker is off. If you are not comfortable working inches from live utility power, hire a licensed electrician to make the main panel connection. For the subpanel feeder installation:
  1. Turn OFF the main breaker in your primary service panel.
  2. Apply a lockout/tagout (LOTO) device to the main breaker to prevent accidental re-energizing.
  3. Use your Fluke 1AC-II tester to verify the branch breakers are dead.
  4. Use your Fluke 117 multimeter to test across the main breaker load terminals and from each load terminal to the neutral bar. You must read 0.0V. Never assume a panel is dead based solely on a breaker toggle position.
Note: Always follow NFPA 70 National Electrical Code guidelines and consult your local Authority Having Jurisdiction (AHJ) for permit requirements.

Step-by-Step: Routing and Terminating the Feeder

With the main panel verified dead, you can safely route your conduit and pull your four 6 AWG THHN wires (Black, Red, White, Green) from the main panel to the subpanel. Leave at least 24 inches of slack inside both enclosures. Below is the exact termination sequence.

Step 1: Main Panel Terminations

  1. Ground (Green): Strip 3/4 inch of insulation from the Green wire. Terminate it to an open slot on the main panel's grounding bus bar. Torque to 35 in-lbs.
  2. Neutral (White): Strip 3/4 inch of insulation from the White wire. Terminate it to an open slot on the main panel's neutral bus bar. (In a main service panel, neutral and ground are bonded, but they must still land on separate physical bar slots if the bar is not explicitly rated for two wires per hole). Torque to 35 in-lbs.
  3. Hot Leg 1 (Black): Strip 3/4 inch of insulation. Insert the Black wire into the left load terminal of the new 60-Amp 2-pole breaker. Torque to 35 in-lbs.
  4. Hot Leg 2 (Red): Strip 3/4 inch of insulation. Insert the Red wire into the right load terminal of the 60-Amp 2-pole breaker. Torque to 35 in-lbs.
  5. Snap the 60-Amp breaker into the main panel bus stabs, ensuring it seats firmly.

Step 2: Subpanel Preparation (The Bonding Check)

Before terminating any wires in the subpanel, you must isolate the neutral from the ground. In a main panel, neutral and ground are bonded. In a subpanel, they must remain completely isolated.

  1. Locate the green bonding screw or copper bonding strap inside the Square D or Eaton subpanel.
  2. Remove the bonding screw entirely, or cut and remove the metal bonding strap connecting the neutral bar to the panel enclosure.
  3. Verify with a multimeter on continuity mode that there is no continuity (OL) between the neutral bus bar and the metal panel enclosure/ground bus bar.

Step 3: Subpanel Terminations

  1. Ground (Green): Terminate the Green feeder wire to the subpanel's ground bus bar (which is bolted directly to the metal enclosure). Torque to 35 in-lbs.
  2. Neutral (White): Terminate the White feeder wire to the subpanel's isolated neutral bus bar. Ensure it does not touch the metal enclosure. Torque to 35 in-lbs.
  3. Hot Leg 1 (Black): Terminate the Black wire to the L1 main lug on the subpanel's main breaker or main lug assembly. Torque to 45 in-lbs (check the panel label for exact main lug torque specs).
  4. Hot Leg 2 (Red): Terminate the Red wire to the L2 main lug. Torque to 45 in-lbs.
Conduit Fill Check: Pulling four 6 AWG THHN wires through a 1-inch PVC conduit utilizes roughly 18% of the conduit's cross-sectional area. This is well below the NEC Chapter 9 maximum fill limit of 40% for three or more wires, making the pull smooth and preventing insulation damage.

The Most Common Botch: Neutral and Ground Bonding

The single most frequent and dangerous mistake DIYers make when wiring a subpanel is failing to remove the neutral-to-ground bonding screw or strap in the subpanel.

The Symptom: If the bonding strap is left in place, the neutral bar and ground bar become electrically connected at the subpanel. Because current always seeks multiple paths back to the source, a portion of your 120V neutral return current will flow backward through the Green ground wire, through the earth, and back to the main panel. This causes three major failures:

  1. Nuisance Tripping: Upstream GFCI or AFCI breakers will detect the current imbalance and trip constantly.
  2. Shock Hazard: The metal enclosures of your tools, the subpanel door, and any metal conduit will become energized with return current. Touching a plugged-in table saw could result in a lethal shock.
  3. Inspector Fail: Any AHJ inspector will immediately red-tag the installation for violating NEC 250.142.

The Fix: Always physically verify the isolation of the neutral bar from the panel chassis with a multimeter before energizing the circuit.

Verify and Test: Expected Meter Readings

Once all terminations are torqued, the bonding strap is removed, and the panel covers are replaced, it is time to energize and test. Remove the LOTO device, stand to the side of the panel (to avoid arc flash vector), and flip the main breaker ON, followed by the new 60-Amp subpanel feeder breaker.

Set your Fluke 117 multimeter to AC Voltage (V~) and take the following measurements at the subpanel:

Test Points Expected Reading What it Verifies
L1 (Black) to L2 (Red) 238V - 242V Both hot legs are live and properly phased.
L1 (Black) to Neutral (White) 118V - 122V Leg 1 is delivering standard single-phase voltage.
L2 (Red) to Neutral (White) 118V - 122V Leg 2 is delivering standard single-phase voltage.
Neutral (White) to Ground (Green) < 1.0V (ideally 0.0V - 0.5V) Confirms no neutral current is leaking onto the ground wire; proves the subpanel bonding strap was successfully removed.
L1 (Black) to Ground (Green) 118V - 122V Confirms the grounding path is intact back to the main panel.

If your Neutral-to-Ground reading is higher than 2.0V, immediately de-energize the panel. You likely have a loose neutral connection at the main panel, or the subpanel bonding strap was not fully removed. Do not plug in any tools or appliances until the Neutral-to-Ground voltage reads under 1.0V at no-load.

Finally, if your detached garage subpanel is fed from a separate building, NEC 250.32 requires a local grounding electrode system. Drive two 5/8-inch copper ground rods at least 6 feet apart outside the garage, connect them with a continuous #6 AWG bare copper wire, and terminate that wire to the subpanel's ground bus bar using an acorn clamp. This provides a local path for lightning and surge dissipation, completing a safe, code-compliant 60-amp subpanel installation.