To wire up a circuit breaker box for a standard detached garage, workshop, or home addition, install a 100-amp subpanel fed by 3 AWG copper THHN wires protected by a 100A 2-pole breaker at the main panel. Land the black and red hots on the main breaker lugs, the white neutral on the subpanel's isolated neutral bar, and the bare/green ground on the subpanel's grounding bar. This configuration provides 120V/240V split-phase power while maintaining the critical neutral-to-ground separation required by the NEC for downstream panels.

CRITICAL MAINS SAFETY PROTOCOL: Wiring a circuit breaker box involves lethal 120V/240V AC mains voltage. Before opening any panel, turn off the main service disconnect, apply a lockout/tagout device, and verify the bus bars are dead using a CAT III (600V minimum) non-contact voltage tester and a digital multimeter. If you are replacing your main service entrance panel or working on the utility meter side of the main disconnect, stop immediately. Service entrance upgrades require a licensed electrician and utility coordination. This guide strictly covers wiring a downstream subpanel and branch circuits.

The Decision Path: Sizing Your Breaker Box and Feeder

Before pulling wire, you must match the subpanel's main lug rating, the feeder wire gauge, and the upstream breaker. Undersizing causes voltage drop and thermal runaway; oversizing wastes copper. Use this decision matrix to select your exact materials.

Target Load Profile Subpanel Rating Upstream Breaker Copper THHN Feeder (Hots/Neutral) Copper Ground (NEC 250.122)
Basic shed (lighting, 1-2 receptacles) 60A 60A 2-Pole 6 AWG 10 AWG
Standard Garage/Shop (EV charger, welder, table saw) 100A 100A 2-Pole 3 AWG 8 AWG
Heavy shop (CNC, large air compressor, heat) 125A 125A 2-Pole 1 AWG 6 AWG
The Default Pick: For 90% of residential workshop and garage builds, choose the 100A configuration. Buy a Square D QO 100A 6-space or 12-space main lug load center (Part # QO612L100PG), a 100A 2-pole breaker for your main panel (Part # QO2100), and 3 AWG copper THHN. This provides enough headroom for a 50A EV charger and a 30A 240V receptacle simultaneously without derating issues.

Tools and Materials Checklist

Do not start until you have the correct torque tools. The NEC (110.14(D)) now strictly requires calibrated torque tools for terminations. Guessing the tightness causes loose connections that arc and melt lugs under load.

  • Wire: 3 AWG Copper THHN (Black, Red, White) + 8 AWG Copper THHN (Green) or bare copper.
  • Conduit: 1.25-inch PVC Schedule 80 or EMT with appropriate sweep bends.
  • Breakers: 100A 2-pole (upstream), plus branch breakers (e.g., 20A 1-pole QO120).
  • Tools: CAT III Digital Multimeter, Non-Contact Voltage Tester, 1/4-inch drive torque screwdriver (inch-pound scale), wire strippers (capable of 3 AWG), torque wrench (for main lugs if required by panel label).
  • Hardware: Nylon cable ties, 1-inch bushings, THHN pulling lubricant.

Step-by-Step: Terminating the Feeder Wires

This phase connects your main panel to the new subpanel. We are assuming the conduit is already pulled and the wires are fished through to both boxes.

  1. Strip the Feeder Wires: Strip exactly 3/4 inch of insulation from the 3 AWG black, red, and white wires, and 1 inch from the 8 AWG green ground. Do not nick the copper strands; a nicked strand creates a hot spot.
  2. Terminate at the Main Panel (Upstream):
    • Land the Black wire on one terminal of the new 100A 2-pole breaker.
    • Land the Red wire on the other terminal of the 100A 2-pole breaker.
    • Land the White (neutral) wire on an available lug on the main panel's neutral/ground bar.
    • Land the Green (ground) wire on the main panel's ground bar.
    • Torque: Tighten the breaker terminals to the value printed on the breaker label (typically 45 in-lbs for 3 AWG on Square D QO breakers).
  3. Terminate at the Subpanel (Downstream):
    • Land the Black wire on Main Lug 1 (Phase A).
    • Land the Red wire on Main Lug 2 (Phase B).
    • Land the White wire on the subpanel's isolated neutral bar. (Crucial: Ensure the green bonding screw or strap connecting the neutral bar to the panel chassis is REMOVED).
    • Land the Green wire on the subpanel's dedicated ground bar (which must be bolted directly to the panel chassis).
    • Torque: Tighten the main lugs to the panel's specified torque (usually 45-50 in-lbs or 4 ft-lbs depending on the lug type; read the sticker inside the panel door).
  4. Dress the Wires: Use nylon cable ties to bundle the hots together, keeping the neutral and ground separated. Route them neatly along the sides of the panel gutter, ensuring no wires cross over the breaker faces.

Wiring 120V and 240V Branch Circuits

With the feeder secured, you can now wire individual branch circuits (like a 20A receptacle circuit or a 30A dryer circuit) into the subpanel.

  1. Snap in the Breaker: Align the 1-pole or 2-pole branch breaker with the bus bar stab. Press firmly on the clip side until it snaps onto the bus bar, then pivot the other side down. It should seat with a solid click.
  2. Prepare the Branch Wire: For a standard 12 AWG NM-B (Romex) 20A circuit, strip 1/2 inch of insulation from the black and white wires, and leave 3/4 inch of bare ground exposed.
  3. Land the Ground First: Loop the bare copper ground clockwise under a screw on the subpanel's ground bar. Tighten to 12-15 in-lbs. Always land grounds first so the circuit is referenced to earth before applying power.
  4. Land the Neutral: Insert the white neutral wire into an open hole on the isolated neutral bar. Tighten the set screw to 12-15 in-lbs. Do not double-lug neutrals; one wire per hole.
  5. Land the Hot: Insert the black hot wire into the terminal on the branch breaker. Tighten to the breaker's specified torque (typically 12-15 in-lbs for 14-10 AWG wire).
  6. For 240V Circuits (e.g., 30A Welder): Use 10 AWG wire. Land the black on one pole of the 2-pole breaker, the red (or white re-identified with black tape) on the other pole, and the bare ground on the ground bar. No neutral is needed for pure 240V loads.

Verify, Test, and the Most Common Botch

Never flip the main breaker without verifying your terminations. Set your digital multimeter to AC Voltage (V~) and perform these exact measurements at the subpanel.

Expected Meter Readings

  • Phase A to Neutral (Black to White): 120V (Acceptable range: 114V - 126V)
  • Phase B to Neutral (Red to White): 120V (Acceptable range: 114V - 126V)
  • Phase A to Phase B (Black to Red): 240V (Acceptable range: 228V - 252V)
  • Neutral to Ground (White to Green): < 1.0V (Ideally 0.0V to 0.2V. Anything higher indicates a loose neutral connection or excessive voltage drop on the feeder).

The Most Common Botch: Bonding the Subpanel Neutral

The single most frequent and dangerous mistake DIYers make when wiring a subpanel circuit breaker box is leaving the main bonding jumper (a green screw or metal strap) installed, which connects the neutral bar to the panel chassis (ground).

The Symptom: You will measure 2V to 5V between neutral and ground at downstream receptacles. More critically, any GFCI outlets on the circuit will nuisance-trip randomly, and you create a parallel neutral path where return current flows on the bare ground wires, energizing appliance chassis and conduit.

The Fix: De-energize the panel. Locate the green bonding screw or strap on the subpanel's neutral bar. Remove it entirely. The neutral bar must 'float' (be electrically isolated from the metal box), while the ground bar remains solidly bolted to the box. The NEC strictly mandates this separation for all panels downstream of the main service disconnect (NEC 250.24(A)(5)). Always verify isolation with an ohmmeter (reading should be 'OL' or infinite resistance between the neutral bar and the panel chassis when power is off).

Pro-Tip on Torque: If you do not own a calibrated torque screwdriver, buy one before starting. Loose terminations on 3 AWG feeder wires cause high resistance. At a 50A continuous load (like an EV charger), a loose lug will generate enough heat to melt the insulation and start a fire within weeks. Always torque to the manufacturer's spec, such as those outlined in the Schneider Electric/Square D torque guides.