To install a 100A breaker box (subpanel) in a garage or workshop, you need a 100A double-pole breaker in your main panel, a 4-wire feeder (typically #1 AWG aluminum XHHW-2), and a main-lug panel like the Square D HOM1224L125PC. The most critical rule: you must isolate the neutral and ground bars in the new breaker box. This guide covers the exact wire gauges, torque specs, and termination color codes required to safely feed a subpanel from an existing 200A service.

CRITICAL MAINS SAFETY PROTOCOL: Working inside a main panel exposes you to lethal 240V AC and high available fault current. Before removing any panel covers, turn off the main service disconnect, apply a lockout/tagout device, and verify the bus bars are dead using a Category III or IV non-contact voltage tester and a multimeter. If you are upgrading your main service entrance panel rather than adding a subpanel, stop here: service entrance work requires utility coordination and a licensed electrician. This guide applies strictly to downstream subpanels.

Sizing the Feeder: Wire Gauge and Breaker Ratings

When figuring out how to install a breaker box, the feeder sizing dictates everything else. Per NFPA 70 (NEC) Article 310.16, we size subpanel feeders using the 75°C ampacity column, as most modern breakers and panel lugs are rated for 75°C. While copper THHN is common for short runs, aluminum XHHW-2 is the professional standard for feeders over 60A due to cost and flexibility.

Subpanel Rating Feeder Breaker Size Copper THHN (75°C Col.) Aluminum XHHW-2 (75°C Col.) Min. Conduit Size (EMT)
60 Amp 60A (2-Pole) #6 AWG #4 AWG 3/4 inch
100 Amp 100A (2-Pole) #3 AWG #1 AWG 1 inch
125 Amp 125A (2-Pole) #1 AWG #1/0 AWG 1.25 inch
200 Amp 200A (2-Pole) #2/0 AWG #4/0 AWG 2 inch

For a standard 100A garage subpanel, we will use a 100A double-pole breaker and four strands of #1 AWG Aluminum XHHW-2. This setup costs roughly $1.50 per foot for the wire, compared to $4.50+ per foot for the copper equivalent, saving hundreds of dollars on a 50-foot run.

Tools and Materials Checklist

  • Panel: Square D HOM1224L125PC (Homeline 125A main-lug, 12/24 circuits, flush mount) or HOM1224L125PG (surface mount).
  • Breaker: Square D HOM2100 (100A 2-pole, 120/240V).
  • Wire: 4 individual strands of #1 AWG Aluminum XHHW-2 (Black, Red, White, Green).
  • Conduit: 1-inch EMT or Schedule 80 PVC with appropriate fittings and sweep elbows.
  • Stripping: Klein Tools 11063 or equivalent heavy-duty wire stripper.
  • Torquing: CDI 401SM or Milwaukee M18 torque screwdriver (inch-pound settings required).
  • Testing: Fluke 117 True-RMS Multimeter and a CAT III NCVT (Non-Contact Voltage Tester).

Step-by-Step Breaker Box Installation

1. Mount the Panel and Pull the Feeder

Mount the new breaker box so the center of the operating handle is approximately 54 inches above the finished floor, per NEC 240.24 accessibility requirements. Secure it to the studs using #10 or #12 structural screws. Run your 1-inch conduit from the main panel knockout to the subpanel knockout. Pull the four #1 AWG aluminum wires through the conduit, leaving at least 6 inches of slack past the panel's interior deadfront.

2. Terminate at the Main Panel

With the main breaker OFF and the bus verified dead, route your wires neatly along the side gutters of the main panel. Strip exactly 3/4 inch of insulation from the ends of all four wires.

  1. Black (Hot 1): Land the black wire on one of the brass load terminals of the new 100A double-pole breaker. Torque to the manufacturer's spec (typically 40 in-lbs for #1 Al).
  2. Red (Hot 2): Land the red wire on the second brass load terminal of the 100A breaker. Torque to 40 in-lbs.
  3. White (Neutral): Land the white wire on an available silver terminal on the main panel's neutral bar. Torque to spec.
  4. Green (Ground): Land the green wire on an available terminal on the main panel's ground bar (which is bonded to the neutral bar in the main service disconnect). Torque to spec.

3. Prepare the Subpanel (Isolate the Neutral)

This is the most crucial physical step. By default, main-lug panels ship with a green bonding screw or a metal bonding strap connecting the neutral bar to the metal panel enclosure. You must remove this bonding screw or strap. In a subpanel, the neutral and ground must remain strictly separated. Keep the removed screw in a plastic bag taped to the inside of the panel door for inspector verification.

4. Terminate at the Subpanel

Route the wires into the subpanel gutters, keeping hots, neutrals, and grounds separated to prevent inductive heating and make future troubleshooting easier.

  1. Black & Red (Hots): Land the black and red wires on the two main aluminum lugs at the top of the subpanel. Ensure no bare wire is exposed outside the lug barrel, and no insulation is caught inside. Torque both lugs to the value printed on the panel label (usually 40 to 45 in-lbs for #1 Al).
  2. White (Neutral): Land the white wire on the silver terminal of the isolated neutral bar. Do not land it on the ground bar. Torque to spec.
  3. Green (Ground): Land the green wire on the green terminal of the equipment grounding bar, which remains bonded to the metal enclosure. Torque to spec.

Verification and Meter Testing

Never skip the testing phase. Before installing the deadfront cover, double-check all torque marks. Once the cover is on and branch circuits are ready, turn ON the 100A feeder breaker at the main panel.

Set your Fluke 117 multimeter to AC Voltage (V~) and take the following readings at the subpanel's main lugs and bus bars:

  • Black to White (L1 to Neutral): Expect 120V (Acceptable range: 114V - 126V).
  • Red to White (L2 to Neutral): Expect 120V (Acceptable range: 114V - 126V).
  • Black to Red (L1 to L2): Expect 240V (Acceptable range: 228V - 252V).
  • White to Green (Neutral to Ground): Expect < 1.0V. If you read 120V here, you have an open neutral or a cross-wired connection. Shut down immediately.

The Most Common Botch: Subpanel Bonding

The most frequent and dangerous mistake DIYers make when learning how to install a breaker box is leaving the neutral-to-ground bonding screw in place at the subpanel. According to Schneider Electric's technical guidelines and NEC Article 250.32, a subpanel must have a 4-wire feed with isolated neutrals.

The Symptom: If you leave the bonding screw in, the neutral current has two parallel paths back to the main panel: the white neutral wire and the green/bare ground wire (along with the conduit and building steel). This causes the metal panel enclosure, appliance chassis, and ground wires to become energized with stray current. You may experience nuisance tripping of downstream GFCI receptacles, a tingling shock when touching a plugged-in appliance, or a severe shock hazard if the neutral wire ever breaks or loosens at a terminal.

The Fix: Always pull the green bonding screw on subpanels. Ensure your 4-wire feeder includes a dedicated equipment grounding conductor, and verify with your multimeter that the neutral bar reads 0V to the panel enclosure while under load.