How to Install a Circuit Breaker Box: 100A Subpanel Wiring Guide

Learning how to install a circuit breaker box for a garage, workshop, or detached building almost always means installing a subpanel fed from your main service. To install a standard 100-amp subpanel, you need a 100A main breaker panel, 4-wire feeder cable (3 AWG copper or 1 AWG aluminum), and you must physically isolate the neutral and ground bus bars inside the new box. This guide covers the exact wire sizing, color-coded termination sequence, and testing protocols required for a safe, code-compliant 100A subpanel installation.

Critical Mains Safety & Code Requirements

Feeder Wire, Breaker, and Panel Sizing Chart

Before pulling wire, you must match your feeder breaker, conductor gauge, and panel rating. The NFPA 70 (National Electrical Code) Table 310.16 dictates ampacities based on the 75°C column for standard residential terminations. Always use a 4-wire system (two hots, one neutral, one ground) for subpanels.

Subpanel Rating	Feeder Breaker Size	Copper AWG (75°C)	Aluminum AWG (75°C)	Min. Conduit Size (PVC Sch 80)
60 Amp	60A (2-Pole)	6 AWG	4 AWG	1 inch
100 Amp	100A (2-Pole)	3 AWG	1 AWG	1-1/4 inch
125 Amp	125A (2-Pole)	2 AWG	1/0 AWG	1-1/4 inch
200 Amp	200A (2-Pole)	2/0 AWG	4/0 AWG	2 inch

Note: If the subpanel is in a detached building, NEC Article 250.32 requires a separate grounding electrode system (like two ground rods) at the detached structure, bonded to the subpanel's ground bus.

Tools and Materials List

• Panel: 100A main breaker load center (e.g., Square D Homeline or Siemens), 24-space minimum.
• Feeder Breaker: 100A 2-pole breaker matching your main panel's brand and bus bar type.
• Feeder Wire: Four individual THHN/THWN-2 conductors in conduit (3 AWG Copper: Black, Red, White, Green) OR 4-wire URD/SER cable if running outdoors/underground.
• Conduit & Fittings: 1-1/4" PVC Schedule 80 or EMT with appropriate sweep elbows and pull boxes.
• Torque Screwdriver: Calibrated inch-pound torque screwdriver (NEC 110.14(D) mandates torqued terminations to prevent arc faults).
• Wire Prep: Heavy-duty wire strippers (Klein 11055 or similar), cable cutter, fish tape, and 3M Scotch 35 vinyl electrical tape for phase marking.
• Testing: True-RMS CAT III/IV digital multimeter and a clamp-on ammeter.

Step-by-Step Subpanel Installation

1. Mount the Subpanel: Secure the panel box to the studs using 1/4" x 3" lag screws. Ensure the center of the panel is roughly 55 to 60 inches from the finished floor to meet ADA and NEC working clearance requirements.
2. Remove the Bonding Strap (CRITICAL): Before pulling any wire, locate the green bonding screw or metal bonding strap inside the new subpanel that connects the neutral bus bar to the steel chassis. Remove and discard it. In a subpanel, the neutral (current-carrying) and ground (safety) paths must remain completely isolated. Schneider Electric's subpanel bonding guidelines explicitly detail how to identify and remove this strap on Square D panels.
3. Pull the Feeder Conductors: Use fish tape to pull your four THHN wires through the conduit from the main panel to the subpanel. Leave at least 24 inches of slack in both boxes. If using white wire for a hot leg (not recommended, but sometimes done in older cable pulls), you must wrap both ends with black or red electrical tape to re-identify it as a hot conductor.
4. Terminate at the Main Panel (Source):
   • Black & Red (Hot): Strip 3/4" of insulation. Land the Black wire on the brass/black screw of Terminal 1, and the Red wire on Terminal 2 of the new 100A feeder breaker. Torque to the breaker manufacturer's spec (typically 45-50 in-lbs).
   • White (Neutral): Strip 3/4" of insulation. Land this on an available silver screw on the main panel's neutral bus bar. Torque to spec.
   • Green (Ground): Strip 3/4" of insulation. Land this on an available green screw on the main panel's equipment grounding bus bar (or the combined neutral/ground bar if it's a main service panel). Torque to spec.
5. Terminate at the Subpanel (Destination):
   • Black & Red (Hot): Strip 3/4" of insulation. Land the Black wire on the left main lug and the Red wire on the right main lug of the 100A subpanel's main breaker. Torque to the exact inch-pound rating printed on the panel label (usually 50 in-lbs for 3 AWG).
   • White (Neutral): Strip 3/4" of insulation. Land this on an available silver screw on the isolated neutral bus bar. Do not mix ground wires on this bar.
   • Green (Ground): Strip 3/4" of insulation. Land this on an available green screw on the equipment grounding bus bar, which is bolted directly to the steel chassis.
6. Dress the Wires: Use zip ties or nylon cable staples to neatly route the wires along the panel gutters. Keep hot, neutral, and ground wires grouped but separated to maintain a clean, professional installation that inspectors look for.

Verification and Testing

Before closing the dead front cover or turning on the feeder breaker, perform a visual inspection to ensure no bare copper is exposed outside the lugs and that the neutral and ground bars are physically isolated. Once verified, remove your LOTO device, stand to the side of the panel (arc flash safety posture), and energize the 100A feeder breaker.

Set your True-RMS multimeter to AC Voltage (V~) and take the following measurements at the subpanel's main lugs and bus bars:

• Line 1 (Black) to Line 2 (Red): Expected reading is 240V (acceptable range 230V–250V). This confirms both poles are live.
• Line 1 (Black) to Neutral (White): Expected reading is 120V.
• Line 2 (Red) to Neutral (White): Expected reading is 120V.
• Neutral (White) to Ground (Green/Chassis): Expected reading is < 1.0V (typically 0.2V to 0.5V due to normal voltage drop on the neutral conductor). If you read 120V here, your neutral is loose or disconnected. Shut down immediately.

Finally, use a clamp-on ammeter around the Black and Red feeder wires simultaneously (with a known load applied downstream) to ensure the current is balanced and within the 100A rating.

The Most Common Botch: Subpanel Bonding

The single most frequent and dangerous mistake DIYers make when learning how to install a circuit breaker box for a subpanel is failing to remove the neutral-to-ground bonding screw or strap.

The Physics of the Botch: In your main service panel, the neutral and ground are bonded together. This is the only place in your electrical system where this connection is allowed. If you leave the bonding strap installed in a subpanel, you create a parallel path for neutral current. Every time you turn on a 120V tool or light in the garage, the return current splits: some flows back on the white neutral wire, and some flows back on the bare green ground wire and the metal conduit.

The Symptoms: 1. Tingling Shocks: Because the grounding system is now carrying load current, it develops a voltage gradient. If you touch the metal casing of a table saw or a grounded appliance, you may feel a mild tingling shock. 2. Nuisance Tripping: Upstream GFCI or AFCI breakers may trip randomly because they detect current returning on the ground path instead of the neutral path, interpreting it as a ground fault. 3. Inspection Failure: An AHJ inspector will check for the bonding strap immediately. If it is present, you will fail the rough-in or final inspection and be forced to de-energize and correct the panel.

Always verify the isolation of the neutral bar from the steel chassis using your multimeter on the Ohms (Ω) setting before energizing the panel. With the power off, place one probe on the silver neutral bar and one on the steel panel box. The meter should read 'OL' (Open Loop / Infinite Resistance). If it reads 0.0Ω or continuity, the bonding strap is still installed and must be removed.