How to Install a Breaker Panel: 100A Subpanel Wiring Guide

When learning how to install a breaker panel for a garage, workshop, or detached outbuilding, you are almost always installing a subpanel fed from your home's main service disconnect. The direct answer for 90% of residential outbuilding projects is to install a 100-Amp main-breaker subpanel fed by 2 AWG copper THHN wire pulled through 1.5-inch PVC conduit. This configuration provides ample headroom for 240V tools, EV chargers, and standard 120V lighting circuits without requiring a costly utility service upgrade.

This guide provides the exact decision framework, material list, and termination sequence to install a 100A subpanel safely and in accordance with NEC-style guidance. Note: Your local Authority Having Jurisdiction (AHJ) has final authority over code compliance, and permits are required for all panel installations.

Decision Tree: Sizing Your Feeder and Panel

Do not guess your panel size based on the physical number of slots. Size the panel based on the continuous and non-continuous load calculations of the outbuilding, then select the wire gauge based on the 75°C column of NEC Table 310.16.

Target Load	Feeder Wire (Copper 75°C)	Min. Conduit	Main Breaker	Verdict
60A	6 AWG	1 inch	60A	Undersized for modern EV/welders
100A	2 AWG	1.5 inch	100A	DEFAULT PICK: Best balance of cost/headroom
125A	1 AWG	1.5 inch	125A	Overkill for standard garages
200A	2/0 AWG	2 inch	200A	Requires full main service upgrade

The Concrete Pick: Buy a Square D Homeline 100A 20-Space Main Breaker Load Center (Model HOM20M100C). Feed it with four individual 2 AWG Copper THHN wires (Black, Red, White, Green) inside 1.5-inch Schedule 80 PVC conduit. Copper is specified here over aluminum to eliminate DIY oxidation risks and terminal loosening over time, though 1/0 AWG Aluminum SER cable is a valid, cheaper alternative if you use anti-oxidant paste.

Critical Safety & Code Boundaries

Tools and Materials List

Having the right tools prevents damaged lugs and failed inspections. Do not substitute a standard screwdriver for a torque tool.

• Panel: Square D HOM20M100C (100A Main Breaker, 20 spaces)
• Feeder Breaker (at Main Panel): Square D HOM2100 (100A 2-pole)
• Wire: 2 AWG Copper THHN/THWN-2 (Black, Red, White, Green) - approx. 150 ft
• Conduit & Fittings: 1.5" Schedule 80 PVC, sweeping elbows, liquid-tight connectors for panel knockouts
• Torque Tool: CDI Torque Screwdriver (10-100 in-lbs) or equivalent, with 5/16" nut driver attachment
• Wire Prep: Klein Tools 11055 wire strippers, cable cutter, wire brush for lug cleaning
• Testing: Fluke 117 True-RMS Multimeter, CAT III NCV tester
• Hardware: 1/4" x 1.5" lag screws, panel bonding bushings (if using flexible conduit at any point)

Step-by-Step Installation and Termination

The following sequence assumes the conduit is already mounted, pulled, and the panel enclosure is securely lagged to the wall studs. We are focusing strictly on the electrical terminations.

1. Prepare the Main Panel Feeder Breaker

1. With the main panel de-energized and verified dead, snap the 100A 2-pole feeder breaker into the designated bus stabs.
2. Strip exactly 5/8" of insulation from the Black (Hot 1) and Red (Hot 2) 2 AWG wires.
3. Land the Black wire into the left lug of the 100A feeder breaker. Torque to 250 in-lbs (verify against the breaker label).
4. Land the Red wire into the right lug of the 100A feeder breaker. Torque to 250 in-lbs.

2. Terminate the Subpanel Main Breaker (Hot Conductors)

1. Route the Black (Hot 1) and Red (Hot 2) wires into the new subpanel enclosure, leaving at least 18 inches of slack.
2. Strip 5/8" of insulation from both wires.
3. Land the Black wire into the top-left lug of the subpanel's 100A main breaker. Apply your torque screwdriver and tighten to exactly 250 in-lbs.
4. Land the Red wire into the bottom-right lug of the subpanel's 100A main breaker. Torque to 250 in-lbs.

3. Terminate the Neutral Conductor

1. Route the White (Neutral) wire to the isolated neutral bus bar. Crucial: In a subpanel, the neutral bar must NOT be bonded to the metal enclosure. Ensure the green bonding screw or strap has been removed.
2. Strip 1/2" of insulation from the White wire.
3. Land the White wire onto an empty silver screw on the isolated neutral bus bar.
4. Torque the neutral lug to 45 in-lbs (standard for Square D neutral bars accepting up to 2 AWG).

4. Terminate the Ground Conductor

1. Route the Green (Equipment Grounding Conductor) to the dedicated ground bus bar. This bar must be physically bonded to the metal panel enclosure via a green bonding screw or a grounding strap.
2. Strip 1/2" of insulation from the Green wire.
3. Land the Green wire onto an empty green screw on the ground bus bar.
4. Torque the ground lug to 45 in-lbs.

Verify and Test: Expected Meter Readings

Do not skip the testing phase. Before installing the panel deadfront cover, restore power to the main panel, turn ON the 100A feeder breaker, and turn ON the subpanel main breaker. Set your Fluke multimeter to AC Voltage (V~) and take the following measurements:

Test Points (Probes)	Expected Reading	Diagnostic Meaning
Main Breaker Lug L1 to Lug L2	240V (±5%)	Both hot legs are energized and out of phase.
Lug L1 to Neutral Bus	120V (±5%)	Leg 1 is properly referenced to neutral.
Lug L2 to Neutral Bus	120V (±5%)	Leg 2 is properly referenced to neutral.
Neutral Bus to Ground Bus	< 2.0V	Confirms no illegal neutral-ground bond in subpanel and no heavy neutral return fault.
Ground Bus to Known Earth (e.g., cold water pipe)	0.0V	Confirms equipment grounding path is continuous back to the main service ground.

If your Neutral-to-Ground reading is above 2V, you either have an illegal bond in the subpanel, a loose neutral at the main panel, or a heavy fault current on a downstream branch circuit. Turn off the breaker and investigate immediately.

The Most Common Botch: Leaving the Bonding Strap Installed

The single most frequent failure in DIY subpanel installations is failing to remove the main bonding jumper (a green screw or a metal strap) that connects the neutral bus bar to the metal panel enclosure.

The Symptom: Randomly tripping GFCI outlets in the outbuilding, a mild tingling shock when touching metal tools plugged into the panel, or a neutral wire that feels warm to the touch.

The Physics: According to NEC Article 250.142, the neutral and ground can only be bonded at the first point of disconnect (the main service panel). In a subpanel, neutral carries the unbalanced return current. If you bond neutral to ground in the subpanel, you create a parallel path. Return current will flow back to the main panel on both the white neutral wire and the bare/green ground wire. This energizes the ground wire, the metal panel enclosure, and any metal tool chassis plugged into the building, creating a severe shock hazard and causing ground-fault protection devices to misread the current imbalance and trip.

The Fix: Before terminating any wires, locate the green bonding screw or metal strap on the subpanel's neutral bar. Unscrew it, remove it entirely from the enclosure, and throw it away. Ensure the neutral bar sits on plastic insulators, completely floating from the metal box. The ground bar, conversely, must be bolted directly to the bare metal of the enclosure.

By strictly following the 100A default pick, terminating the Black, Red, White, and Green wires to their exact designated lugs, and verifying your torque and voltage readings, your breaker panel installation will be safe, functional, and ready to pass inspection.