How to Wire an Electrical Panel: 100A Subpanel Installation Guide

When you ask how to wire an electrical panel, the safest and most common DIY-adjacent project is installing a 100-amp subpanel to expand your circuit capacity for a garage, workshop, or addition. To wire a 100A subpanel, you need a 4-wire feeder (two hots, one neutral, one ground) routed from a 100A double-pole breaker in your main panel. For a run up to 50 feet, you will use 3 AWG copper or 1 AWG aluminum THHN wire in conduit. The neutral and ground bars in the subpanel must remain physically isolated.

Scope Note: This guide covers wiring a subpanel fed from an existing main service panel. Upgrading your main service entrance, swapping the meter base, or working on the utility side of the main disconnect is strictly for licensed electricians and your local utility provider.

The Direct Answer: Sizing Your Subpanel Feeder

The most frequent mistake when figuring out how to wire an electrical panel is undersizing the feeder wire or using the wrong insulation type. Subpanel feeders are calculated based on the National Electrical Code (NEC) Article 310 ampacity tables, specifically the 75°C column, because most modern breakers and panel lugs are rated for 75°C terminations.

Below is the exact sizing matrix for common subpanel installations. This table assumes a standard residential 120/240V single-phase system, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in the raceway.

Subpanel Rating	Feeder Breaker Size	Copper THHN (75°C)	Aluminum XHHW (75°C)	Min. PVC Conduit
60 Amp	60A (2-Pole)	6 AWG	4 AWG	1 inch
100 Amp	100A (2-Pole)	3 AWG	1 AWG	1.25 inch
125 Amp	125A (2-Pole)	1 AWG	1/0 AWG	1.5 inch
200 Amp	200A (2-Pole)	2/0 AWG	4/0 AWG	2 inch

Voltage Drop Note: If your subpanel is more than 100 feet from the main panel, you must upsize the wire by one gauge to keep voltage drop under the NEC recommended 3% for feeders. For a 150-foot run to a 100A subpanel, step up to 2 AWG copper or 1/0 AWG aluminum.

Tools and Materials Checklist

Do not start pulling wire until you have the exact hardware. Mixing brands of breakers and panels (e.g., putting a Siemens breaker in a Square D Homeline panel) is a code violation and a fire hazard unless the breaker is specifically UL-classified for that panel (like Eaton CL).

• Panel: Square D Homeline 100A, 12-space main lug subpanel (Model: HOM1224L100C) or equivalent.
• Feeder Breaker: 100A 2-pole breaker matching your main panel brand (e.g., HOM2100 for Square D).
• Wire: Four individual conductors of 3 AWG Copper THHN/THWN-2 (Black, Red, White, Green) OR 4-wire SER cable if running through framing without conduit.
• Conduit: 1.25-inch Schedule 40 PVC or EMT with appropriate fittings and sweep elbows.
• Torque Tool: Insulated torque screwdriver (e.g., Klein Tools or Milwaukee) capable of measuring inch-pounds.
• Testers: Category III or IV Digital Multimeter (e.g., Fluke 117) and a Non-Contact Voltage Tester (NCVT).
• Hand Tools: Wire strippers rated for 3 AWG, lineman pliers, fish tape, and a cordless drill with step bits for conduit knockouts.

Step-by-Step: How to Wire the Electrical Panel

Follow these steps sequentially. The 2008 NEC revision mandated 4-wire feeds for all subpanels, meaning the neutral and ground must remain strictly separated at the subpanel.

1. Prep the Main Panel and Install the Feeder Breaker: Following the safety callout above, kill the main power. Snap the 100A 2-pole breaker into an available slot on the main panel bus bar. Ensure it seats firmly. Do not wire it yet.
2. Pull the 4-Wire Feeder: Route your conduit from the main panel to the subpanel location. Pull the four THHN wires (Black, Red, White, Green) through the conduit using fish tape and wire pulling lubricant. Leave at least 3 feet of slack at both ends.
3. Isolate the Subpanel Neutral Bar (Crucial): Open the subpanel. Locate the green bonding screw or bonding strap that connects the neutral bar to the panel's metal enclosure. Remove it completely. In a subpanel, the neutral bar must float (be isolated from ground). The ground bar should remain bolted directly to the panel enclosure. If your panel didn't come with a separate ground bar, buy an accessory ground bar (e.g., Square D HOMGB2) and install it now.
4. Terminate Feeder at the Subpanel: Strip 3/4 inch of insulation from the feeder wires.
   • Land the Black wire on Main Lug A (Phase 1). Torque to the manufacturer's spec (typically 45 in-lbs for 3 AWG).
   • Land the Red wire on Main Lug B (Phase 2). Torque to spec.
   • Land the White (Neutral) wire on the isolated neutral bar. Torque to spec.
   • Land the Green (Ground) wire on the dedicated ground bar bonded to the enclosure. Torque to spec.
5. Terminate Feeder at the Main Panel: Route the wires neatly along the wire gutters.
   • Land the Black wire on Terminal 1 of the 100A feeder breaker.
   • Land the Red wire on Terminal 2 of the 100A feeder breaker.
   • Land the White (Neutral) wire on the main panel's neutral bar.
   • Land the Green (Ground) wire on the main panel's ground bar (or neutral bar if they are bonded at the main service disconnect).
6. Secure and Close: Tuck all wires neatly, ensuring no bare copper is exposed outside the lugs. Replace both panel covers and install all blank filler plates in unused slots.

The Verify and Test Sequence

Before plugging in any tools or turning on lights in the new subpanel, you must verify the wiring with a multimeter. Set your Fluke 117 (or equivalent) to AC Voltage (V~).

1. Turn on the main breaker, then turn on the new 100A subpanel feeder breaker.
2. Test Line-to-Line: Place one probe on the Black feeder lug and the other on the Red feeder lug at the subpanel. Expected Reading: 240V (acceptable range 228V - 252V).
3. Test Line-to-Neutral: Place one probe on the Black lug and the other on the White (Neutral) bar. Expected Reading: 120V. Repeat for Red to Neutral. Expected Reading: 120V.
4. Test Neutral-to-Ground: Place one probe on the White (Neutral) bar and the other on the Green (Ground) bar. Expected Reading: 0V to 1.5V. If you read 120V here, you have an open neutral or a crossed wire. Kill power immediately and re-check terminations.

The Most Common Botch (And How to Avoid It)

The single most dangerous and frequent mistake DIYers make when learning how to wire an electrical panel is failing to remove the bonding screw/strap in the subpanel.

The Symptom: You might notice a faint tingling sensation when touching metal tools plugged into the subpanel, or you might experience nuisance tripping of GFCI or AFCI breakers downstream. In severe cases, a clamp meter on the ground wire will show current flowing under normal load.

The Physics: According to Schneider Electric's technical guidelines and NEC Article 250.142, the neutral and ground can only be bonded at the first point of disconnect (the main service panel). If you leave the bonding strap in the subpanel, you create a parallel path for neutral return current to flow back to the main panel along the bare ground wire and the metal conduit. This energizes the grounding system. If the neutral wire ever breaks or loosens at the main panel, the full 120V return current will seek a path through the ground wire, potentially electrifying the metal enclosure of your subpanel and any tools plugged into it.

The Fix: Always verify the neutral bar is floating. Give the neutral bar a gentle wiggle with an insulated screwdriver before terminating the white wire; it should not be physically attached to the metal backplane of the panel. Only the ground bar should be rigidly bolted to the enclosure.