To wire a 100A breaker box (subpanel) correctly, you must use #2 AWG copper or #1 AWG aluminum feed wire. The black and red hot wires land on the main breaker lugs, the white neutral wire lands on the isolated neutral bus bar, and the bare/green ground wire lands on the bonded ground bus bar. Unlike a main service panel, the neutral and ground bars in a subpanel must remain strictly separated to prevent shock hazards and nuisance tripping.
The 100A Subpanel Decision: Wire Gauge and Breaker Sizing
Before pulling any wire, you must match your feed breaker, wire gauge, and subpanel rating. The National Electrical Code (NEC) requires that the wire ampacity meets or exceeds the breaker rating, based on the 75°C column of NEC Table 310.16. While many big-box stores sell '2-2-2-4' aluminum SER cable labeled for 100A, it is technically only rated for 90A at 75°C. For a true, code-compliant 100A feed, use the following exact specifications:
| Component | Copper Option (Premium) | Aluminum Option (Budget) |
|---|---|---|
| Feed Breaker (Main Panel) | 100A 2-Pole (Square D HOM2100) | 100A 2-Pole (Square D HOM2100) |
| Wire Gauge (Hots & Neutral) | #2 AWG (Rated 115A @ 75°C) | #1 AWG (Rated 130A @ 75°C) |
| Equipment Grounding Conductor | #8 AWG Copper | #6 AWG Aluminum or #8 Copper |
| Conduit Size (Minimum) | 1.25-inch Schedule 40 PVC | 1.5-inch Schedule 40 PVC |
Tools, Materials, and the Non-Negotiable Safety Protocol
Wiring a breaker box involves exposed, potentially lethal bus bars. Even if the main breaker is off, the utility feed lugs at the top of the main panel remain energized. If you are uncomfortable working inches away from live utility power, hire a licensed electrician to make the final connection at the main panel.
1. De-energize: Turn off the main breaker in your primary service panel.
2. Lock/Tag: Apply a physical lockout/tagout device to the main breaker so no one accidentally flips it while you are working.
3. Verify Dead: Use a non-contact voltage tester (NCVT) and a CAT III rated digital multimeter to verify 0V across all bus bars before touching any terminal.
Always reference OSHA electrical safety guidelines for proper lockout/tagout procedures.
Required Tools & Materials
- Subpanel: Square D HOM1624L125CP (125A Main Lug, 16-space) or Eaton BR2100.
- Wire Strippers: Rated for #2 AWG to #10 AWG (e.g., Klein 11057).
- Torque Screwdriver: Insulated, 10-50 in-lbs range (e.g., CDI 401SMF).
- Multimeter: CAT III 600V minimum (e.g., Fluke 117).
- Fasteners: 1/4-inch nut driver for bus bar lugs, #2 Robertson (square) screwdriver for breakers.
Step-by-Step: How to Wire a Breaker Box
Follow these exact termination steps. Do not skip the torque verification; loose connections cause arcing and fires, while over-torqued connections snap the terminal screws.
- Prepare the Feed Cable: Strip 3/4-inch of insulation from the black, red, and white feed wires. Strip the bare/green ground wire to the exact length needed to reach the ground bar without excess looping.
- Land the Ground Wire (Bare/Green): Insert the bare copper or green insulated ground wire into the ground bus bar (the bar physically bolted directly to the metal panel enclosure). Tighten the set screw to 20 in-lbs. Note: In a subpanel, the ground bar is bonded to the box by default.
- Isolate the Neutral Bar: Locate the green bonding screw or bonding strap that connects the neutral bar to the panel enclosure. Remove it completely. The neutral bar must float freely, isolated from the metal box.
- Land the Neutral Wire (White): Insert the stripped white neutral wire into an open terminal on the isolated neutral bus bar. Tighten to 25 in-lbs (or the specific value printed on the panel label). Ensure no bare copper is exposed outside the terminal block.
- Land the Hot Wires (Black and Red): Insert the black wire into the 'A' phase main lug and the red wire into the 'B' phase main lug. If your subpanel uses a main breaker instead of main lugs, land them on the main breaker's line terminals. Tighten to the manufacturer's specified torque (typically 45-50 in-lbs for #2 AWG copper).
- Dress the Wires: Route the wires neatly along the wire gutters. Do not let them cross over the center knockout area where branch breakers will snap in.
Branch Circuit Decisions: Sizing Your First Outlets
Once the feeder is landed, you need to populate the panel with branch circuit breakers. Stop guessing which breaker and wire to use. Use this decision matrix to pick the exact parts for your most common loads.
| Load Type | Breaker Pick (Square D Homeline) | Wire Gauge & Colors | Termination Notes |
|---|---|---|---|
| Standard 120V Receptacles (Bedroom/Living) | 20A AFCI/GFCI Dual Function (HOM120CAFI) | 12 AWG Copper (Black, White, Bare) | Black to breaker, White to neutral bar, Bare to ground bar. |
| 240V Electric Dryer | 30A 2-Pole (HOM230) | 10 AWG Copper (Black, Red, White, Bare) | Black/Red to breaker, White to neutral bar, Bare to ground bar. |
| 240V Baseboard Heater (No Neutral) | 20A 2-Pole (HOM220) | 12 AWG Copper (Black, White re-identified) | Wrap black tape on both ends of the white wire. Land Black and White on breaker. Bare to ground. |
| 120V Dedicated Fridge Circuit | 20A Standard (HOM120) | 12 AWG Copper (Black, White, Bare) | Do NOT use AFCI/GFCI for fridges to prevent nuisance tripping on compressor startup. |
Verify and Test: Meter Readings Before Energizing Loads
Never blindly flip the main breaker and plug in a TV. You must verify the voltage geometry of your new panel. Set your multimeter to AC Voltage (V~) and take the following readings at the subpanel's main lugs and bus bars after the main panel's feed breaker is turned back on.
• Hot A to Hot B (Black to Red): 240V (Acceptable range: 228V - 252V)
• Hot A to Neutral (Black to White): 120V
• Hot B to Neutral (Red to White): 120V
• Neutral to Ground (White to Bare): 0V (Anything above 2V indicates a loose neutral or shared neutral fault upstream).
If you read 120V between Neutral and Ground, immediately shut off the feed breaker. You either have a crossed wire upstream, or you failed to isolate the neutral bar in this subpanel.
The Most Common Botch (and How to Spot It)
The single most frequent mistake DIYers make when learning how to wire a breaker box for a subpanel is leaving the neutral-to-ground bonding strap installed.
The Symptom: When you turn on a 120V load (like a vacuum cleaner), the upstream GFCI or AFCI breaker in the main panel trips immediately. Furthermore, you may feel a slight tingle when touching the metal subpanel enclosure or nearby grounded plumbing.
The Physics: In a subpanel, the neutral wire is the designated return path for 120V current. If the neutral bar is bonded to the ground bar (and the panel enclosure), the return current splits. Half flows back on the white neutral wire, and half flows back on the bare ground wire. An upstream GFCI detects this imbalance (current leaving on the hot but not fully returning on the neutral) and trips to protect you from a perceived ground fault.
The Fix: De-energize the panel, remove the green bonding screw or metal strap connecting the neutral bar to the enclosure, and ensure the white neutral wire lands only on the floating neutral bar. For more on the physics of separate building grounding, refer to Schneider Electric's technical guidance on subpanel bonding.
By strictly separating your neutral and ground paths, torquing your lugs to spec, and verifying your voltage geometry with a meter, your 100A subpanel will provide safe, code-compliant power for decades.
