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

Installing a breaker box—specifically a subpanel to extend circuits to a garage, workshop, or addition—requires precise wire sizing, strict adherence to color codes, and an absolute understanding of grounding physics. While replacing a main service entrance panel requires utility coordination and a licensed master electrician, mounting and wiring a downstream 100A subpanel is a standard procedure for advanced DIYers and trade students. The direct answer for a standard 100A feed is to use a 100A main-breaker subpanel (like the Square D QO2100NRB), feed it with 3 AWG copper or 1 AWG aluminum THHN wire, and terminate the neutral and ground on strictly separated bus bars.

Feeder Wire, Breaker, and Conduit Sizing Matrix

Before purchasing materials, you must match your overcurrent protection device (the breaker at the source panel) to the correct wire ampacity and equipment grounding conductor (EGC). The table below uses the 75°C column of NEC Table 310.16, which is the standard rating for most modern breakers and lugs. Sizing the ground wire is governed by NEC Table 250.122, not the 310.16 ampacity table.

Feeder Breaker Rating	Copper Hots & Neutral (THHN)	Aluminum Hots & Neutral (THHN)	Copper EGC (Ground)	Min. PVC Conduit Size (Sch 40)
60 Amp	6 AWG	4 AWG	10 AWG	1 inch
100 Amp	3 AWG	1 AWG	8 AWG	1.25 inch
125 Amp	1 AWG	1/0 AWG	6 AWG	1.5 inch
200 Amp	2/0 AWG	4/0 AWG	4 AWG	2 inch

Note: If running 4-wire SER (Service Entrance) cable inside a wall cavity instead of THHN in conduit, the cable must be sized by the 60°C column per NEC 334.80, requiring 2 AWG Copper or 1/0 AWG Aluminum for a 100A feed.

Tools, Materials, and Safety Protocol

Working inside a live main panel to install the feeder breaker carries a lethal arc-flash and shock hazard. If you are not comfortable working inches from exposed, uninsulated main bus bars, hire a licensed electrician for the source termination.

Required Tools and Materials

• Panel: Square D QO 100A 12-Space Main Breaker Subpanel (QO2100NRB) or Siemens P2S100B100CU.
• Feeder Breaker (Source): 100A 2-pole breaker matching your main panel brand (e.g., Square D QO2100, Homeline HOM2100, or Siemens Q2100).
• Wire: Four individual THHN/THWN-2 conductors (Black, Red, White, Green). 3 AWG for Hots/Neutral, 8 AWG for Ground (Copper).
• Conduit: 1.25-inch Schedule 40 PVC, sweeps, and fittings.
• Tools: Calibrated torque screwdriver (inch-pound scale), wire strippers for 3 AWG, fish tape, CAT III digital multimeter, NCVT, cordless drill, 1/4" nut driver.

Step-by-Step Termination and Wiring

Proper termination requires stripping exactly 3/4-inch of insulation and torquing the lugs to the manufacturer's specification (typically 40 to 50 in-lbs for 3 AWG copper). Loose connections cause high resistance, leading to thermal expansion, arcing, and melted bus bars.

1. Mount the Enclosure: Secure the subpanel to the studs using 1/4" x 2" lag screws. Ensure the center of the panel is mounted at a height where the operating handle of the highest breaker will not exceed 6 feet 7 inches from the floor (NEC 240.24).
2. Pull the Feeder Wires: Route your 1.25" PVC conduit from the source panel to the subpanel. Use fish tape to pull the Black, Red, White, and Green THHN wires simultaneously. Leave at least 24 inches of slack inside both panels to allow for neat dressing and future re-termination.
3. Terminate at the Source Main Panel:
   • Black (Hot 1): Terminates on one of the load lugs of the new 100A double-pole feeder breaker.
   • Red (Hot 2): Terminates on the second load lug of the 100A feeder breaker.
   • White (Neutral): Terminates on the main panel's silver neutral bus bar. (In a main service panel, neutral and ground are bonded, so either bar is acceptable, but the neutral bar is preferred for load tracking).
   • Green (Ground): Terminates on the main panel's green equipment grounding bus bar.
4. Terminate at the New Subpanel:
   • Black (Hot 1): Route to the subpanel's 100A main breaker. Terminate on Lug L1. Torque to 45 in-lbs.
   • Red (Hot 2): Route to the subpanel's 100A main breaker. Terminate on Lug L2. Torque to 45 in-lbs.
   • White (Neutral): Route to the isolated silver neutral bus bar. Terminate under a dedicated screw. Do not land this on the ground bar.
   • Green (Ground): Route to the green equipment grounding bus bar. Terminate under a dedicated screw. Ensure the green bonding strap or screw that connects this bar to the panel chassis is intact.
5. Dress the Wires: Use cable ties or adhesive-backed zip tie mounts to route the wires neatly along the wire gutters on the sides of the panel. Keep the white and green wires separated from the black and red wires to minimize inductive heating and make future troubleshooting obvious.

The Most Common Botch: Subpanel Bonding

The single most frequent and dangerous mistake when learning how to install a breaker box for a subpanel is failing to isolate the neutral bus bar from the panel chassis. Out of the box, many main-lug panels come with a green bonding strap or a green bonding screw that physically connects the neutral bar to the metal enclosure (and thus, to the ground bar).

The Symptom: If you leave this bonding strap in place in a subpanel, the neutral current will split and travel back to the source on both the white neutral wire and the green ground wire. This creates "objectionable current" on the grounding system. If the white neutral wire ever breaks or loosens at the source panel, the entire metal chassis of your subpanel, the conduit, and any plugged-in appliance frames will become energized at 120V, seeking a path back to the transformer through you or the earth.

The Fix: Per NEC Article 250.142 and 250.32, the neutral and ground must only be bonded at the main service disconnect. In your subpanel, you must physically remove the green bonding strap or unscrew the main bonding jumper before terminating any wires. The white neutral bar must "float" completely isolated from the metal panel box. The green ground bar, however, must remain firmly bolted to the metal chassis.

Verification and Expected Meter Readings

Never assume the wiring is correct just because the breakers snap into place. Before energizing the new subpanel, perform a dead-check. After energizing, perform a live-check with your CAT III multimeter.

Phase 1: Dead Verification (Power OFF)

1. Set your multimeter to Continuity/Ohms (Ω).
2. Place one probe on the subpanel's metal chassis and the other on the silver neutral bus bar. Expected Reading: OL (Open Line) or infinite resistance. If you read < 1 ohm, you failed to remove the bonding strap. Fix it immediately.
3. Place one probe on the chassis and the other on the green ground bus bar. Expected Reading: < 0.5 ohms (confirming the ground bar is bonded to the chassis).

Phase 2: Live Verification (Power ON)

Turn on the 100A feeder breaker at the main panel, then turn ON the 100A main breaker inside the new subpanel. Set your multimeter to AC Voltage (V~).

1. Line 1 to Neutral (Black to White): Expected reading is 120V (acceptable range 114V - 126V).
2. Line 2 to Neutral (Red to White): Expected reading is 120V.
3. Line 1 to Line 2 (Black to Red): Expected reading is 240V. This confirms you landed the hots on opposite phases.
4. Neutral to Ground (White to Green): Expected reading is < 1.0V (typically 0.2V to 0.5V due to normal voltage drop on the neutral wire under zero load). If you read 120V here, your neutral connection is open or failed at the source panel. Turn off the power immediately and check the source terminations.
5. Ground to Chassis: Expected reading is 0.0V.

Once these readings are confirmed, you can safely begin installing branch circuit breakers (like 20A AFCI/GFCI combo breakers for living spaces or standard QO breakers for dedicated 240V equipment) and routing your branch wiring to outlets and fixtures.