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

The Direct Answer: Sizing and Safety for a 100A Breaker Box

To install a 100-amp circuit breaker box (configured as a subpanel fed from a main service), you need a 100A double-pole breaker in your main panel, 1/0-1/0-1/0-2 AWG Aluminum SER cable (or 3 AWG Copper), and a 100A main-lug load center. The black and red wires land on the hot breaker lugs, the white wire lands on the isolated neutral bar, and the bare/green wire lands on the equipment ground bar. In the new subpanel, the neutral and ground bars must remain physically separated.

Tools, Materials, and Wire Sizing Matrix

Undersizing feeder wire or using the wrong insulation type is a primary cause of voltage drop and terminal fires. The National Electrical Code (NEC) requires sizing based on the 75°C column for standard residential terminations. Below is the exact material list for a 100A subpanel installation, followed by a sizing matrix for other common loads.

Required Tools and Materials

• Subpanel: Square D QO or Homeline 100A, 12-space or 24-space main-lug load center.
• Feeder Breaker: 100A 2-pole breaker matching your main panel's brand and bus bar type.
• Feeder Cable: 1/0-1/0-1/0-2 AWG Aluminum USE-2/RHH/RHW-2 SER cable (rated 75°C/90°C).
• Torque Screwdriver: Calibrated inch-pound torque driver (e.g., CDI 40-200 in-lb) with 1/4-inch hex adapter.
• Wire Prep: Klein Tools 1/0 AWG wire strippers, anti-oxidant compound (Noalox) for aluminum terminations.
• Hardware: 3/4-inch or 1-inch cable clamps, 1/2-inch knockout seals, 10-32 green grounding screw (if not pre-installed).

Feeder Wire Sizing Matrix (75°C Column)

Breaker Size	Copper Wire (THHN/THWN)	Aluminum Wire (SER/USE-2)	Grounding Conductor (Cu)
100 Amp	#3 AWG	#1/0 AWG	#6 AWG (or #2 Al in SER)
60 Amp	#6 AWG	#4 AWG	#10 AWG
50 Amp	#8 AWG	#6 AWG	#10 AWG

Source: NFPA 70 National Electrical Code Table 310.16. Always verify local amendments.

Step-by-Step Installation: Feeding the New Breaker Box

Follow these numbered steps to route and terminate the feeder cable. We are assuming a 100A feed using 1/0 AWG Aluminum SER cable.

1. Mount the Subpanel Enclosure: Secure the empty subpanel box to the wall studs using 2.5-inch structural screws. Ensure it is plumb and positioned so the center of the box is roughly 54 to 60 inches from the finished floor (NEC 404.8 accessibility requirements).
2. Prepare the SER Cable: Strip back the outer black jacket of the SER cable by exactly 8 inches from the point it enters the knockout. Do not nick the inner insulation of the black, red, white, or bare wires. Apply a pea-sized amount of Noalox anti-oxidant paste to the stripped ends of the aluminum black, red, and white conductors to prevent galvanic corrosion at the brass/screw terminals.
3. Terminate at the Main Panel (Breaker End):
   • Land the black wire into one lug of the 100A double-pole breaker.
   • Land the red wire into the other lug of the 100A double-pole breaker.
   • Land the white (neutral) wire into an available silver-plated terminal on the main panel's neutral/ground bar.
   • Land the bare (ground) wire into an available terminal on the main panel's ground bar (or the combined neutral/ground bar if it is the main service disconnect).
   • Torque Check: Use your torque screwdriver to tighten the breaker lugs to the exact spec printed on the breaker label (typically 40 to 50 in-lbs for 1/0 Al wire).
4. Terminate at the Subpanel (Main Lug End):
   • Route the cable into the subpanel through a 1-inch cable clamp. Tighten the clamp locknut securely.
   • Land the black wire into the top-left main lug (Phase A).
   • Land the red wire into the top-right main lug (Phase B).
   • Land the white wire into the isolated silver neutral bar. Critical: Ensure the green bonding screw or bonding strap is REMOVED from this neutral bar.
   • Land the bare wire into the green equipment ground bar. If the panel did not come with a separate ground bar, install an accessory ground bar kit (e.g., Square D PK7GBTA) directly to the metal enclosure using the pre-tapped 10-32 hole.
   • Torque Check: Torque all main lugs and bar terminals to the manufacturer's specification (usually 40 in-lbs for 1/0 Al).

Verify and Test: Meter Readings and Torque Checks

Never assume a termination is correct just because the wire is seated. Perform this verification sequence before installing the panel cover and energizing the branch circuits.

1. Visual and Physical Inspection: Tug firmly on every terminated black, red, white, and bare wire. If a wire pulls out, the torque was insufficient or the wire was stripped too short. Re-terminate immediately.
2. Bonding Verification: Use your multimeter on the continuity (ohms) setting. Place one probe on the subpanel's neutral bar and the other on the ground bar. The meter should read 'OL' (Open Loop / Infinite resistance). If it reads near 0 ohms, you forgot to remove the bonding screw. Fix this before applying power.
3. Energize and Measure: Turn on the main service, then flip the new 100A feeder breaker to ON. Set your multimeter to AC Voltage.
   • Probe black to white: Expect 120V (acceptable range 114V–126V).
   • Probe red to white: Expect 120V.
   • Probe black to red: Expect 240V.
   • Probe white (neutral) to bare (ground): Expect 0V to 2V. A reading higher than 5V under no-load indicates a loose neutral termination at the main panel.

The Most Common Botch: Subpanel Bonding Errors

The single most frequent and dangerous mistake when learning how to install a circuit breaker box as a subpanel is failing to isolate the neutral from the ground.

In a main service panel, neutral and ground are bonded together. In a subpanel, they must be kept strictly separate. If you leave the green bonding screw installed in the subpanel, you create a parallel path for neutral return current to flow back to the main panel through the bare ground wire, the panel enclosures, and any metallic conduit.

The Symptom: You will experience a tingling shock when touching grounded appliances (like a table saw or refrigerator) in the subpanel building. Furthermore, the white neutral wire and the bare ground wire will share the return load. Because the ground wire in SER cable is often sized smaller than the neutral (e.g., #2 AWG ground vs #1/0 AWG neutral), the ground wire can silently overheat and melt its insulation inside the wall cavity without ever tripping the 100A breaker.

For a deeper technical breakdown of parallel neutral paths, refer to Penn State Extension's electrical wiring safety resources.

Frequently Asked Questions

Can I install a main circuit breaker box myself?

Legally and practically, replacing a main service circuit breaker box (the one the utility meter feeds directly into) is almost universally restricted to licensed electricians. You must coordinate a utility disconnect, pull municipal permits, and pass an AHJ inspection. Furthermore, working upstream of the main breaker exposes you to unprotected utility fault currents capable of causing arc flash explosions. However, installing a subpanel (fed from an existing main breaker) is a common advanced DIY task, provided you follow NEC feeder sizing and bonding rules.

What size wire do I need for a 60-amp circuit breaker box?

For a 60-amp subpanel feeder, you must use #6 AWG Copper or #4 AWG Aluminum wire, rated at 75°C. The grounding conductor must be a minimum of #10 AWG Copper. If you are running individual THHN wires in PVC conduit, you will pull four wires: two hots, one neutral, and one ground. If using jacketed SER cable, ensure the cable jacket is marked with the correct AWG sizes for all four internal conductors.

Does a detached garage breaker box need a ground rod?

Yes. Under NEC Article 250.32, any circuit breaker box installed in a detached structure (like a garage, barn, or workshop) requires a local grounding electrode system. This typically means driving two 5/8-inch copper-clad ground rods at least 6 feet apart and connecting them to the subpanel's ground bar using a continuous #6 AWG bare copper wire. This protects the structure from lightning strikes and static buildup, completely separate from the equipment ground wire running back to the main house.

How much does it cost to install a circuit breaker box?

If you are hiring an electrician to install a 100A subpanel in an attached garage (within 30 feet of the main panel), expect to pay between $800 and $1,500 in 2026, factoring in labor, the 1/0 SER cable, and the panel itself. For a detached structure requiring trenching, PVC conduit, and ground rods, costs typically range from $2,500 to $4,500. DIY material costs for a basic 100A attached subpanel hover around $350 to $500, assuming you already own a torque screwdriver and heavy-duty wire strippers.