How to Wire a Circuit Breaker Box: Subpanel Feeder & Installation Guide

When homeowners and apprentices search for how to wire a circuit breaker box, they are almost always looking to install a subpanel to expand their electrical capacity. Wiring a main service entrance panel involves the utility drop, the meter base, and the service grounding electrode system—work that legally requires a licensed electrician and utility coordination. However, wiring a subpanel (a secondary breaker box fed from your main panel) is a standard procedure for competent DIYers and trade students, provided you pull a permit and follow NEC-style guidance.

The direct answer: For a standard 100-amp subpanel fed up to 100 feet from your main panel, use a 4-wire feeder consisting of 2 AWG copper THHN in conduit (or 1/0 AWG aluminum). Land the black and red wires on the hot lugs, the white wire on the isolated neutral bar, and the green/bare wire on the equipment grounding bar. You must remove the neutral-to-ground bonding screw in the subpanel.

Tools, Materials, and Feeder Sizing

Sizing your feeder wire correctly prevents voltage drop and prevents the wire insulation from melting under load. According to NEC Table 310.16, you must size breakers and wire based on the 75°C temperature column for standard residential terminations, even if the wire insulation is rated for 90°C.

Feeder Sizing Chart (75°C Column, Max 3% Voltage Drop at 100 ft)

Subpanel Rating	Copper THHN AWG	Aluminum THHN AWG	Conductor Count	Equipment Ground (Copper)
60 Amp	6 AWG	4 AWG	4-Wire	10 AWG
100 Amp	2 AWG	1/0 AWG	4-Wire	8 AWG
125 Amp	1/0 AWG	2/0 AWG	4-Wire	6 AWG
200 Amp	2/0 AWG	4/0 AWG	4-Wire	4 AWG

Required Tools & Materials

• Subpanel: Square D Homeline 100-Amp 12-Space 24-Circuit Indoor Main Lug Load Center (Model: HOM1224L125PGC).
• Feeder Breaker (Main Panel): Square D 100-Amp 2-Pole breaker (Model: HOM2100).
• Feeder Wire: 4 individual THHN/THWN-2 conductors (Black, Red, White, Green) pulled through 1.25-inch PVC or EMT conduit.
• Torque Screwdriver: Klein Tools 1412 or equivalent (NEC 110.14(D) requires calibrated torque for terminations).
• Multimeter: Fluke 117 True-RMS or equivalent CAT III/IV meter.
• Wire Strippers: Klein 11063 (for 2 AWG) or a specialized cable skinning knife for larger aluminum.

Step-by-Step: Wiring the Breaker Box

Since the 2008 NEC revision, all subpanels require a 4-wire feeder. The neutral and ground must remain strictly separated downstream of the main service disconnect. Follow these steps to terminate your conductors correctly.

1. Mount the Enclosure and Route Conduit: Secure the subpanel to the studs using 1/4-inch lag screws. Run your 1.25-inch conduit from the main panel knockout to the subpanel knockout, securing it with compression fittings and pulling the four THHN wires (Black, Red, White, Green) through using wire pulling lubricant.
2. Terminate at the Main Panel (Source):
   • Land the Black wire on one pole of the new 100A 2-pole breaker.
   • Land the Red wire on the second pole of the 100A 2-pole breaker.
   • Land the White (neutral) wire on the main panel's neutral bus bar.
   • Land the Green (ground) wire on the main panel's equipment grounding bus bar.
3. Prepare the Subpanel (Crucial Step): Locate the green bonding screw or bonding strap inside the subpanel. Remove it completely. In a subpanel, the neutral bar must float (be isolated from the metal enclosure). The ground bar is bonded to the enclosure by default via the mounting screws.
4. Terminate the Hots at the Subpanel: Strip 3/4-inch of insulation from the Black and Red wires. Insert the Black wire into the top main lug and the Red wire into the bottom main lug. Using your torque screwdriver, tighten the lug set-screws to the exact value printed on the panel label (typically 45 in-lbs for 2 AWG copper on Square D Homeline panels).
5. Terminate the Neutral at the Subpanel: Strip 3/4-inch of insulation from the White wire. Land it on an empty terminal on the isolated neutral bus bar. Torque to the manufacturer's specification (usually 20-25 in-lbs for the smaller bar screws).
6. Terminate the Ground at the Subpanel: Strip 3/4-inch of insulation from the Green wire. Land it on an empty terminal on the equipment grounding bus bar (the bar directly bonded to the metal panel chassis). Torque to spec.
7. Dress the Wires: Use cable ties to neatly route the wires around the perimeter of the panel, keeping the hot wires on the outside bends and the neutral/ground wires tucked behind the hot wires to prevent interference and ensure easy access to branch circuit spaces.

Verify and Test: Meter Readings & Common Botches

Never assume the wiring is correct just because the breakers fit. You must verify the electrical characteristics before plugging in any loads.

The Testing Sequence

1. Ensure all branch circuit breakers in the new subpanel are in the OFF position.
2. Turn ON the 100A feeder breaker in the main panel.
3. Set your multimeter to AC Voltage (V~).
4. Measure Hot-to-Hot (Black lug to Red lug): Expected reading is 240V (acceptable range 228V–252V).
5. Measure Hot-to-Neutral (Black lug to White neutral bar): Expected reading is 120V.
6. Measure Hot-to-Ground (Black lug to Green ground bar): Expected reading is 120V.
7. Measure Neutral-to-Ground (White neutral bar to Green ground bar): Expected reading is 0V to 2V. If you read 120V here, your feeder is broken or miswired upstream.

The Most Common Botch: Leaving the Bonding Screw In

The single most frequent mistake when learning how to wire a circuit breaker box for a subpanel is failing to remove the neutral-to-ground bonding screw.

The Symptom: If the bonding screw is left in, the neutral current will split and travel back to the main panel on both the white neutral wire and the green ground wire. This creates 'objectionable neutral current' on the grounding path. You may notice a tingling shock when touching grounded appliances in the outbuilding, and GFCI breakers upstream may nuisance-trip because the current returning on the ground wire creates an imbalance that the GFCI interprets as a ground fault.

The Fix: De-energize the feeder, remove the panel cover, unscrew and discard the green bonding screw, and re-test.

Frequently Asked Questions

How to wire a circuit breaker box in a detached garage?

Wiring a breaker box in a detached garage follows the exact same 4-wire feeder rules as an attached subpanel, with one major addition: NEC Article 250.32 requires a separate Grounding Electrode System for the detached building. You must drive two 5/8-inch copper ground rods at least 6 feet apart outside the garage, connect them together with a continuous 6 AWG bare copper wire, and land that wire on the subpanel's equipment grounding bar. Do not bond this ground rod wire to the neutral bar.

How to wire a circuit breaker box with a main breaker as a subpanel?

You can absolutely use a panel with a main breaker as a subpanel; it simply acts as a local disconnect switch. Feed the wires into the main breaker's lugs instead of main feed lugs. The critical rule remains identical: you must remove the neutral-to-ground bonding screw or strap. The presence of a main breaker handle does not change the physics of the neutral-ground bond. If you cannot find the pull-out bonding strap, consult the manufacturer's diagram on the inside of the panel door to locate the specific bonding screw.

What size wire to use when you wire a circuit breaker box for a 60-amp load?

For a 60-amp subpanel feeder, use 6 AWG copper THHN or 4 AWG aluminum THHN. You must still pull four individual wires (Black, Red, White, Green) through conduit. If you are using Aluminum SER (Service Entrance Round) cable instead of THHN in conduit, local codes may permit 4-4-4-6 Aluminum SER for a 60A feed, but THHN in conduit is the universal standard for wet/dry locations and offers better heat dissipation. Always verify the termination torque specs for 6 AWG, which typically require 25 to 30 in-lbs.