To wire a standard 100-amp residential subpanel, use a 4-wire feeder consisting of two hot conductors (2 AWG copper or 1/0 AWG aluminum), one neutral (same size), and one equipment grounding conductor (8 AWG copper or 6 AWG aluminum). Land the black and red hots on a 100A double-pole breaker in the main panel, the white neutral on the isolated neutral bar, and the green/bare ground on the grounding bar. In the subpanel, keep the neutral and ground bars strictly separated.
Tools, Materials, and Feeder Sizing Spec Sheet
Before pulling any wire, verify your materials against the load calculation. A 100-amp subpanel is typically fed by a 100A double-pole breaker. The National Electrical Code (NEC) requires a 4-wire feed for any subpanel in a separate building or structure, and modern best practice mandates 4 wires for all interior subpanels to prevent neutral current from traveling on grounding paths.
Required Tools:
- Non-contact voltage tester (NCVT) and CAT III digital multimeter
- Insulated screwdrivers (e.g., Wiha or Klein Tools 1000V rated)
- Calibrated torque screwdriver and torque wrench (inch-pound rated)
- Wire strippers and cable cutters rated for 2 AWG / 1/0 AWG
- Fish tape and wire pulling lubricant (for conduit runs)
100-Amp Subpanel Feeder & Termination Specifications
| Conductor Function | Copper Size (THHN) | Aluminum Size (XHHW-2) | Insulation Color | Typical Torque Spec |
|---|---|---|---|---|
| Hot 1 (Line A) | 2 AWG | 1/0 AWG | Black | 250 in-lbs (verify label) |
| Hot 2 (Line B) | 2 AWG | 1/0 AWG | Red | 250 in-lbs (verify label) |
| Neutral (Grounded) | 2 AWG | 1/0 AWG | White | 250 in-lbs (verify label) |
| Equipment Ground | 8 AWG | 6 AWG | Green / Bare | 35 in-lbs (for 8 AWG) |
| Main Feed Breaker | 100A 2-Pole | 100A 2-Pole | N/A | 40 in-lbs (breaker lugs) |
Note: Always check the manufacturer’s torque label inside the panel door or on the breaker. The values above are typical for Eaton and Square D residential load centers, but the manufacturer's printed spec always overrides general tables. For conduit sizing, three 2 AWG and one 8 AWG THHN conductors require a minimum 1.25-inch PVC or EMT conduit to meet NEC Chapter 9 fill capacity limits.
Critical Safety Protocols Before Opening the Main Panel
Working inside an energized main panel exposes you to lethal 240V and 120V potentials. Arc flash and electrocution risks are severe. You must de-energize the main panel before removing the dead front cover.
- Turn off the main service disconnect breaker.
- Apply a lockout/tagout (LOTO) device to the main breaker if possible, or tape it in the OFF position and post a warning sign.
- Remove the panel dead front cover carefully, keeping your body clear of the bus bars.
- Verify Dead: Use a tested NCVT and a CAT III digital multimeter to confirm 0V across the main breaker load terminals and between the neutral bus and ground bus. Never assume a panel is dead just because the main breaker is off; utility backfeeds or solar tie-ins can energize bus bars from the bottom.
This guide provides NEC-style guidance for educational purposes. Your local Authority Having Jurisdiction (AHJ) or a licensed electrician has final authority on permitting and code compliance.
Step-by-Step: How to Wire the Electric Panel
With the main panel verified dead, proceed with the physical termination. Always land grounding conductors first, then neutrals, and finally the hot conductors.
- Route and Strip the Feeder: Pull the four feeder wires through the conduit into both the main panel and the subpanel. Strip exactly 3/4-inch to 1-inch of insulation from the ends of the black, red, and white wires, matching the depth of the breaker and bus bar lugs. Strip 1/2-inch from the green (or bare) ground wire.
- Land the Grounds (Both Panels): In the main panel, terminate the green/bare ground wire to an available terminal on the main grounding bar (usually bonded directly to the panel enclosure). Tighten to 35 in-lbs. In the subpanel, terminate the other end of the green/bare wire to the subpanel's dedicated grounding bar. Ensure the green bonding screw or strap is completely removed from the subpanel's neutral bar.
- Land the Neutrals (Both Panels): In the main panel, terminate the white neutral wire to an available terminal on the main neutral/ground bar. In the subpanel, terminate the other end of the white wire to the isolated neutral bar. The neutral bar in a subpanel must float on plastic standoffs and have no physical connection to the metal enclosure.
- Terminate the Hots in the Main Panel: Install the 100A double-pole breaker onto the main panel bus bars. Route the black hot wire to one breaker lug and the red hot wire to the other breaker lug. Ensure no bare copper is exposed outside the lug and that the wire insulation is not pinched inside the lug. Torque both breaker screws to the manufacturer's spec (typically 40 in-lbs for standard 100A plug-on breakers, or up to 250 in-lbs if using a sub-feed lug kit).
- Terminate the Hots in the Subpanel: Route the black and red hot wires into the main lugs of the subpanel's main breaker (if equipped) or the sub-feed lugs. Torque these heavy lugs to the spec printed on the subpanel label (typically 250 in-lbs for 2 AWG copper). Use a calibrated torque wrench; hand-tightening is a code violation and a fire hazard.
- Install Branch Circuit Grounding Bond: If the subpanel is in a separate building, you must drive two 5/8-inch copper ground rods spaced at least 6 feet apart and connect them to the subpanel's grounding bar using a continuous 6 AWG bare copper grounding electrode conductor.
The Most Common Botch: Bonded Neutrals and Undertorqued Lugs
When inspectors fail a subpanel installation, it almost always comes down to two specific errors. Understanding these failure modes will save you a rework trip.
Botch 1: Leaving the Green Bonding Screw in the Subpanel
Every new subpanel ships with a green bonding screw or copper strap that connects the neutral bar directly to the metal panel enclosure. This is required for a main panel, but it is strictly forbidden in a subpanel.
The Symptom: If left in place, neutral return current will split and travel back to the main panel on both the white neutral wire and the green/bare ground wire. This energizes the grounding system, creating a shock hazard on appliance chassis and causing nuisance tripping of upstream GFCI breakers due to current imbalance.
Botch 2: Undertorquing Feeder Lugs
Many DIYers tighten 2 AWG lugs with a standard screwdriver until they feel "tight enough." Aluminum and copper expand and contract at different rates under load.
The Symptom: Over a few months of heavy loading (like running a welder or EV charger), the thermal cycling causes the undertorqued connection to loosen. This increases contact resistance, leading to localized heating, arcing, and eventually a melted bus bar or a panel fire. The 2017 NEC update (Article 110.14(D)) explicitly mandates the use of a torque tool for all terminations. As noted by the NFPA, proper torquing is non-negotiable for preventing thermal failures.
Verify and Test: Expected Meter Readings
Do not energize the panel until you have visually inspected every termination. Tug gently on each wire to ensure it is mechanically locked in the lug. Once verified, remove all LOTO devices, ensure the subpanel main breaker is OFF, and turn the main service disconnect back ON. Turn the new 100A feeder breaker ON.
Set your digital multimeter to AC Voltage (600V range) and take the following measurements at the subpanel:
- Line 1 to Neutral (Black to White): Expected reading is 120V (acceptable range 114V - 126V).
- Line 2 to Neutral (Red to White): Expected reading is 120V (acceptable range 114V - 126V).
- Line 1 to Line 2 (Black to Red): Expected reading is 240V (acceptable range 228V - 252V).
- Neutral to Ground (White to Green/Bare): Expected reading is 0V to 2V. If you read 120V here, you have an open neutral or a cross-wired connection. Shut down immediately.
- Line 1 to Ground (Black to Green/Bare): Expected reading is 120V.
- Line 2 to Ground (Red to Green/Bare): Expected reading is 120V.
If all readings match expectations, turn off the 100A feeder breaker, install the subpanel dead front cover, and restore power. For further details on grounding and bonding electrode requirements for detached structures, consult Mike Holt's NEC grounding resources or your local building department. Always ensure your load center and breakers are from the same manufacturer (e.g., Eaton BR with Eaton BR breakers) to maintain the UL listing and warranty, as detailed in Eaton's load center specifications.
