Wiring a 100-amp subpanel to expand your home's electrical capacity requires a 100-amp double-pole breaker in the main panel, 3 AWG copper wire for the two hot legs and the neutral, and an 8 AWG copper wire for the equipment ground. The black and red wires land on the main breaker's load lugs, the white wire terminates on the isolated neutral bus bar, and the green or bare wire lands on the ground bus bar, which must be physically bonded to the metal enclosure. Getting these terminations right—and keeping the neutral and ground isolated in the subpanel—is the difference between a safe, code-compliant installation and a dangerous shock hazard.
This guide walks through the exact materials, safety protocols, and termination sequence for feeding a 100-amp subpanel from an existing 200-amp main service. Note that while this covers the technical execution, local jurisdictions often require a licensed electrician to pull permits and perform service-side work. Always defer to your local Authority Having Jurisdiction (AHJ) for final code compliance.
Tools, Materials, and Feeder Sizing
Before pulling any wire, you need to verify your main panel has the physical space and the calculated load capacity to support a new 100-amp feeder. Assuming you have cleared a load calculation, gather the following specific materials. Wire sizing here is based on the 75°C column of NEC Table 310.16, which is the standard rating for most modern breakers and lugs.
Required Tools
- Insulated Screwdrivers & Nut Drivers: Rated for at least 1000V to protect against accidental contact.
- Calibrated Torque Screwdriver: Essential for terminating lugs to manufacturer specifications (typically measured in inch-pounds).
- Wire Strippers & Cable Cutter: Capable of cleanly stripping 3 AWG and 8 AWG wire without nicking the copper.
- Non-Contact Voltage (NCV) Tester & Digital Multimeter: Category III (CAT III) rated for verifying dead circuits and testing AC voltage.
Materials and Sizing Table
| Component | Specification / Rating | NEC Reference / Notes |
|---|---|---|
| Feeder Breaker (Main Panel) | 100-Amp, 2-Pole, 240V | Must match the subpanel's main lug rating. |
| Hot Conductors (2x) | 3 AWG Copper THHN/THWN-2 | Rated 100A at 75°C. Colors: Black, Red. |
| Neutral Conductor (1x) | 3 AWG Copper THHN/THWN-2 | Must be same gauge as hots. Color: White (or grey). |
| Equipment Ground (1x) | 8 AWG Copper THHN/THWN-2 | Per NEC 250.122 for a 100A overcurrent device. Color: Green or Bare. |
| Subpanel Enclosure | 100-Amp Main Lug, 12-24 Spaces | Must include a separate, isolated neutral bus bar. |
Mains Safety Protocol: De-Energize and Verify
Working inside a main breaker panel exposes you to lethal voltage. The utility feed entering the top of the main breaker remains energized even when the main breaker is turned off. Never touch the utility lugs or the bus stabs directly above the main breaker.
- De-energize: Switch off the main breaker to cut power to all branch circuits and bus bars.
- Lock/Tag: Apply a lockout/tagout device to the main breaker handle so no one accidentally turns it back on while you are working.
- Verify Dead: Use a CAT III NCV tester and a multimeter to verify zero voltage on the bus bars and the load side of the main breaker before touching any internal components.
Step-by-Step: Routing and Terminating the Feeder Wires
With the main panel verified dead, you can begin routing your conduit and pulling the four feeder wires (Black, Red, White, Green) from the main panel to the subpanel. Leave at least 3 feet of slack inside both enclosures to ensure you have enough length to reach the farthest termination points without splicing.
- Prepare the Subpanel Neutral and Ground Bars: Before terminating any wires in the subpanel, locate the green bonding screw or bonding jumper strap that connects the neutral bus bar to the metal enclosure. Remove it. In a subpanel, the neutral must be completely isolated from the ground. The ground bus bar, however, must remain bonded to the enclosure. (For more on the physics of this requirement, refer to Mike Holt's explanation of NEC 250.32 regarding separate buildings).
- Terminate the Hot Wires at the Main Panel: Strip 3/4 inch of insulation from the Black and Red 3 AWG wires. Insert the Black wire into Lug A (Line 1) of the new 100-amp double-pole breaker, and the Red wire into Lug B (Line 2). Tighten using your torque screwdriver to the value printed on the breaker label (typically 40-50 in-lbs for 3 AWG copper).
- Terminate the Neutral at the Main Panel: Strip the White 3 AWG neutral wire and land it on an available terminal on the main panel's neutral/ground bus bar. Because this is the main service disconnect, the neutral and ground bars are bonded here, so either bar is acceptable, but the neutral bar is preferred for load balancing.
- Terminate the Ground at the Main Panel: Strip the Green (or bare) 8 AWG ground wire and land it on the main panel's ground bus bar. Tighten to the manufacturer's torque specification.
- Terminate the Hots at the Subpanel: Route the Black and Red wires to the subpanel's main lugs (often labeled L1 and L2). Strip, insert, and torque to the subpanel manufacturer's specified inch-pounds.
- Terminate the Neutral at the Subpanel: Route the White wire exclusively to the isolated neutral bus bar (the one with the plastic insulators separating it from the metal enclosure). Do not land it on the ground bar.
- Terminate the Ground at the Subpanel: Route the Green (or bare) 8 AWG wire to the ground bus bar that is physically bolted directly to the subpanel's metal enclosure.
Verify and Test: Meter Readings Before Power-On
Never flip the breaker on without performing a pre-power continuity check. This step catches wiring errors that could result in a catastrophic short circuit or a bonded subpanel.
Pre-Power Continuity Test (Main Breaker OFF)
Set your digital multimeter to the Ohms (Ω) or continuity setting.
- Hot-to-Ground: Place one probe on the subpanel's L1 lug (Black wire) and the other on the ground bus. Expected reading: OL (Open Line) or infinite resistance. Repeat for L2 (Red wire). If you read near 0 ohms, you have a short circuit. Stop and trace the wire.
- Neutral-to-Ground (The Critical Test): Place one probe on the subpanel's isolated neutral bus and the other on the ground bus. Expected reading: OL (Open Line). If you read 0 ohms or hear a continuity beep, your neutral and ground are bonded. This means you forgot to remove the green bonding screw in the subpanel, or a white neutral wire is accidentally touching the metal enclosure somewhere in the conduit.
Post-Power Voltage Test (Main Breaker ON)
Remove lockout/tagout, ensure all panel covers are temporarily secured, and switch the 100-amp feeder breaker ON. Set your multimeter to AC Voltage (V~).
- L1 to Neutral (Black to White): Expected reading: 120V (acceptable range 114V - 126V).
- L2 to Neutral (Red to White): Expected reading: 120V.
- L1 to L2 (Black to Red): Expected reading: 240V.
- Neutral to Ground (White to Green/Bare): Expected reading: 0V to 2V. If you read 120V here, your neutral connection is loose or broken upstream, and the ground is carrying the return current. Turn off the breaker immediately and check terminations.
Frequently Asked Questions
What is the most common mistake when wiring a subpanel, and what are the symptoms?
The most common botch is failing to isolate the neutral bus bar from the ground bus bar in the subpanel (leaving the green bonding screw installed). In a main panel, neutral and ground are bonded so that fault current has a low-impedance path back to the utility transformer to trip the breaker. In a subpanel, they must be kept separate.
The Symptom: If bonded, normal neutral return current will split and flow back to the main panel on both the white neutral wire and the bare copper ground wire. This energizes the grounding system. You may experience nuisance tripping of upstream GFCI or AFCI breakers, a faint tingling shock when touching grounded appliances in the subpanel building, and in a severe fault, the ground wire could overheat and melt because it is carrying continuous load current it was not sized for.
Can I use aluminum SER cable instead of individual copper THHN wires?
Yes, and it is often more cost-effective for long runs. If you choose aluminum, you must upsize the wire to compensate for aluminum's lower conductivity. For a 100-amp feeder using the 75°C column, you will need 1 AWG Aluminum for the two hots and the neutral. The equipment grounding conductor must be 6 AWG Aluminum (or you can pull a separate 8 AWG copper ground). When terminating aluminum wire, you must apply an antioxidant compound (like Noalox) to the stripped wire before inserting it into the lug to prevent oxidation, which increases resistance and causes heat buildup over time.
Does a detached garage subpanel require a separate grounding rod?
Yes. According to NEC Article 250.32, if the subpanel is located in a separate building or structure (like a detached garage, barn, or workshop), you must install a local grounding electrode system. This typically means driving two 5/8-inch copper-clad ground rods, spaced at least 6 feet apart, and connecting them to the subpanel's ground bus bar using a minimum 6 AWG bare copper grounding electrode conductor. This local rod does not replace the 8 AWG equipment ground wire running back to the main panel; both are required to safely clear faults and equalize potential during lightning strikes or utility surges.
