A 100 amp subpanel takes four feeder wires from a 2-pole 100 amp breaker, and keeps the neutral bar completely separate from ground.
Wiring a 100 amp subpanel comes down to four feeder wires and one rule that fails more inspections than anything else: once you leave the main panel, neutral and ground never connect again. The circuit is fed from a 2-pole 100 amp breaker in the main panel, and the subpanel’s neutral bar must stay isolated from the metal enclosure.
This guide covers minimum wire sizes, the wiring order, and the mistakes that get panels rejected. Local code and the panel’s own instructions are the final word, and because working inside a live enclosure can be fatal, many jurisdictions require a licensed electrician for this work.
What Wire Size Does a 100 Amp Subpanel Need?
The feeder needs four conductors: two hot wires, one neutral, and a separate equipment grounding conductor.
The two hot conductors land on the 2-pole 100 amp breaker in the main panel. NEC conductor sizing guidance for 100 amp feeders treats copper and aluminum separately, and the insulation rating matters as much as the gauge.
Distance changes the answer. Past roughly 100 feet, bump the feeder up a size or run a voltage-drop calculation, because a long code-minimum run can lose noticeable voltage under load.
| Feeder Detail | Requirement | Why It Matters |
|---|---|---|
| Conductors | 2 hots, 1 neutral, 1 ground | Ground must be a separate wire past the main panel |
| Copper minimum | #4 AWG, 75°C-rated | THHN/THWN-2 or XHHW-2 typical |
| Aluminum minimum | #2 AWG, 75°C-rated | Saves cost while meeting code |
| Over 100 feet | Upsize or calculate voltage drop | Long runs lose voltage under load |
| Underground rigid PVC | 24 in cover minimum | NEC Table 300.5 burial depth |
| EMT conduit | Support every 10 ft, within 3 ft of boxes | Keeps runs secure and to code |
Lug ratings, wiring diagrams, and torque values differ by model, so the panel label beats any generic guide. If you are still choosing the box, our roundup of the best 100 amp subpanels makes it easier to compare tested options before you commit.
100 Amp Subpanel Wiring: The Step Order That Works
The sequence that passes inspection is short, but every step has a consequence. Install in this order:
- Turn off the main breaker and verify zero voltage with a tester before opening any panel. Lock the breaker out if you can, so nobody restores power while you work.
- Mount the subpanel in a dry, accessible spot with the working clearance the code requires around the enclosure.
- Run the feeder from main to subpanel in conduit or another approved wiring method.
- Terminate the feeder in the main panel: the two hots go on the 2-pole 100 amp breaker, and the neutral and equipment grounding conductor land on their separate buses.
- Terminate the load end in the subpanel. The hots connect to the main lugs or the subpanel’s own main breaker, the neutral goes to the isolated neutral bar, and the equipment ground goes to the ground bar bonded to the enclosure.
- Remove the bonding screw or strap in the subpanel. It ships installed for main-panel use, and leaving it in bonds neutral to ground — the failure inspectors catch most.
- Tighten every termination to the manufacturer’s torque specification. After the inspector signs off, you can energize the feeder knowing the subpanel is wired correctly.
What Fails a 100 Amp Subpanel Inspection?
Inspectors reject subpanels for a short list of repeated problems, and fixing them is usually faster than explaining them. Watch these:
- Bonded neutral and ground. If the neutral bar is connected to the enclosure through a screw or strap, remove it. The neutral-ground bond belongs only at the service equipment.
- Feeder sized wrong for the run. #4 copper or #2 aluminum is the minimum for 100 amps, but long distances need upsizing for voltage drop.
- No separate equipment ground. The neutral wire can never double as the ground path; the feeder must include a dedicated grounding conductor.
- Skipped permit and inspection. Most areas require both before the circuit gets energized.
