Field guide: installing a subpanel, for apprentices (edition 1)

Plan the load before you plan the pipe

Before you pull a permit or cut a knockout, work the math. A subpanel exists to extend capacity, not to hide a shortage. Confirm the feeder ampacity against the calculated load per NEC Article 220, and size the panel with headroom. Apprentices get burned when they spec a 60A feeder for a shop that already pulls 48A on a dusty day.

Pull the main panel's directory, verify the available breaker space, and check the service calculation. If the existing service is near its limit, a subpanel does not solve the problem. You are just moving the bottleneck downstream.

• Calculate the load per NEC 220.40 through 220.61.
• Verify the feeder overcurrent device matches the conductor ampacity per NEC 240.4.
• Confirm the main panel can host the feeder breaker without violating the 120% rule in NEC 705.12(B) if solar is present.
• Size for the next ten years of loads, not today's.

Feeder conductors and the grounding story

This is where most apprentice installs fail inspection. A subpanel fed from the same structure requires four wires: two hots, a neutral, and a separate equipment grounding conductor. Neutrals and grounds bond at the service disconnect only, per NEC 250.24(A)(5). At the subpanel, the neutral bar floats and the ground bar bonds to the enclosure.

If the subpanel is in a separate structure, older code allowed a three-wire feeder with the neutral bonded at the remote panel. That exception was removed in the 2008 NEC. Today, NEC 250.32(B)(1) requires an equipment grounding conductor run with the feeder, and the neutral stays isolated.

Pull the green bonding screw out of the subpanel and tape it to the inside of the door. Inspectors check for that screw first, and so should you.

Conductor sizing, and the 83% rule that trips people up

Feeder ampacity comes from NEC Table 310.16 after applying any adjustment or correction factors. For a typical 100A subpanel fed from a 100A breaker, #3 copper THHN at the 75C column is the standard answer. Aluminum requires #1.

The 83% rule in NEC 310.12 applies only to the single main feeder supplying a dwelling unit's entire load, not every subpanel in the house. Apprentices misapply this constantly. A subpanel in a detached garage is not a dwelling service, so use the full Table 310.16 ampacity. Do not shrink the conductor because you read a trick online.

• 100A feeder: #3 Cu or #1 Al at 75C.
• 60A feeder: #6 Cu or #4 Al at 75C.
• Equipment grounding conductor sized per NEC Table 250.122.
• Neutral sized for the maximum unbalanced load per NEC 220.61.

Mounting, working space, and the stuff inspectors actually look at

Working clearance is not negotiable. NEC 110.26(A) requires 36 inches of depth in front of the panel, 30 inches of width or the width of the equipment (whichever is greater), and 6.5 feet of headroom. A subpanel tucked behind a water heater or inside a closet with shelving will fail, even if the wiring is perfect.

Mount the enclosure plumb, flush with the finished wall surface in combustible construction per NEC 312.3, or within 1/4 inch in noncombustible. Use proper connectors at every knockout. A plastic bushing on a metallic raceway into the panel is required by NEC 300.4(G) when conductors are 4 AWG and larger.

If you cannot open the deadfront all the way without hitting a stud, a pipe, or your own knee, the panel is in the wrong place. Move it now, not after the drywall goes up.

Breaker installation and circuit labeling

Use breakers listed for the panel. A Square D breaker in a Siemens panel is not a judgment call, it is a NEC 110.3(B) violation and an inspector will fail it on sight. Check the panel label for approved breaker types and stick to the list.

Torque every lug to the manufacturer's spec. NEC 110.14(D) now requires a calibrated torque tool. Feeder lugs are typically 250 to 275 inch-pounds for a 100A panel, but always read the sticker inside the door. Loose lugs cause the fires that make the news.

1. Land the feeder with the panel de-energized and locked out.
2. Torque the feeder lugs and branch breaker terminals to spec.
3. Verify the neutral bar is isolated from the enclosure.
4. Install AFCI and GFCI protection per NEC 210.8 and 210.12 for covered circuits.
5. Fill the directory with clear, specific circuit descriptions. "Bedroom" is not a description. "North bedroom receptacles and lights" is.

Before you close the cover

Energize the feeder, meter the voltage leg to leg and leg to neutral, and confirm 240V and 120V are where they should be. Check for voltage between the neutral and ground bar. You should read zero. Anything else means the neutral is bonded somewhere it should not be.

Walk every branch circuit and verify polarity, grounding, and GFCI trip function. Photograph the inside of the panel with the directory visible before you put the cover on. That photo will save you on a callback six months from now when the homeowner wants to add a circuit and cannot remember which breaker feeds the freezer.