Field guide: installing a subpanel, for apprentices (edition 4)

Plan the Load and Feeder First

Before you cut a single knockout, size the feeder to the calculated load, not the panel rating. Run the calc per NEC Article 220, apply demand factors, and then pick conductor size from 310.16 with the correct termination temperature column (75°C for most modern lugs per 110.14(C)). A 100A subpanel fed with #4 copper is fine at 75°C; aluminum needs #2.

Voltage drop kills more subpanel installs than ampacity does. For runs over 100 feet, do the math. NEC 210.19 Informational Note recommends 3% on branch circuits and 5% total, and the AHJ will ask if the run looks long.

Confirm the main panel has capacity. If the service is maxed on a load calc, a subpanel does not create new capacity, it just relocates it. Check the main breaker, the bus rating, and the available spaces before you promise anyone anything.

Four Wires, Always, on a Separate Structure or Interior Sub

Since the 2008 NEC, feeders to subpanels require four wires: two hots, a neutral, and a separate equipment grounding conductor. The neutral and ground are isolated at the subpanel. That means the bonding screw comes out, and the neutral bar floats on its insulated standoffs.

This trips up apprentices every time. The main panel is the only place neutral and ground are bonded together, per 250.24(A)(5) and 250.142. Anywhere downstream, they stay separate or you create parallel neutral paths through the EGC, which will heat up metal raceways and trip AFCIs for no obvious reason.

If you find a subpanel humming or a GFCI tripping when nothing is loaded, the first thing to check is whether some previous hack left the bonding screw in. I have pulled that screw out of 20 year old panels and watched the problem vanish.

Grounding Electrodes at Detached Structures

Feeding a detached garage, barn, or shed triggers 250.32. You need a grounding electrode system at the second structure: ground rods, a Ufer if the slab is new, or a metal underground water pipe if one exists. Two rods driven 6 feet apart unless you can prove 25 ohms or less with one, which you cannot without a clamp meter most days.

The EGC runs with the feeder from the main building. Do not bond neutral to ground at the detached structure. The old "three wire feeder with bond" method is gone for new installs, though you may still see it on existing work that was legal under older code.

• Two 8 foot ground rods, 6 feet minimum spacing, per 250.53(A)(3).
• #6 copper to the rods, irreversible connection or listed clamp rated for direct burial per 250.70.
• Bond to the subpanel ground bar, not the neutral.
• If there is metal water or gas piping at the detached building, bond it per 250.104.

Working Space, Mounting, and Physical Protection

NEC 110.26 is not negotiable. 36 inches deep, 30 inches wide or the panel width if wider, and 6.5 feet of headroom. No storage, no shelving, no water heater in the dedication zone above the panel up to the structural ceiling per 110.26(E).

Mount the panel so the highest breaker handle is no higher than 6 feet 7 inches above the floor, per 240.24(A). In a garage or unfinished basement, the feeder needs physical protection where subject to damage: NM cable in exposed stud bays below 7 feet needs running boards or conduit per 334.15.

Breaker Sizing, AFCI, GFCI, and the Common Gotchas

The subpanel main breaker or main lug rating must match or exceed the calculated load, and the feeder breaker at the main panel must protect the feeder conductors. A 100A subpanel fed through a 60A breaker is legal, the subpanel is just limited to 60A of load. Apprentices often oversize the subpanel breaker assuming bigger is better. It is not. The breaker protects the wire.

AFCI and GFCI requirements follow the circuit, not the panel. Dwelling unit bedroom, living room, kitchen, and laundry circuits need AFCI per 210.12. Kitchens, bathrooms, garages, outdoors, basements, laundry areas, and within 6 feet of a sink need GFCI per 210.8(A). A detached garage subpanel feeding receptacles means almost every 15 and 20A circuit is GFCI.

Buy the dual function breakers once. Yes they cost more. No, you do not want to come back and swap them after the inspector fails you for a missing AFCI on the laundry.

1. Torque every lug to the label value. Use a calibrated torque screwdriver, not feel. 110.14(D) requires it.
2. Label every breaker clearly and specifically per 408.4. "Lights" is not a description.
3. Anti-oxidant on aluminum feeder terminations. Brush the strands, apply the compound, torque, done.
4. Keep the neutral bar and ground bar clearly separated and labeled in your own head before the inspector arrives.

Before You Call for Inspection

Walk the install cold. Pull the dead front, verify the bonding screw is removed, confirm neutrals and grounds are on separate bars, check torque on every lug including the feeder and the main bonding jumper at the service. Measure voltage line to line and line to neutral. Trip test every GFCI and AFCI at the panel and at the first device.

Take photos of the grounding electrode connections before anything gets buried or covered. Inspectors will ask, and a photo on your phone beats digging up a rod. Keep your load calc and conductor sizing notes with the permit. When the inspector asks why you ran #2 aluminum, you hand them the math and the conversation ends.