Field guide: installing a subpanel, residential version (edition 4)

Plan the load and the feeder before you touch a knockout

Start with a load calc under NEC 220, Part III. Add up the fixed loads in the detached structure or the remodel area, apply the demand factors, and size the feeder to the result, not to the breaker you happen to have on the truck. A 60A feeder to a garage is almost never enough once the customer adds a mini-split and an EVSE.

Pick the panel location with 110.26 working space in mind: 36 inches depth, 30 inches width, 6.5 feet headroom, and a clear path to the panel. No shelving over it, no washer in front of it, no water heater leaking onto it. If the panel is in a bathroom or a clothes closet, move it. 240.24(D) and (E).

Confirm the main panel can actually host another breaker. Check the bus rating, the 120% rule if there is a solar interconnect, and whether the existing service has the capacity for the new subpanel load.

Feeder conductors, conduit, and the grounding rules that trip people up

Size feeder conductors per 215.2 and 310.16, then check the 83% dwelling feeder rule in 310.12 when the feeder serves the entire dwelling. For a typical 100A subpanel inside the same structure, 1 AWG copper or 1/0 aluminum SER works, but derate anything running through insulation per 310.15(C) and 334.80.

The rule that gets missed most: a subpanel is fed with four wires. Two hots, a neutral, and a separate equipment grounding conductor. The neutral and ground bars must be isolated. Remove the main bonding jumper. 250.24(A)(5) and 408.40.

If the old subpanel in the garage has a green screw through the neutral bar, you are looking at a parallel neutral path. Every metal raceway and every ground wire between the two panels is carrying return current. Fix it before you energize anything new.

• Four-wire feeder, always, for a separately located subpanel
• Neutral bar floating, isolated from the enclosure
• EGC landed on a ground bar that is bonded to the can
• Main bonding jumper removed and bagged with the paperwork

Detached structures and the grounding electrode system

Feeding a subpanel in a detached garage, shop, or barn triggers 250.32. You still run four wires, and you still install a grounding electrode system at the detached building: two ground rods 6 feet apart, or a Ufer if the slab is new, or a ground ring. Bond the EGC from the feeder to that electrode system at the subpanel.

Do not re-bond neutral to ground at the detached building. The 2008 NEC closed that loophole and it has not come back. If an inspector cites an older edition, verify the jurisdiction and then follow the current code anyway because you will be the one the insurance company calls.

Sleeve the ground rod connection if the panel is surface-mounted on a finished wall. Acorn clamps below grade are listed for direct burial, but they still corrode faster than anyone expects in wet soil.

Overcurrent protection, breakers, and panel listings

The feeder breaker at the main panel protects the feeder conductors. Size it to the conductor ampacity after all adjustments, not above it. 240.4. If the subpanel is rated 100A and you feed it with a 60A breaker, the panel is a 60A panel for load-calc purposes.

Use the breakers the panel is listed for. A CH panel takes CH breakers. A BR panel takes BR breakers. Classified breakers are legal under 110.3(B) only if the panel label explicitly allows them, and most do not. Inspectors will pull covers to check.

1. Verify panel listing on the inside of the door
2. Match breaker manufacturer and series
3. Torque every lug and breaker connection to the label spec
4. Record the torque values on the as-built if the AHJ asks

AFCI, GFCI, and the branch circuit trap

A subpanel does not get you out of 210.8 or 210.12. Every 15A and 20A, 120V branch circuit in a dwelling area covered by AFCI rules still needs AFCI protection, whether it lands in the main or the sub. Same for GFCI in garages, unfinished basements, outdoors, kitchens, and laundry areas.

If the subpanel feeds a detached garage with a single receptacle circuit and a lighting circuit, the receptacle circuit needs GFCI, and if the garage is part of a dwelling, the lighting circuit needs AFCI. Dual-function breakers at the subpanel are the cleanest path.

Label the panel directory with the actual room names, not "lights" and "plugs". The next electrician, or the homeowner at 2 a.m., will thank you. 408.4(A) requires it anyway.

Commissioning and handoff

Before you energize, megger the feeder if the run is long or fished through questionable spaces. Verify phase rotation is not a concern on single-phase residential, but do verify L1 and L2 are actually on opposite legs by measuring 240V across the two hots at the sub.

Test every AFCI and GFCI with the test button and with a plug-in tester. Check neutral-to-ground voltage under load: you want a few hundred millivolts or less. Anything above a volt or two means you have a neutral problem, often a missed isolation at the sub.

Close it up, take photos of the inside of both panels for your file, update the directory, and walk the customer through the main disconnect location. That last step is 230.70 in spirit and a callback-preventer in practice.