Field guide: installing a subpanel, after the job (edition 4)

Walking it before you energize

Before the main breaker goes back on, walk the subpanel like you're the inspector. Every landed conductor, every knockout, every label. Field experience says 90% of callbacks trace back to something you could have caught in a five-minute walk-around.

Check torque on every lug. Yes, every one. Aluminum feeders creep, and a lug that felt tight at rough-in can loosen after the bus heats and cools a few times. NEC 110.14(D) requires torque to manufacturer's spec, and inspectors are increasingly asking to see the wrench.

Verify the neutral and ground are separated. If this is a subpanel fed from a main, the bonding screw or strap must be removed or left out. NEC 250.24(A)(5) and 408.40 are clear: only one neutral-to-ground bond per system, at the service.

Labeling that survives five years

Pencil on painter's tape is not labeling. Neither is a Sharpie scrawl that the next guy has to decode with a flashlight. NEC 408.4(A) requires every circuit to be legibly identified as to purpose or use, and the identification has to be durable.

Print the directory. Laminate it if the panel lives in a garage, crawlspace, or anywhere humidity gets at it. Match circuit numbers to the breakers, not to the order you happened to land them.

• Room and load, not just room. "Kitchen SABC 1, counter south" beats "Kitchen."
• Note dedicated circuits clearly: disposal, dishwasher, microwave, fridge.
• Flag AFCI and GFCI breakers so future troubleshooting starts in the right place.
• Date the directory. The homeowner will remodel, and you want the next electrician to know when this snapshot was taken.

Testing before the cover goes on

Cover off, meter out. Verify voltage leg to leg and leg to neutral at the bus. You're looking for 240V and 120V nominal, within a few percent. Anything outside that range points back to the feeder or the utility, and you want to know now, not after drywall.

Check voltage leg to ground. It should match leg to neutral. If it doesn't, you have a neutral issue, a bonding issue, or both. Run it down before you close up.

A journeyman I worked under drilled this in: "If the numbers don't make sense, the panel doesn't get closed." Walked off a job once because a builder wanted us to energize with a 4V neutral-to-ground reading. Turned out to be a loose lug on the service neutral three hundred feet away.

Megger the feeder if the run is long or if it got pulled through rough conduit. 500V insulation test, conductor to conductor and conductor to ground. NEC doesn't mandate it for residential, but it catches nicks before they become faults.

Grounding electrode system, if this is a detached structure

Subpanels inside the same building share the service grounding electrode. But if this subpanel feeds a detached garage, shop, or ADU, you need a grounding electrode system at the second structure per NEC 250.32(A).

Two ground rods driven 6 feet apart, or a single rod if you can prove 25 ohms or less to earth, which you almost never can. Just drive two and move on. Bond to the equipment ground bar in the subpanel, not the neutral.

1. Drive rods full depth, top flush or below grade.
2. Use listed acorn clamps, not hardware store ground clamps meant for water pipe.
3. Size the grounding electrode conductor per NEC 250.66. For most residential subpanels, #6 copper covers it.
4. Document the rod locations. Inspectors ask, and homeowners dig.

Working clearance and the stuff nobody photographs

NEC 110.26 working clearance: 36 inches deep, 30 inches wide or the width of the equipment, 6.5 feet high. Measured from the face of the panel, not the wall. This trips more final inspections than any wiring error.

Check what got stored in front of the panel after rough-in. Water heaters, shelving, the homeowner's treadmill. If it's in the clearance zone, it has to move, and the homeowner has to know it can't come back. Put that in writing.

Had a panel fail final because the GC built a utility sink 34 inches in front of it after we left. Two inches of plumbing cost him a day of demo. Now I photograph the clearance on my way out and text it to the super.

Paperwork, photos, and the handoff

The job isn't done when the cover is on. Before you leave site, document what you installed and what you verified.

• Photos of the panel open, the directory, the feeder terminations, and the grounding electrode.
• Torque values recorded, with the spec you torqued to.
• Voltage and insulation readings logged.
• A copy of the directory to the homeowner or GC, and one in the panel.
• Permit closed, inspection signed, and the sticker where the AHJ wants it.

Five years from now, someone else will open this panel. Make their job easy, and your name stays on the short list for referrals. Sloppy work follows you too. That's the whole game.