Field guide: installing a subpanel, during the job (edition 6)

Before you pull the main, recheck your load math

You sized this panel at the bid. Now conditions have changed. The HO added a mini split, the shop swapped to a 50A welder receptacle, or the kitchen remodel grew an induction range. Redo the calc on the tailgate before you cut a single knockout. NEC 220.82 for dwelling feeders, 220.40 for the general method. Ten minutes now saves a Monday trip back.

Confirm the feeder ampacity against the OCPD you actually have on the truck. A 100A subpanel fed with #2 SER through an insulated assembly is not the same as #2 THHN in conduit. Table 310.16 versus 310.12 for dwelling services and feeders supplying the main power. Know which column you are in before you terminate.

If the existing main panel is already past 80 percent on the calc, stop. Call the POCO or upsize the service before you land a single subfeed conductor.

Kill it, prove it, lock it

Lockout tagout is not optional, even on a residential remodel. Shut the main, verify dead at the lugs with a known-hot, known-dead, known-hot check on your meter. NFPA 70E drives this but 110.12 and good sense back it up. Your meter can fail between the truck and the panel.

If you cannot kill the main because the homeowner needs the fridge running, you are doing energized work. That means arc-rated PPE, a written plan, and a second set of eyes. Most residential subpanel installs do not justify the risk. Schedule the outage.

• Main breaker off, breaker locked, tag signed and dated
• Meter tested on a live circuit before and after the dead check
• Insulated tools within reach, metal jewelry off
• Flashlight and headlamp staged, because the panel room just went dark

Mounting, clearances, and the stuff inspectors actually check

110.26 working space is the number one callout on subpanel rough. 36 inches deep, 30 inches wide or the width of the equipment, 6.5 feet high. Measure from the face of the deadfront, not the wall. If the washer sits in front of the panel, that is a fail even if you can swing the door.

Mount height matters for the handle: 240.24(A) caps the highest breaker at 6 feet 7 inches above the floor. In a garage or unfinished basement, the panel needs to be readily accessible, which means no shelving, no storage racks, no freezer in front of it. Document the clearance with a photo before the drywaller shows up.

For damp or wet locations, use a NEMA 3R enclosure and keep the knockouts on the bottom or sides, never the top. 312.2 for raintight requirements. A garage panel on an exterior wall is not automatically wet, but if it is subject to splash from a hose bib or driveway runoff, treat it as such.

Feeder conductors, grounds, and the four-wire rule

Subpanels in the same structure as the service need four wires: two hots, a neutral, and an equipment grounding conductor. 250.32(B) was rewritten years ago and the old three-wire feeder to a detached building is gone for new work. Neutrals and grounds are bonded only at the service, never at the subpanel. Pull the bonding screw or strap out of the sub and bag it for the inspector if they want to see it.

Size the EGC from 250.122 based on the feeder OCPD, not the conductor size. A 100A feeder gets a #8 copper EGC minimum. If you upsize the ungrounded conductors for voltage drop, 250.122(B) says upsize the EGC proportionally. This one trips people up on long runs to detached shops and ADUs.

Separate the neutral bar from the ground bar. If the panel only ships with one bar, order a ground bar kit before you leave the supply house. Do not improvise.

Circuit directory, torque, and AFCI/GFCI placement

408.4(A) requires a legible circuit directory. Not pencil scratches, not "lights," not "bedroom stuff." Room, device type, and breaker number. Do it as you land each circuit, not at the end when you are tired and the HO is asking when you will be done.

Torque every lug and every breaker to the manufacturer spec printed inside the can. 110.14(D) now requires a calibrated torque tool, and inspectors in a lot of jurisdictions are asking to see it. Mark each terminal with a paint pen once it hits spec so you know what you have done.

1. Feeder lugs first, to spec, with the main off
2. Neutral and ground terminations, torqued and marked
3. Branch breakers seated fully, listen for the click
4. AFCI on 210.12 circuits, GFCI on 210.8 locations, dual function where both apply
5. Directory filled in, deadfront on, label the panel itself with feeder source and OCPD size

Before you energize, walk it

Megger the feeder if the run is long or the conduit was wet during the pull. At minimum, ring out each branch circuit with the breaker off and the neutral lifted. A crossed neutral on a shared circuit will cook a multiwire load the second you flip the main.

Energize the main first, then the subfeed, then branches one at a time. Watch the panel, listen for buzz, feel the deadfront for heat after ten minutes. If something is wrong, it usually tells you in the first minute. Sign the directory, photograph the finished panel with the deadfront off and on, and leave the torque values on the job folder for the inspector.