Field guide: installing a subpanel, inspector tips (edition 4)

Plan the feeder before you cut drywall

Subpanel work lives or dies on the feeder calc. Before you pick a panel, nail down the load, the distance, and the grounding path. Inspectors sign off faster when the feeder paperwork matches what is in the pipe.

Size the feeder to the calculated load per NEC 220, not to the main breaker of the subpanel. A 100A subpanel fed by a 60A breaker is fine if the load calc supports it. What kills jobs is undersized conductors for the voltage drop on a long run, especially detached structures.

• Run a load calc per NEC 220.40 and keep a copy in the panel.
• Check voltage drop on runs over 100 feet, aim for 3% on the feeder per NEC 210.19 informational note.
• Confirm the feeder OCPD at the source does not exceed the subpanel bus rating (NEC 408.36).
• For detached structures, plan the disconnect at the second building per NEC 225.31.

Four wires, always, and keep them separated

The single most cited subpanel failure is bonded neutrals. A subpanel is a separately derived... no, it is a downstream panel. The neutral and ground must be isolated. Pull the green bonding screw. Remove any factory neutral-to-enclosure strap.

Feeder must be four conductors: two hots, a neutral, and an equipment grounding conductor per NEC 250.32(B)(1). The old three-wire feeder to detached buildings went away in the 2008 cycle and inspectors still catch guys running it.

"If I can put my meter between the ground bar and neutral bar and read anything but zero, I am writing it up. Every time." ... a county inspector in central Ohio.

Ground bar gets bonded to the enclosure. Neutral bar floats on its insulating kit. If the panel did not ship with a separate ground bar kit, order one, do not improvise with a sheet metal screw into the can.

Grounding electrode at detached structures

If the subpanel feeds a detached building, you need a grounding electrode system at that building per NEC 250.32(A). One ground rod is rarely enough. Drive two rods six feet apart or prove 25 ohms or less with a clamp-on tester, and nobody carries that tester, so drive two.

The grounding electrode conductor sizes off NEC Table 250.66 based on the feeder. For a typical 100A feeder with #4 copper or #2 aluminum, a #8 copper GEC to the rods is the floor. Bigger feeder, bigger GEC.

• Two rods, 8 feet, driven full depth, top flush or below grade.
• Acorn clamps rated for direct burial, listed for the conductor.
• GEC unspliced where practical, or irreversible splice per NEC 250.64(C).
• Bond any metal water piping and structural steel at the second building.

Working space and panel location

Inspectors carry a tape measure. You should too. NEC 110.26 wants 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. That space is dedicated. No shelving, no water heater, no furnace flue in the envelope.

Subpanels in closets with easily ignitable material are out per NEC 240.24(D). That means no panels in clothes closets. Pantries, linen closets, same answer. Garages, basements, utility rooms, mechanical closets are fine.

"Homeowners love to frame a shelf right above the panel. I fail it, and the drywaller hates me, but that is the rule." ... a master electrician in Denver.

Terminations and torque

NEC 110.14(D) now requires torque values be applied per the manufacturer. That means a calibrated torque screwdriver or wrench on every lug, and a lot of inspectors want to see you do it or see the tool on site. A torque sticker on the deadfront is cheap insurance.

Aluminum feeders need antioxidant where the listing calls for it, and they need the right lug. Check the panel label for AL/CU rating. Strip length matters, too short and you clamp insulation, too long and you expose conductor outside the lug.

• Torque every feeder lug, breaker lug, and neutral/ground termination.
• Label the torque tool calibration date in your truck log.
• Use listed antioxidant on aluminum, wipe the excess, do not pack the lug full.
• Tag the feeder at both ends per NEC 408.4 with circuit ID and source panel.

Final walkthrough before you call it

Before you call for inspection, walk the panel like the inspector will. Deadfront off. Meter in hand. Check neutral-to-ground isolation, check every breaker is seated, check the AFCI and GFCI requirements per NEC 210.8 and 210.12 for the branch circuits you added.

Fill out the directory. Inspectors hate blank or pencil-scratched directories. Typed or neatly printed, circuit by circuit, room by room. NEC 408.4(A) wants it legible and specific, "lights" is not specific.

1. Verify neutral bar is isolated, bonding screw removed.
2. Verify EGC landed on ground bar only.
3. Torque every lug, mark with a paint pen if that is your habit.
4. Test GFCI and AFCI breakers with the test button and a plug-in tester.
5. Complete the directory, close the deadfront, clean the area.

Subpanels are not hard. They are just unforgiving of shortcuts. Four wires, isolated neutral, torqued lugs, complete directory. Do those four things every time and the red tag stays in the inspector's truck.