Field guide: installing a subpanel, inspector tips (edition 6)

Plan the feeder before you touch a knockout

Subpanel jobs go sideways when the feeder gets sized after the fact. Pull the load calc first, confirm the ampacity you actually need, then size conductors per NEC 215.2 and the 310.16 tables. If the run is long, voltage drop drives the wire size, not the breaker.

Check the main service capacity before promising a 100A subpanel. A 200A service already loaded at 160A of calculated demand will not pass a plan review for another 100A feeder without load management per NEC 750 or a documented calc under 220.83.

• Feeder OCPD sized per NEC 215.3.
• Grounded (neutral) conductor sized to 220.61.
• EGC sized to 250.122 based on the feeder OCPD, not the conductor.
• Voltage drop target: 3% on the feeder, 5% total to the farthest outlet (215.2(A)(1) Informational Note 2).

Separate the neutral and ground, every time

This is the number one red tag on subpanel inspections. At a subpanel, the grounded conductor (neutral) and the equipment grounding conductor must be kept separate. NEC 250.24(A)(5) only permits the neutral-to-ground bond at the service disconnect, nowhere downstream.

Pull the factory bonding screw or bonding strap out of the subpanel enclosure. Add a separate ground bar, kitted from the panel manufacturer, and land every EGC there. Neutrals land on the isolated neutral bar that floats on plastic standoffs.

"I tell apprentices: if you can measure continuity between the neutral bar and the can with your meter, you failed before the inspector walked in. Check it before you close the cover."

Four-wire feeder, no exceptions on new work

New subpanel feeders require four conductors: two ungrounded, one grounded, one equipment grounding. The old three-wire feeder to a detached structure was removed from the code in the 2008 cycle. NEC 250.32(B)(1) is explicit, run the EGC with the feeder.

The grandfathered three-wire feeder under 250.32(B)(2) exception only applies to existing installations with no metallic path and no other circuits between structures. Do not build one new, and do not extend one without bringing it up to current code.

1. Two hots, properly phased to match the main.
2. Insulated neutral, sized per 220.61, kept isolated at the sub.
3. EGC sized per 250.122, bonded to the subpanel can.
4. Grounding electrode system at detached structures per 250.32(A).

Working space, mounting, and the stuff inspectors actually check

NEC 110.26 working space is the fastest fail on a finished install. 30 inches wide, 36 inches deep at 120/240V, 6 feet 6 inches high, clear to the floor. No shelving, no water heater overhang, no laundry sink crowding the front. If the panel is in a closet with clothes storage, NEC 240.24(D) kills it before you start.

Mount the panel so the highest breaker handle is no more than 6 feet 7 inches off the finished floor per 240.24(A). Outdoor subpanels need a NEMA 3R enclosure, drip loops on the feeder, and listed hub fittings where required. Indoor garages and unfinished basements count as damp or wet locations depending on conditions, check the AHJ interpretation.

"Inspectors carry a tape measure. If you eyeballed the 36 inches and came up at 34, you are pulling the panel off the wall on a Friday afternoon."

Breakers, labeling, and the panel schedule

Only use breakers listed for the panel. A UL classified breaker is not the same as a listed breaker for that specific load center, and a lot of AHJs will not accept the classified substitute. Check the panel label for acceptable breaker part numbers.

NEC 408.4(A) requires every circuit to be legibly identified as to purpose at the panel, and the identification must be specific enough to distinguish it from others. "Lights" on a ten-circuit panel does not meet this. Use room names, fixture locations, or outlet groupings. 408.4(B) also requires identification of the source supplying the subpanel, on the subpanel itself.

• AFCI per NEC 210.12 on required dwelling circuits.
• GFCI per 210.8 for locations including kitchens, baths, garages, outdoor, basements.
• Handle ties on multi-wire branch circuits per 210.4(B).
• Torque every lug to the manufacturer spec, 110.14(D), and keep the torque tool on site for the inspection.

Final walk before you call for inspection

Open the cover and look at your own work like an inspector would. Neutrals isolated, grounds on the ground bar, no doubled neutrals under a single lug (408.41), bonding screw removed, feeder conductors landed on the correct terminals, anti-short bushings on MC, and every knockout filled or closed.

Photograph the inside of the panel with the directory visible and the deadfront off. It saves you a trip back when the inspector asks about a terminated conductor six months later, and it documents torque marks if you use a paint pen after tightening.