Field guide: installing a subpanel, common mistakes (edition 6)

Subpanels look simple on paper. Four wires, a feeder breaker, a handful of branch circuits. Then you open up the next guy's work and find the neutral bonded at both ends, a bare EGC sharing a lug with a grounded conductor, and a 100A feeder protected by a 60A breaker "because that's what was in the truck." Here's what actually goes wrong in the field, and the code sections that back up doing it right.

Separating neutrals and grounds

This is the number one callback on subpanel work. At a subpanel (any panel downstream of service disconnect), grounded conductors and equipment grounding conductors must land on separate buses. The main bonding jumper belongs at the service, and only at the service, per NEC 250.24(A)(5) and 250.142(B).

That means pulling the bonding screw, strap, or green bond jumper out of the subpanel before energizing. Every manufacturer ships panels bonded from the factory. If you don't remove it, you've created a parallel path for neutral current on the EGC, and every metallic part downstream becomes a current-carrying conductor.

• Pull the green bonding screw from the neutral bar.
• Add a separate ground bar kit (listed for the panel) and land all EGCs there.
• Verify the neutral bar is isolated from the enclosure with an ohmmeter before closing up.

"If you can read continuity between the neutral bar and the can with the feeder disconnected, you missed the bond. Fix it before you energize, not after the inspector shows up."

Four-wire feeders, not three

The three-wire feeder with a bonded neutral is gone for new installations. NEC 250.32(B) requires a four-wire feeder (two ungrounded, one grounded, one equipment grounding conductor) to a separate building or structure, and the same applies within a building to any subpanel. The EGC gets its own conductor, sized per 250.122.

Old farm shops and detached garages are where this bites people. If you're extending or replacing a feeder to an existing detached structure, you're doing new work, and the exception in prior code cycles for existing three-wire feeders does not apply to the replacement. Pull the fourth wire.

Feeder sizing and overcurrent protection

Size the feeder conductors to the calculated load per Article 220, then protect them at their ampacity per 240.4. The subpanel's bus rating is a ceiling, not a target. A 100A rated panel can be fed with a 60A feeder if the load calc supports it, but you cannot feed a 100A panel with 100A conductors and protect them with a 125A breaker.

1. Calculate load per 220.40 (general lighting, receptacles, fixed appliances, largest motor at 125%).
2. Select conductor from 310.16 at the appropriate temperature column, applying derating for ambient and conduit fill.
3. Size the feeder OCPD at or below conductor ampacity, rounding per 240.4(B) where allowed.
4. Confirm the panel bus rating meets or exceeds the OCPD.

Don't forget 310.12 for dwelling service and feeder conductors. A 100A feeder to a dwelling unit subpanel can often be #4 copper or #2 aluminum under that provision, which catches inspectors who are still sizing from 310.16 alone.

Working space and mounting

Working clearance per 110.26 is 30 inches wide, 36 inches deep (on 120/240V residential), and 6.5 feet high. The space in front of the panel cannot be used for storage, and the dedicated equipment space above the panel extends 6 feet up or to the structural ceiling, whichever is lower, per 110.26(E).

The common violations are mounting in a clothes closet (prohibited per 240.24(D)), over a laundry sink, behind a water heater, or in a bathroom of a dwelling unit (240.24(E)). If the homeowner says "just put it in the pantry," check the pantry isn't really a clothes closet before you drill the first hole.

"36 inches from the face of the deadfront, not from the wall the panel is mounted on. Measure with the door open if it's a flush-mount."

Terminations and torque

Loose lugs start fires. Every breaker manufacturer prints torque values on the breaker or in the label kit, and 110.14(D) now requires a calibrated torque tool for terminations where torque values are provided. That means a torque screwdriver for branch breakers and a torque wrench for feeder lugs, not a ratcheting feel-good.

Aluminum feeders need antioxidant compound rated for the conductor, the lug filled and wire-brushed before insertion. Copper doesn't require it but doesn't suffer from it either. Strip length matters: too short and you're clamping insulation, too long and you've got exposed conductor outside the lug.

• Match the conductor temperature rating to the terminal rating per 110.14(C). Most breakers are 75C terminals, so use the 75C column.
• Don't double-lug a breaker unless it's listed for two conductors.
• Re-torque after the first heat cycle on large aluminum feeders if the manufacturer calls for it.

Grounding electrode at separate structures

Feed a detached garage or shop, and you owe it a grounding electrode system at that structure per 250.32(A). Ground rods, Ufer, or whatever qualifies under 250.52. The EGC from the feeder bonds to that electrode system via the grounding electrode conductor sized from 250.66.

One rod is rarely enough. 250.53(A)(2) requires supplemental electrodes unless you can prove 25 ohms or less, and nobody carries a fall-of-potential tester to a residential job. Drive two rods, six feet apart, bond them together, and move on. It's faster than arguing with the inspector.