Field guide: installing a subpanel, final inspection (edition 2)

Before the inspector rolls up

Final on a subpanel gets kicked back for the same handful of reasons every time. Most are visual, most are fixable in ten minutes, and most got missed because the install crew packed up before walking the panel one more time. Do the walk. Bring a meter, a torque screwdriver, and the approved plans.

Pull the dead front and look at the panel like you have never seen it. Inspectors do. They are checking clearances, bonding, labeling, torque, and conductor management before they ever open a breaker schedule. If any of those fail the eye test, the rest of the visit goes downhill.

Bonding and grounding, the separate-structure question

The single biggest failure point on a subpanel final is the neutral-to-ground relationship. In a subpanel, neutrals and grounds are separated. The bonding screw or strap that came with the enclosure is removed or left out, and the neutral bar floats. Equipment grounds land on a bar bonded to the can. NEC 250.24(A)(5) and 408.40 are the articles the inspector is holding in his head.

If the subpanel feeds a separate structure, check 250.32. Since the 2008 cycle, you need an equipment grounding conductor run with the feeder. No more re-bonding neutral at the detached garage unless you meet the narrow existing-installation exception in 250.32(B)(1) Exception. Four-wire feeder, isolated neutral, grounding electrode system at the second structure, bonded to the ground bar only.

If you pulled a three-wire feeder to a detached shop because that is how the old one was done, stop the inspection and fix it now. Inspectors are not looking the other way on this one anymore.

Conductor sizing, OCPD, and the feeder

Verify the feeder ampacity against the breaker protecting it at the main. Table 310.16 for most dwelling work, with the 75C column for terminations per 110.14(C)(1)(b) on anything over 100A. A 100A subpanel on #4 copper THHN is fine at the 75C column. A 125A sub on #2 aluminum SER in a dwelling gets the 310.12 adjustment, but only for service and main-power feeders to dwelling units, not a general-purpose subpanel downstream.

Walk the OCPD schedule. Every handle should match the conductor on the load side. Double-check any 15A or 20A circuits that got bumped up during rough-in. Tandems only in slots the panel schedule sticker allows. GFCI and AFCI protection per 210.8 and 210.12 where the branch circuits land in kitchens, baths, laundry, bedrooms, and the rest of the 210.8(A) list.

• Feeder conductor sized to breaker, 75C column, 110.14(C)
• Neutral sized per 220.61, not just matched to ungrounded conductors by habit
• EGC sized per 250.122 based on the feeder OCPD, not the conductor
• Tandem breakers only in CTL-listed positions
• Handle ties on multiwire branch circuits, 210.4(B)

Working space, clearances, and mounting

110.26 is the article that catches sloppy installs. 36 inches of depth in front of the panel, 30 inches of width or the width of the equipment if wider, 6'6" of headroom. No storage in that envelope. The inspector will open the door wide and make sure it swings 90 degrees minimum, and that the space is not doubling as a pantry or utility closet shelf.

Check the mounting. Panel plumb, secure to the stud or backing, not floating on drywall anchors. In a garage or unfinished basement, physical protection is not required for the panel itself but any NM cable run on the face of framing below 7 feet should be protected per 334.15(B). Dedicated equipment space above the panel per 110.26(E), nothing foreign passing through.

Terminations, torque, and the stuff inspectors actually touch

Torque is code now, not a suggestion. 110.14(D) since the 2017 cycle. Bring a calibrated driver and hit every lug, every breaker terminal, and every neutral and ground. Manufacturer values are on the label inside the door. If you torqued at rough and the panel has been sitting for three months, re-torque before final. Aluminum especially.

Strip length matters. Too long and you have bare conductor past the lug. Too short and you are biting insulation. Pull each conductor gently after torquing to confirm bite. Dress the conductors in neat bends, keep the wire-bending space per 312.6, and route grounds and neutrals to their respective bars without crossing the breaker field any more than necessary.

Inspectors will grab a handful of neutrals and wiggle. If anything moves, you are re-torquing the whole bar while they watch. Save yourself the humiliation and do it yourself first.

Labeling, directory, and the paperwork

Panel directory filled out, typed or legibly printed, specific room and load. Not "lights" on twelve circuits. 408.4(A) requires it be legible, accurate, and not handwritten in pencil that smears. Identify the subpanel itself per 408.4(B), noting the source panel and feeder location.

Arc fault and ground fault circuits labeled where required. Series-rated equipment labeled per 110.22 if applicable. Available fault current marking at the service per 110.24 if the subpanel install triggered a service evaluation. If the inspection card lists rough-in, service, and final, make sure rough was signed before you buttoned up, because finals do not unwind missed roughs.

• Directory complete and specific
• Subpanel source and location identified
• AFCI and GFCI breakers visible and labeled
• Dead front screws all present, no knockouts open
• Approved plans and load calc on site