Field guide: installing a subpanel, rough-in phase (edition 5)

Rough-in is where a subpanel job lives or dies. Get the feeder sizing, grounding, and cabinet prep right before drywall and the trim-out goes fast. Miss something now and you are cutting rock later.

Load calc and feeder sizing before you pull wire

Run the calc under NEC Article 220 before you touch a hole saw. Subpanel feeders get sized to the calculated load or the overcurrent device, whichever the AHJ wants to see on paper. For most residential detached-structure or basement feeds, a 60A or 100A subpanel on #6 Cu or #4 Al covers the bulk of what you will see.

Match the conductor to the terminal rating. NEC 110.14(C) locks most breakers and lugs to the 75C column. A 100A feeder on #3 Cu or #1 Al at 75C is compliant, but if the panel lugs say 60C only, you downsize the ampacity before you size the wire.

• 100A feeder: #3 Cu or #1 Al THHN/THWN-2 at 75C
• 60A feeder: #6 Cu or #4 Al at 75C
• Neutral sized per 220.61, equipment ground per 250.122
• Voltage drop check on runs over 100 ft, target under 3 percent

Cabinet location, working space, and mounting

Pick the location with NEC 110.26 in mind first, code minimums later. You need 36 inches of depth, 30 inches of width, and 6.5 feet of headroom in front of the panel, clear and dedicated. No shelving above it inside the 6 foot dedicated space per 110.26(E). Garages, utility rooms, and finished basements all work. Bathrooms and clothes closets are out under 240.24(D) and (E).

Mount the cabinet plumb and proud of the stud face by the thickness of your finished wall. Half inch rock means the cabinet front sits half an inch past the studs so the trim ring lands flush. Shim with plywood strips or panel-mount spacers, not washers stacked on drywall screws.

If you are feeding a detached structure, set the subpanel where the feeder lands cleanly, not where the homeowner wants a picture hung. You will be back there in ten years and so will the next guy.

Four-wire feeder and the grounding split

This is the part that trips apprentices and the occasional inspector. A subpanel on the same premises as the service gets a four-wire feeder: two hots, an insulated neutral, and a separate equipment grounding conductor. The neutral bar floats. The ground bar bonds to the cabinet. Never tie them together downstream of the service disconnect. NEC 250.24(A)(5) and 250.32 spell it out.

Pull the green bonding screw or bonding strap out of the neutral bar if the panel shipped with one installed. Add a separate ground bar kit if the cabinet did not come with one. Terminate the EGC on the ground bar, the feeder neutral on the isolated neutral bar, and verify with a meter between neutral and ground at the subpanel: you want continuity through the EGC back to the service, but no bond at the sub.

• Two ungrounded conductors, sized to breaker
• One grounded (neutral) conductor, insulated, on isolated bar
• One equipment grounding conductor, sized per 250.122
• Bonding screw removed, ground bar kit installed and bonded to can

Raceway, cable, and box fill at the rough

Feeder method depends on the run. SER cable is the workhorse for interior residential feeders where permitted, strapped per 334.30 and protected through framing per 300.4. PVC or EMT for exterior, underground, or anywhere you need mechanical protection. Underground PVC at 18 inches minimum cover for 120/240V residential branch and feeder per Table 300.5, deeper if you are under a driveway.

Count your conductors at every box and the panel itself. Panel cabinets are not exempt from fill thinking. Leave 6 to 8 inches of free conductor past the face of the cabinet per 300.14 so the trim-out electrician, which might be you on a Friday, has something to work with. Ream every knockout. Use listed connectors. No romex through a KO without a connector, ever.

GFCI, AFCI, and what gets landed at rough vs trim

At rough-in you are not landing branch circuits yet, but you are deciding feeder and homerun paths. Know where your AFCI and GFCI requirements land so you do not pull a homerun into a space that needs dual-function protection and forget the breaker slot budget. NEC 210.8 covers GFCI locations, 210.12 covers AFCI.

• Kitchen, bath, laundry, garage, outdoor, unfinished basement: GFCI per 210.8(A)
• Dwelling bedrooms, living rooms, kitchens, laundry: AFCI per 210.12(A)
• Dishwasher circuit: GFCI per 210.8(D)
• Dual-function breakers eat one slot each, budget accordingly

Count your slots twice before you close the wall. A 20-space sub fills up fast once you start loading AFCI and GFCI breakers on every kitchen, laundry, and bedroom homerun.

Before you button it up

Walk the rough before the inspector does. Feeder torqued to the label on the lug, not to feel. Neutral isolated, ground bonded, bonding screw in the bag taped to the inside of the door for the next electrician. All KOs filled or plugged. Every homerun labeled at the panel end with room and circuit intent, even in pencil on the sheathing.

Photograph the open panel and the feeder path through framing before rock goes up. Ten minutes with a phone now saves two hours of troubleshooting a year from now when something reads hot that should not.