Field guide: installing a subpanel, rough-in phase (edition 4)

Plan the feeder before you cut a single hole

Rough-in starts at the load calc, not the drill. Size the subpanel feeder per NEC 215.2 and 220, then confirm the host panel has the breaker space and available fault current rating to feed it. A 100A subpanel fed from a 200A main is common, but verify the host bus rating and series-rated combinations before you order parts.

Pull the panel schedule for the existing service. Note any shared neutrals, MWBC circuits, or AFCI/GFCI loads that will migrate. If the subpanel lands in a detached structure, you are now in NEC 225 territory, not just 215, and the disconnect rules at 225.31 through 225.36 apply.

Confirm the conductor type and ampacity table you are pulling from. Most residential subpanel feeders run 75C terminations, so size off NEC Table 310.16 at the 75C column even if the THHN jacket is rated 90C.

Locate the panel for working space, not convenience

NEC 110.26 is non-negotiable. You need 36 inches of depth, 30 inches of width centered on or extending from the panel, and 6 feet 6 inches of headroom. The width does not have to be centered on the panel itself, but the clear space must include the full width of the equipment. Dedicated equipment space above the panel runs 6 feet up or to the structural ceiling per 110.26(E).

Avoid bathrooms (240.24(E)), clothes closets (240.24(D)), and any spot above stairs. In garages and basements, mount so the highest breaker handle sits no more than 6 feet 7 inches off the finished floor per 240.24(A).

If the GC swears the drywall is going up Monday, mark your working space on the subfloor with spray chalk before you leave for the day. Framers and HVAC will respect a painted box. They will not respect a verbal.

Feeder routing and conductor count

A subpanel in the same structure as the service requires four wires: two ungrounded, one grounded (neutral), and one equipment grounding conductor. The neutral and ground must be isolated at the subpanel. This has been the rule since the 2008 cycle closed the old three-wire feeder allowance for separate buildings, and 250.32(B) governs detached structures today.

Pick your wiring method based on the run, not habit. SER cable works for interior dry runs but watch NEC 334.80 derating when bundled or run through thermal insulation. For long runs, EMT or PVC with THHN gives you cleaner derating math and easier future pulls.

• 100A subpanel: typically #2 or #3 copper, or #1 or #2 aluminum SER, per 310.16 at 75C.
• Use 310.12 dwelling service rules only for the main service or main feeder, not arbitrary subpanels.
• EGC sized off NEC Table 250.122 based on the upstream overcurrent device.
• Voltage drop check at 3 percent for feeders if the run exceeds roughly 100 feet on a 100A circuit.

Box fill, knockouts, and bonding hardware

Every conductor entering the panel needs a listed connector matched to the cable type. SER through a non-listed Romex connector is a fail and a fire risk. Use a listed SE/SER connector or a proper strain relief rated for the jacket diameter.

Knock the right knockout the first time. Concentric and eccentric knockouts on the panel can disqualify the enclosure as an effective bonding path per 250.97 when feeding circuits over 250V to ground, which means a bonding bushing or bonding jumper is required at those points. For standard 120/240V residential, the concentric rings are usually fine, but verify the listing.

Land the EGC on the ground bar. Land the neutrals on the neutral bar. Remove the bonding screw or strap that ties neutral to the can. This is the single most common rough-in failure on subpanel inspections, and it is the one that kills people downstream when a neutral opens.

Stub, label, and leave it inspector-ready

Stub all branch circuit cables into the panel with at least 6 inches of free conductor inside the cabinet per NEC 300.14, measured from where the cable emerges from the box. If the opening is over 8 inches in any dimension, you also need 6 inches past the opening edge.

Label every home run at the panel with circuit purpose and room before drywall. Sharpie on the jacket fades. Use printed wire markers or wraparound labels. The trim-out electrician (sometimes future you) will lose hours guessing which black 14/2 is the hallway versus the bath fan.

Photograph the open panel and every wall before insulation. Three pictures per wall, overlapping. When the homeowner adds a TV mount in two years and hits a cable, the photos save the callback argument.

Final rough-in walk before you call for inspection

Verify the feeder is protected from physical damage where it leaves the panel and runs through framing. NEC 300.4(A) requires either a 1.25 inch setback from the framing edge or a steel nail plate. AHJs vary on whether SER through a stud bay needs plates at every penetration; default to plating if you are unsure.

Pre-inspection checklist before you call it in:

1. Feeder conductors sized, identified, and terminated to torque spec (look for the label inside the door).
2. Neutral isolated from ground at the subpanel.
3. EGC continuous and properly sized to 250.122.
4. Working clearances per 110.26 unobstructed by ducts, plumbing, or framing.
5. All knockouts filled or closed with listed plugs.
6. Cables secured within 12 inches of the panel and every 4.5 feet thereafter (334.30 for NM, 338.10 for SE).

Rough-in is where the trim-out either flows or fights you. Spend the extra 20 minutes now on labeling, plating, and torque, and the inspector signs without a second trip.