Field guide: installing a subpanel, rough-in phase (edition 1)

Plan the feeder before you pull the first staple

Rough-in dies on bad feeder math. Before you set the panel, know the load, the distance, and the OCPD at the source. A 100A subpanel 180 feet out on #3 copper is not the same job as a 60A subpanel 40 feet away, and the cable, conduit, and knockouts all shift with that number.

Size the feeder per NEC 215.2 and check voltage drop against 210.19(A) Informational Note No. 4 (recommended 3% on feeders, 5% total). If the run is long, bump a size before you buy wire, not after you have pulled it. Remember 310.12 for dwelling service and feeder conductors supplying the main power feeder, you can use the 83% rule on single-phase 120/240V dwelling feeders when conditions are met.

Grounded (neutral) and equipment grounding conductors run separately to a subpanel. That is 250.32(B)(1) for separate structures and 408.40 inside the same structure. Four wires, always, unless you are working on a legacy exception you can actually document.

Locate and mount the panel

Working space comes first. 110.26(A) gives you 36 inches depth, 30 inches width (or panel width if wider), and 6.5 feet of headroom. That space is dedicated per 110.26(E), nothing above it that is not electrical. Do not set the can behind a swing door, under a stair soffit that eats headroom, or in a clothes closet (240.24(D)).

Top breaker handle no higher than 6 feet 7 inches per 240.24(A). On a finished basement job, aim for the handle of the main at 60 to 66 inches, it keeps everything reachable and leaves room below for the gutter space you will need for feeders and branch circuits.

Set the can proud of the finished wall thickness. 312.3 and 312.4 require it flush or within 1/4 inch recessed in non-combustible, and flush in combustible. Measure the rock, add the mud, then shim.

Knockouts, connectors, and conduit entries

Every penetration is a code decision. Romex connectors for NM, listed cord grips for SO, and proper conduit fittings for EMT or PVC. No double-lugging a connector with two cables unless the fitting is listed for it, 110.3(B). On metal cans, bonding bushings are required where concentric or eccentric knockouts remain on circuits over 250V to ground, or anywhere you break the grounding path per 250.97.

Fill and bend math still applies on the feeder. Chapter 9 Table 1 for fill, 344.26 or 358.26 for bends (360 degrees max between pull points). If you are stubbing PVC up into the bottom of the can, use a male adapter and a listed connector, not a coupling and a chopped end.

• Label the feeder conductors at both ends before you close up. 200.6 for neutral identification, 250.119 for EGC, and phase tape for anything not factory colored.
• Leave at least 6 inches of free conductor at each outlet and device box per 300.14, measured from where the conductor emerges from the cable sheath.
• Torque nothing at rough-in. Dress the conductors, leave them long, torque at trim with a calibrated driver per 110.14(D).

Bonding and grounding at the subpanel

Remove the main bonding jumper. This is the number one inspector callout on a sub. The neutral bar floats, the ground bar bonds to the can. 250.24(A)(5) prohibits a neutral-to-case connection on the load side of service disconnect, and 408.40 says the EGC terminal bar must be bonded to the enclosure.

If the subpanel feeds a separate structure, 250.32 governs. Drive your ground rods (or use the existing grounding electrode system) and land the GEC on the ground bar, not the neutral. Two rods 6 feet apart unless a single rod tests at 25 ohms or less per 250.53(A)(2) Exception.

On a detached garage or shop, run the EGC with the feeder and do not re-bond neutral. The old three-wire feeder allowance in 250.32(B)(1) Exception is gone for new installs since 2008. Pull four wires.

Circuit planning and box fill at rough-in

Lay out homeruns before you start drilling. Kitchen needs two 20A small appliance circuits (210.52(B)), bathroom needs a dedicated 20A (210.11(C)(3)), laundry needs a 20A (210.11(C)(2)), and dwelling unit bedrooms, living rooms, and similar need AFCI protection per 210.12. GFCI per 210.8 where applicable, and the 2020 and later cycles expanded that list, know which code year your AHJ is on.

Box fill per 314.16 is non-negotiable. Count each conductor, each device (double), each clamp (one), and each EGC (one total). A 3x2x2.5 device box is 12.5 cubic inches, that is four 12 AWG conductors with a device and one EGC and you are full.

• Staple NM within 12 inches of a single-gang plastic box and within 8 inches of a box without cable clamps, and every 4.5 feet after that per 334.30.
• Keep cables 1.25 inches back from the stud face or use a nail plate, 300.4(A)(1).
• Bore holes in the center third of the stud when you can, it keeps you out of drywall screws and framing nails.

Prep for inspection

Rough-in inspection is a paperwork job as much as an install job. Have the panel schedule started, circuits identified at the can, and all cables labeled at the panel end with circuit intent. Inspectors want to see bonding, box fill, cable support, and working clearance. Give them that in the first 30 seconds and the rest of the walk goes fast.

Photograph every cable entry, every bonding point, and the inside of the can with the cover off before drywall. When the rocker covers something you missed, those photos save a demo bill. Keep them on the job folder, not just your phone.