Field guide: installing a subpanel, rough-in phase (edition 2)

Plan the feeder before you cut a single hole

Rough-in starts at the panel schedule, not the drill. Calculate the subpanel load per NEC 220, size the feeder conductors to 215.2, and confirm the feeder overcurrent protection at the supply end per 408.36. If the subpanel is in a separate structure, you are also dealing with 250.32 for the grounding electrode system at that building.

Pull the prints and walk the route before you commit. Note every stud bay, fire-rated assembly, and plumbing chase. A feeder that looks clean on paper gets ugly when it crosses three joists and a waste stack.

• Verify ampacity at the 75C column of Table 310.16 unless terminations say otherwise (110.14(C)).
• Confirm the neutral is sized for the maximum unbalanced load per 220.61.
• Pick the EGC size from Table 250.122 based on the feeder OCPD, not the conductor.
• Decide 3-wire or 4-wire now. Separate structures and same-structure subpanels both require 4-wire feeders with isolated neutral and ground (250.32(B), 250.142).

Set the can where it will pass inspection and serve the work

Working space is non-negotiable. 110.26(A) gives you 36 inches of depth, 30 inches of width or the width of the equipment (whichever is greater), and 6.5 feet of headroom. That space is measured from the face of the enclosure and it stays clear, no shelving, no water heater, no laundry sink creeping in after drywall.

Height matters too. The highest operating handle cannot exceed 6 feet 7 inches above the floor or working platform (404.8(A)). Mount the can plumb, flush to the finished wall surface you are expecting, and leave the trim ring proud enough for your mud depth.

If you are roughing in before the drywallers spec their board, set the can out 5/8 inch and shim the mounting ears. Beats grinding tabs off a flush mount after the fact.

Feeder routing and cable protection

Every hole you drill is a code decision. Bored holes in studs and joists need to keep the nearest edge of the cable at least 1.25 inches from the face, or you install a 1/16 inch steel plate (300.4(A)(1)). NM cable run through metal framing needs grommets or bushings (300.4(B)(1)). Running along the face of framing members requires the same 1.25 inch setback or physical protection.

Support intervals are not suggestions. NM is secured within 12 inches of every cabinet and every 4.5 feet along the run (334.30). EMT is supported within 3 feet of each box and every 10 feet thereafter (358.30). If you are running conduit, mind the fill tables in Chapter 9 and deduct for each additional 90 you bend past the first.

• Keep the feeder at least 6 inches off recessed can housings that are not IC-rated (410.116).
• Derate for conductor bundling past 24 inches when more than three current-carrying conductors share a raceway (310.15(C)(1)).
• Seal penetrations through fire-rated assemblies with a listed firestop system (300.21).

Bonding, grounding, and the neutral question

This is where rough-ins fail inspection. In a subpanel fed from the same structure, the neutral is isolated from the enclosure. The main bonding jumper belongs at the service, not downstream. Remove the factory bond screw or strap in the subpanel can and land the EGC on a separate ground bar that is bonded to the enclosure.

For a subpanel in a separate building or structure, 250.32(B)(1) requires the same 4-wire feeder plus a grounding electrode system at that structure, usually a pair of ground rods spaced at least 6 feet apart, bonded together and to the EGC bar. The neutral stays isolated. The old 3-wire feeder rule for separate structures is gone and has been for years, but inspectors still catch it on additions and outbuildings.

Label the neutral and ground bars with a paint pen before drywall. Saves five minutes of head-scratching when the trim crew calls you back for a tripping issue six months later.

Box fill, knockouts, and the stuff that fails rough-in

Every conductor, device, and clamp counts toward box fill per 314.16. Undersized boxes at the feeder landing are a common callback. Count the feeder conductors, the EGC as one, and any internal clamps or fittings that take up space. Cable terminations in the subpanel can itself are exempt, but pull boxes and junctions upstream are not.

Knockouts need the right connector. Concentric and eccentric knockouts break the fault current path if you rely on a locknut alone, so a bonding bushing or bonding locknut is required on feeders over 250 volts to ground and often a good idea at 208 and 240 too (250.97). Strip enough jacket to get NM past the clamp and into the can, but not so much that you have more than 1/4 inch of unsheathed cable inside the enclosure.

• Torque lugs to the value stamped on the can. 110.14(D) makes this a code requirement, not a suggestion.
• Leave 6 inches of free conductor at every outlet and at the panel, measured from where it emerges from the cable sheath (300.14).
• Cap and label unused conductors. Do not leave a spare neutral floating in the can.

Before you call for rough inspection

Walk the job with the prints in hand. Check that every staple is seated without crushing the jacket, every plate is installed where cable is less than 1.25 inches from the face, and every box is set to the right depth for the finish. Pull a tape on the working clearance one more time.

Photograph the open can with the feeder landed, the ground bar bonded, and the neutral isolated. If the inspector has a question three weeks later when the cover is on, you have the answer in your phone.