Field guide: installing a subpanel, time estimates (edition 4)

Scope and what "subpanel" means for time estimates

This is a 100A or 125A subpanel fed from an existing main, mounted in a finished or semi-finished space, with a feeder run under 50 feet. Residential or light commercial. If you're pulling 200A through a detached structure with a ground rod array, add hours. If you're fishing walls in a 1920s plaster house, add a day.

Time estimates below assume one journeyman, one helper, tools on the truck, and permit pulled before arrival. Inspection time is tracked separately because AHJ response varies wildly.

Code baseline: feeder conductors sized per NEC 215.2, overcurrent per 215.3, grounding and bonding per 250.32 when the subpanel is in a separate structure, and working clearance per 110.26.

Pre-job: load calc, permit, and materials (1.5 to 3 hours)

The job is won or lost at the kitchen table. A proper load calc using NEC 220 Part III (or Part IV for optional) tells you feeder ampacity, conductor size, and whether the existing service can even carry the new load. Skip this and you'll be back swapping a 100A feeder for a 125A on your own dime.

Permit paperwork takes 30 to 60 minutes if the jurisdiction has online submittal, longer if you're standing at a counter. Material staging at the supply house runs another 45 minutes once you know exactly what you need.

• Load calc and panel schedule: 30 to 45 min
• Permit application and fees: 30 to 60 min
• Material pickup and staging: 30 to 75 min
• Customer walk-through and shutoff planning: 15 min

Mounting the enclosure and roughing the feeder (2 to 4 hours)

Flush-mount in drywall adds an hour over surface-mount. Confirm working space per NEC 110.26(A): 36 inches deep, 30 inches wide, 6.5 feet high, and dedicated equipment space above per 110.26(E). If the panel lands under a plumbing stack or inside a bathroom, stop and relocate.

Feeder pathway drives the clock. EMT through an unfinished basement with open joists is fast. Romex fished through finished walls with fireblocking and a mid-run floor crossing doubles the time. For aluminum SER in a residential feeder, verify the 83% rule under 310.12 if you're trying to land #2 aluminum on a 100A breaker.

"If the homeowner says the basement ceiling is 'mostly open,' bring a ladder and a drill extension anyway. 'Mostly' means there's a soffit in the way of your straightest run."

Terminations, grounding, and the bonding screw (1.5 to 2.5 hours)

This is where journeymen separate from apprentices. The subpanel neutral bar must be isolated from the enclosure. Remove the main bonding jumper, the green screw, whatever the manufacturer calls it. NEC 250.24(A)(5) and 408.40 are explicit: neutrals and grounds bond at the service disconnect only, never downstream.

Grounding electrode conductor sizing comes from 250.66 when it's a service, but for a feeder to a subpanel in the same structure you size the equipment grounding conductor per 250.122 based on the feeder OCPD. Four-wire feeder, always, for subpanels installed after 2008 in separate structures (250.32(B)(1)).

• Land feeder conductors with proper torque per label and 110.14(D): 20 min
• Verify neutral isolation and remove bonding screw: 10 min
• Install and label equipment grounding bar kit if not present: 15 min
• Populate branch breakers and label directory per 408.4(A): 45 to 90 min

Energizing, testing, and inspection prep (1 to 2 hours)

Before you throw the feeder breaker, megger the feeder if the run was long or the environment damp. Insulation resistance to ground should read well into the megohms. A rotation check matters only if you're on a 3-phase feed, but verifying L1-to-L2 at 240V and each leg to neutral at 120V is non-negotiable on single-phase.

Torque-mark every lug. Inspectors in a growing number of jurisdictions will fail you on the spot for missing torque marks, citing 110.14(D). A $4 paint marker saves a $95 re-inspection fee.

"I keep a one-page punch list taped inside my panel cover: bonding screw out, neutrals isolated, grounds to ground bar, torque marks, directory filled, AFCI/GFCI where required. Five minutes before the inspector shows up, I walk the list."

Realistic total and what blows the estimate

For a clean surface-mount 100A subpanel, 25 feet of feeder, open basement, eight branch circuits moved over: 7 to 9 working hours plus 1 to 2 hours of office and materials time. Call it a long day for a two-person crew, or a day and a half solo.

The estimate gets destroyed by three things: discovering the existing service is undersized after you open the main, finding aluminum branch wiring that needs CO/ALR terminations or pigtails, and fireblocking in walls that weren't disclosed. Pad your quote accordingly or bill time and material when the scope is fuzzy.

1. Undersized service discovered at cutover: add 4 to 8 hours
2. Aluminum branch circuits requiring remediation: add 2 to 4 hours
3. Finished-ceiling fishing not disclosed in walkthrough: add 2 to 6 hours
4. AHJ requiring a second trip for inspection: add 1 to 2 hours plus drive time

Track your actuals. After ten of these jobs, your personal numbers beat any published estimate, including this one.