Field guide: installing a subpanel, time estimates (edition 3)

Scope and the clock you're racing

A 100A subpanel in a detached garage, fed from an existing service, with a 60 foot run and eight circuits lands between 8 and 14 hours for a two person crew. Solo, expect 12 to 20. The spread is real. Drywall condition, panel location, and whether the service has a free double pole breaker drive most of the variance.

Before quoting, walk the path. Measure the feeder run, confirm the main panel has bus space and a knockout, verify grounding electrode availability at the remote structure per NEC 250.32, and check whether the AHJ requires a permit sign off before energizing. If any of those is a question mark, add two hours to the estimate.

Load calc and feeder sizing

Run the calc per NEC Article 220 before you touch a tool. For a typical detached garage with lighting, a few receptacles, an EVSE, and a small compressor, 60A is often enough, but 100A gives headroom and costs maybe $80 more in copper. Size the feeder per NEC 310.16 and adjust for ambient and conduit fill.

Grounding at the subpanel trips up more installs than any other single item. Separate structure, separate grounding electrode system, four wire feeder, and the neutral bar isolated from the enclosure. NEC 250.32(B) is clear, and inspectors check it first.

• 100A feeder, 75C terminals: #3 Cu or #1 Al, THHN/THWN-2
• Equipment ground: #8 Cu minimum per NEC 250.122
• Grounding electrode conductor at remote structure: #6 Cu to ground rods per NEC 250.66
• Two ground rods, 6 feet apart, unless one tests under 25 ohms per NEC 250.53(A)(2)

If the existing main panel is a Federal Pacific or Zinsco, stop. Price the service upgrade, have that conversation with the homeowner before you pull wire. Adding load to a panel you wouldn't certify is a liability you don't want.

Time breakdown, honest numbers

These are journeyman hours with a helper pulling wire and handling material. Adjust for access, height, and whether you're cutting into finished space.

1. Site walk, load calc, permit paperwork: 1.0 to 1.5 hours
2. Mount subpanel, install feeder breaker at main: 1.0 to 1.5 hours
3. Conduit or cable run, 60 feet with two bends: 2.0 to 3.5 hours
4. Pull feeder, make up terminations: 1.0 to 1.5 hours
5. Ground rods, GEC, bonding at remote structure: 1.0 hour
6. Branch circuits inside the structure, 8 circuits roughed: 2.5 to 4.0 hours
7. Device trim, breaker install, labeling per NEC 408.4: 1.0 to 1.5 hours
8. Testing, megger if required, inspection prep: 0.5 to 1.0 hour

Total: 10 to 14.5 hours on a clean job. Add two hours for fished walls, another two if the feeder path crosses a finished ceiling, and a full day if you're trenching for PVC under a driveway.

Trenching and conduit, where estimates die

Direct burial USE-2 at 24 inches or PVC at 18 inches per NEC 300.5. Rent the trencher, don't hand dig 60 feet unless you're punishing yourself. A walk behind trencher handles 60 feet of clean soil in 45 minutes. Rocky or root bound ground doubles that easily.

If the run crosses a driveway, schedule around the concrete cut and patch. That's a separate trade most of the time, and it'll gate your inspection. Price the sleeve, coordinate the cut, and don't promise a finish date until the concrete sub commits.

Always pull a pull string with the feeder, even on a straight run. Future you will thank present you when the homeowner adds a hot tub in 2029.

Terminations, torque, and the things inspectors flag

Torque every lug to spec. NEC 110.14(D) requires a calibrated torque tool since the 2017 cycle, and inspectors in most jurisdictions now ask to see it. A $120 click wrench pays for itself the first time you avoid a callback.

Label the panel schedule legibly, not in pencil. Identify the feeder disconnect at the supply end and at the subpanel per NEC 408.4(B). If the subpanel serves a separate structure, the disconnect requirements of NEC 225.31 through 225.33 apply, and that disconnect needs to be readily accessible at the structure served.

• Anti oxidant on aluminum terminations, even when the lug doesn't require it
• Neutral and ground bars separated, bonding screw removed at the subpanel
• Working clearance per NEC 110.26, 36 inches deep, 30 inches wide, 6.5 feet high
• AFCI and GFCI per the branch circuit requirements, don't forget garage receptacles per NEC 210.8(A)(2)

Quoting the job

Material on a 100A subpanel with 60 feet of feeder, eight branch circuits, and standard devices runs $650 to $950 depending on copper pricing that week. Add $150 to $250 for permit and inspection fees depending on jurisdiction. Labor at a 12 hour midpoint and a $125 per hour billing rate puts the ticket around $2400 to $2800 all in for a clean install.

Pad the quote 15 percent for the unknown. Finished basements hide surprises, and the homeowner who says "the walls are open" usually means three of the four walls are open. Walk it first, quote it second, and never commit to a price over the phone for a subpanel install.