Field guide: installing a subpanel, time estimates (edition 5)

Scope and prep: what you're actually signing up for

A 100A subpanel in a detached garage, 60 feet from the main, is a full day for a two-person crew if the trench is already open. Solo, with drywall cut-in on the feed side, plan on a day and a half. The estimates below assume residential, single-phase, copper feeders, and a service that can actually support the added load per NEC 220.

Before you pull a permit, walk the path. Confirm the main panel has a breaker slot or double-lugged location, check available fault current, and verify the grounding electrode system at the separate structure per NEC 250.32. If it's a detached building, you're running four wires (two hots, neutral, equipment ground) and driving rods or using an existing electrode, with the neutral isolated from ground at the subpanel.

If the existing service is a 100A and the homeowner wants a 100A sub, stop and do a load calc. Nine times out of ten the answer is a service upgrade first, or a 60A sub sized to actual load.

Load calc and feeder sizing

Run the calc per NEC 220.82 for dwellings or 220.42 for general. Size the feeder conductors to the calculated load, not to the breaker. For a 100A feed, #3 copper THHN at 75C per NEC 310.16 and Table 310.12 is the working answer; #1 aluminum if you're going that route. Size the EGC per NEC 250.122, which lands you at #8 copper for a 100A OCPD.

Voltage drop matters past 50 feet. Use 3% as the working ceiling for feeders per the NEC 210.19 informational note. At 80A continuous on 100 feet of #3 copper, you're around 2.1%. Push to 150 feet and you'll want to upsize to #1 or run aluminum #1/0.

• 100A feeder: #3 Cu or #1 Al, #8 Cu EGC, 1.25" PVC or EMT
• 60A feeder: #6 Cu or #4 Al, #10 Cu EGC, 1" conduit
• Neutral sized to calculated unbalanced load, NEC 220.61
• Isolate neutral from ground at the subpanel, remove bonding screw

Time estimates by task

These are honest numbers from job sheets, not catalog marketing. Add 20% if the structure is finished, 30% if you're working around occupied space, and another hour per unexpected fish.

1. Permit, load calc, materials pull: 1.5 to 2 hours
2. Shutoff, main panel prep, feeder breaker install: 1 hour
3. Trenching 60 ft at 18" (rental trencher): 1.5 hours, plus 30 min setup
4. Conduit run and pulling feeders: 2 to 3 hours for 60 ft
5. Subpanel mounting, terminations, bonding: 1.5 hours
6. Grounding electrode, two rods at 6 ft apart per NEC 250.53(A)(3): 45 min
7. Branch circuit landings (assume 8 existing): 1 hour
8. Label, torque, test, inspection prep: 45 min

Total working time, no surprises: 9 to 11 hours. Call it 12 on the invoice and you'll sleep better. If the inspector wants to see the trench open, add a half day for their schedule, not your work.

The stuff that eats your day

Torque specs. Every lug, every breaker, every neutral bar. NEC 110.14(D) makes it enforceable. A calibrated torque screwdriver adds 20 minutes across a small panel and saves you a callback. Write the torque values on the inside of the door with a paint pen.

The other killer is the main panel interior. Old Federal Pacific, Zinsco, or a crowded Square D QO from 1978 will turn a two-hour feeder install into a four-hour fight. Price the possibility of a main panel replacement into the bid conversation before you commit to a fixed number.

On detached structures, the #1 callback reason is a bonded neutral at the sub. Pull the green screw, cut it off, tape the hole. Do it before you energize, not after the inspector finds it.

Common inspector hits

Inspectors see the same five things. Knowing them saves a re-inspection fee and a trip back.

• Neutral and ground bonded at subpanel (must be separated, NEC 250.24(A)(5) and 250.142)
• Missing or undersized EGC for detached structure, NEC 250.32(B)
• No working space, 30" wide by 36" deep, NEC 110.26
• Missing AFCI or GFCI on branch circuits that now require them, NEC 210.8 and 210.12
• Unlabeled breakers, NEC 408.4(A)
• Conduit fill over 40% for three or more conductors, NEC Chapter 9 Table 1

Label every circuit with the room and the load, not just "lights." If you're adding a subpanel, you're likely adding circuits that fall under current code cycles even if the rest of the house is grandfathered. The added circuits get AFCI or GFCI per the adopted code edition.

Pricing the job honestly

Flat-rate software undersells subpanels. Materials for a 100A detached sub with 60 ft of conduit, feeders, the panel itself, ground rods, and fittings runs $650 to $900 depending on copper pricing. Labor at 12 hours, two-person crew, bills out to whatever your shop rate supports, but $2,400 to $3,200 labor is typical in most markets.

Quote in writing, list exclusions (drywall repair, trench restoration, permit fees), and get a signature before the first hole. Subpanels look simple on paper and eat weekends when they aren't.