Field guide: installing a subpanel, time estimates (edition 6)

Scope and site walk

Before quoting hours, walk the job. A 100A subpanel feeding a detached garage at 60 feet runs different than a 60A subpanel mounted on the other side of the main panel wall. Measure the feeder run, count the turns, check for fire-rated assemblies, and locate the grounding electrode you will bond to per NEC 250.32 if the structure is separate.

Confirm the main panel has a breaker slot or spare capacity. If the service is maxed, you are quoting a load calc per NEC 220 and possibly a service upgrade, which is a different job. Verify the existing bonding, check for a main bonding jumper already landed, and photograph the dead front before you unscrew anything.

Ask the customer what is feeding the subpanel in five years. A welder, a mini-split, an EV charger on a shared neutral circuit... size the feeder and conduit for the next load, not today's load.

Typical time estimates

These are shop-floor numbers from journeyman work, not book rates. Add time for old construction, finished walls, and inspector backlog. Residential, single-story, attached structure:

• Surface-mount subpanel, same room as main, 10 ft feeder: 3 to 4 hours rough, 1 hour trim.
• Flush-mount subpanel in finished wall, 25 ft fished feeder: 6 to 8 hours, plus drywall patch.
• Detached garage subpanel, 60 ft underground in PVC per NEC 300.5: 10 to 14 hours over two days (trench, inspection, backfill, terminations).
• 100A subpanel upgrade replacing existing 60A: 4 to 6 hours if feeder is reusable, double that if feeder is undersized.
• Permit pickup, rough inspection wait, final: plan 1 to 2 calendar weeks regardless of labor hours.

Commercial and multi-family run longer. Fire-caulking penetrations per NEC 300.21, EMT bends, and coordination with the GC will add 30 to 50 percent. If you are pulling THHN through a full raceway system, budget the pull by conductor count and distance, not by the panel itself.

Materials and feeder sizing

Pick the feeder based on the calculated load, not the panel rating. A 100A subpanel does not mean a 100A feeder if the load calc comes in at 62A. That said, most guys oversize one step for voltage drop on runs over 50 feet. NEC 215.2 sets the minimum ampacity; NEC Chapter 9 Table 8 gives you the drop math.

For a 100A feeder at 100 feet, 1 AWG aluminum SER or 3 AWG copper THHN is the common call, with a separate equipment grounding conductor per NEC 250.122. Do not use the neutral as a ground in a subpanel. Separate the neutrals and grounds on their own bars, and remove the main bonding jumper. This is the single most common failure on inspection.

"I keep a Sharpie in my pouch and write the feeder size, breaker size, and date on the inside of the dead front. Saves the next guy an hour of guessing." ... IBEW Local 26 journeyman

Grounding and bonding for separate structures

If the subpanel feeds a detached building, NEC 250.32 requires a grounding electrode system at that building. Two ground rods 6 feet apart, or a Ufer if the slab is new, bonded to the subpanel ground bar with 6 AWG copper minimum. The equipment grounding conductor from the main still runs with the feeder.

Do not bond neutral to ground at the detached subpanel. Four-wire feeder, period. The old three-wire exception was removed in the 2008 NEC and inspectors will fail it on sight. If you are working on a pre-2008 install and the existing feeder is three-wire, price the repull into the quote and explain why to the customer.

Common inspection fails

Most subpanel red-tags come from the same handful of issues. Walk the install yourself before you call for inspection. Check these in order:

1. Neutral and ground bonded at the subpanel (remove the bonding screw or strap).
2. Missing or undersized equipment grounding conductor per NEC 250.122.
3. No working space per NEC 110.26 (30 inches wide, 36 inches deep, 6.5 feet high).
4. Double-tapped breakers or neutrals on the same terminal.
5. Missing arc fault or ground fault protection on branch circuits per NEC 210.8 and 210.12.
6. No handle ties on multiwire branch circuits per NEC 210.4(B).
7. Feeder conductors not protected from physical damage, or missing anti-short bushings on SER.

Torque every lug to the manufacturer spec. NEC 110.14(D) requires it, and more inspectors are asking to see a calibrated wrench on site. A loose lug is a callback and a fire risk.

Quoting and labor

Flat-rate the common jobs and hourly the weird ones. A standard 100A subpanel in an attached garage, 20 feet of feeder, no drywall work, runs around $1,800 to $2,800 in most markets including permit and materials. Detached with a trench adds $1,500 to $3,000 depending on soil and length.

Build in an inspection contingency. If the AHJ wants a different grounding method, a different conductor, or a cover letter for a non-listed assembly, you eat that time unless your contract says otherwise. Two hours of contingency on every subpanel job pays for itself within three calls.

"Every subpanel I quote, I add one hour for 'stuff I will find when I open the main.' Aluminum branch wiring, a federal pacific main, a hidden junction behind the panel... it is always something."