Field guide: installing a subpanel, time estimates (edition 2)

Scope and prep before you pull the truck in

A 100A subpanel feeding a detached garage or finished basement runs 6 to 10 hours for a clean install, assuming the main panel has space, the run is under 60 feet, and the drywall is open. Add time for trenching, concrete cuts, or fishing walls. This guide assumes single-phase residential, 120/240V, copper or aluminum feeders sized per NEC 215.2 and 310.16.

Walk the job before you quote. Check main panel bus rating, available breaker spaces, grounding electrode system condition, and the path from A to B. A 15 minute site walk saves two hours of surprises. Pull permits before you buy wire. Some AHJs want a load calc per NEC 220 attached to the application, so have it ready.

Gear to stage at the truck:

• Subpanel with main breaker or main lug kit, rated for the calculated load
• Feeder: 4 wire (two hots, neutral, equipment ground) for separate structures and same structure subpanels per NEC 250.32(B)
• Ground rods and #6 bare copper if the subpanel feeds a separate structure
• Anti-oxidant compound if pulling aluminum SER or XHHW
• Torque screwdriver and calibrated torque wrench, NEC 110.14(D)

Load calc and feeder sizing: 45 to 60 minutes

Run the load calc per NEC 220.82 for dwelling units or 220.42 for general. Do not eyeball it. A 100A subpanel with electric dryer, range circuit, and general lighting usually calcs to 60 to 75A demand, which still warrants 100A service for headroom. Size the feeder to the OCPD, not the calc.

Common feeder pairings: 100A with #3 copper THHN or #1 aluminum, 60A with #6 copper, 125A with #1 copper or 2/0 aluminum. Apply 75C column values in NEC 310.16 when terminations are rated 75C, which most modern panels are. If you are using SER cable inside a dwelling, check NEC 338.10(B)(4)(a) for the 60C adjustment, it trips up a lot of guys.

Old timer tip: if you are running aluminum, brush the strands with a stainless wire brush, coat with Noalox, and torque to spec with a calibrated wrench. Half the aluminum callbacks I get are loose lugs, not the metal.

Mounting and the physical install: 2 to 3 hours

Set the subpanel at working clearance per NEC 110.26: 30 inches wide, 36 inches deep, 6.5 feet high. Do not mount it in a clothes closet or bathroom, NEC 240.24(D) and (E). Garages and unfinished basements are fine. Height of the highest breaker handle stays under 6 feet 7 inches, NEC 240.24(A).

Knock out the top or bottom as your run dictates, install the proper connector, and leave 6 inches of free conductor inside the box per NEC 300.14. For EMT or PVC runs, ream every cut. Stranded feeders get terminated with anti-short bushings if you used MC or AC.

Sequence that keeps you moving:

1. Mount enclosure, level and plumb
2. Install feeder connector and pull feeder with 8 to 12 inches of slack
3. Land equipment ground on the ground bar, neutral on the isolated neutral bar
4. Land hots on the main lugs or main breaker, torque to label spec
5. Install branch breakers after feeder is dead and verified

The neutral-ground separation that trips inspections

In any subpanel, neutrals and grounds must be isolated. Remove the bonding screw or strap between the neutral bar and the enclosure. NEC 250.24(A)(5) and 408.40 are clear: the grounded conductor and equipment grounding conductor bond only at the service disconnect, not downstream.

If the subpanel feeds a separate structure, drive two ground rods 6 feet apart, bond them with #6 bare copper to the ground bar in the subpanel, per NEC 250.32(A) and 250.53(A)(2). The equipment ground still runs back to the main in the feeder. Do not rebond neutral at the separate structure on new work, that changed in NEC 2008.

Inspector will yank the cover and check for the bonding screw first. I keep a sticky note on the dead front reading "bond screw removed, date, initials" so there is no debate.

Energize, test, label: 45 minutes

Before you flip the main, torque check every lug, verify neutral and ground separation, and meg the feeder if the run was pulled through damp conduit. Energize the feeder breaker at the main, then walk to the subpanel and verify 240V across the hots, 120V hot to neutral on each leg, and zero volts neutral to ground.

Label every breaker with the actual circuit, not "kitchen". Write "kitchen SABC north wall" or "garage GFCI receptacles." NEC 408.4(A) requires specific identification. Fill in the panel schedule in pen, not pencil, and date it.

Realistic totals and where jobs blow up

Baseline 100A subpanel, same structure, open walls: 6 to 8 hours, one electrician plus a helper for the pull. Detached structure with trench: add 3 to 5 hours for 40 feet of trench at 18 inch depth for PVC direct bury per NEC 300.5, more if you are hand digging or hitting roots.

Where estimates die: undersized existing service forcing a load calc upgrade, finished walls needing fish tape work, main panel with no spare double pole slot, and aluminum feeders getting kinked during the pull. Pad your quote 15% for any of those conditions. Quote hourly if the scope is unclear, flat rate only when you have eyes on the whole path.