Field guide: installing a subpanel, tool list (edition 3)

Before you leave the shop

A subpanel install lives or dies on what's in the van. Nothing kills a day faster than driving back for a knockout punch or the wrong lug torque bit. Build the list the night before, check it against the permit scope, and stage it by phase: demo, rough, termination, trim.

Confirm the feeder calc first. Load study per NEC 220 Part III, then size the feeder conductors and OCPD before you touch a tool. If the subpanel is in a separate building, remember NEC 225.30 and the disconnect rules at 225.31 through 225.33. Detached structure means you're also dealing with NEC 250.32 for grounding electrodes at the second building.

Verify panel clearance per NEC 110.26 before committing to a location. 30 inches wide, 36 inches deep, 6 foot 6 headroom. If the HVAC guy already claimed the wall, settle it now, not after the panel is screwed to the stud.

Core hand tools

This is the non-negotiable kit. Everything here earns its spot on a subpanel day, and most of it stays on the belt for trim too.

• Klein 11055 or equivalent solid/stranded stripper, plus a dedicated large-gauge stripper for 4 AWG and up
• Linesman pliers, needle nose, diagonal cutters, Channellocks
• 11-in-1 screwdriver, plus a stubby #2 Phillips for tight gutter work
• Insulated torque screwdriver (inch-pound range) and a torque wrench that covers lug specs up to 375 in-lb
• Greenlee or Klein cable cutters rated for the feeder size you're pulling
• Tape measure, Sharpie, speed square, torpedo level
• Fish tape and a decent set of glow rods
• Non-contact tester, Wiggy or solenoid tester, and a true-RMS meter

Torque matters. NEC 110.14(D) has been enforceable language since the 2017 cycle, and AHJs are writing it up. A calibrated torque tool is not optional anymore. If yours hasn't been checked in a year, send it out or buy a new one.

Power tools and the holes they make

Most subpanel jobs need at least one fresh knockout, and usually a run of conduit or cable protection. Plan the penetrations before you start cutting.

• Hammer drill with SDS bits for masonry feeds, plus a rotary hammer if you're going through a foundation
• Hole saw kit, bi-metal, covering 7/8 through 2-1/2 inch. A Milwaukee Switchblade set pays for itself
• Greenlee Slug-Buster knockout set, 1/2 through 2 inch minimum. Hydraulic ram if you're running 2-1/2 or larger
• Right-angle drill with a ship auger set for joist and stud boring
• Reciprocating saw, oscillating multitool for clean drywall cuts around the panel opening
• Cordless impact for lag bolts into the mounting surface, not for lugs

Never torque a lug with an impact driver. Ever. You will crush the conductor, strip the lug, or both, and the AHJ will fail you on sight. Impact for mounting, torque wrench for terminations.

Materials staged at the truck

Pull the material list straight from your feeder calc and the panel schedule. Bring 10 to 20 percent extra on wire and a few spare breakers in the amperages you're using. Nothing stalls a rough like being one 20A single-pole short on a Friday afternoon.

1. Subpanel can and cover, main lug or main breaker depending on whether the feeder OCPD is upstream
2. Feeder conductors sized per NEC 215.2 and 310.16, with a separate equipment grounding conductor per NEC 250.122. Remember: four-wire feeder, isolated neutral, bonded ground at the service only (NEC 250.24 and 250.142)
3. Breakers matched to the panel's listing. No classified breakers unless the panel label explicitly allows them
4. Conduit, fittings, locknuts, bushings, anti-short bushings for MC, listed cable connectors
5. Anti-oxidant compound for aluminum terminations (Noalox or Penetrox)
6. Panel directory, permanent marker, and a label maker for circuit identification per NEC 408.4

If the feed is aluminum, bring the right compound and actually use it. Brush the strands, apply, then torque. Skipping this step is why aluminum gets a bad reputation it mostly doesn't deserve.

Testing and safety gear

Lockout-tagout is not a suggestion. The main panel gets locked open before you touch the feeder lugs, and your meter proves dead before gloves come off.

• Class 0 rubber gloves with leather protectors, in-date on the test stamp
• Arc-rated shirt and pants appropriate to the incident energy at the source panel
• Safety glasses, and a face shield for any work inside an energized enclosure
• LOTO kit: breaker lockouts for common brands, padlock, tag, and a sharpie
• Insulated mat if you're working in a damp basement or on a concrete slab
• Megohmmeter for feeder insulation testing on longer runs before energizing

Test the meter on a known live source before and after you test the dead feeder. A meter that failed silently between tests has killed electricians who skipped the second check.

Paperwork and the finish

The install isn't done when the cover goes on. Fill out the panel directory completely, with circuit numbers matching what you actually landed. NEC 408.4(A) requires legible, specific descriptions. "Lights" is not a description. "Kitchen recessed, south half" is.

Get your torque values documented if the spec calls for it, photograph the completed terminations, and walk the homeowner or GC through the subpanel location and main disconnect. File the permit closeout the same day if your jurisdiction lets you. A clean paper trail on a subpanel job is what separates the electricians who get called back from the ones who don't.