Field guide: installing a subpanel, tool list (edition 1)

Core hand tools

A subpanel install lives or dies by the basics. Miss one tool and you are either making a parts-house run or improvising with pliers, which is how knuckles get opened up. Stage everything on a clean drop cloth before you kill power.

Keep your strippers matched to the conductor you will actually pull. For a 100A feeder in 4 AWG copper or 2 AWG aluminum, standard Klein 11055 strippers will not touch it. You need a cable stripper rated for larger conductors, or a sharp utility knife and a steady hand.

• Lineman's pliers (9 inch, insulated 1000V)
• Long-nose and diagonal cutters
• Wire strippers, 10-18 AWG and a large-conductor stripper or cable ripper
• #2 Phillips, 1/4 and 3/8 slotted, square drive bits
• Nut drivers: 1/4, 5/16, 3/8, 7/16
• Insulated screwdrivers for anything live adjacent
• Tape measure, torpedo level, speed square
• Sharpie, pencil, label maker or wraparound labels

Metering and verification

You are not energizing anything until you prove dead. A true-RMS meter and a non-contact voltage tester are the minimum. The NCVT is a screening tool, not a verification tool. Lockout the feeder breaker at the main, tag it, then meter phase to phase, phase to neutral, and phase to ground at the subpanel lugs before you touch a lug screw.

NEC 110.16 covers arc flash labeling, and your PPE should match. For residential 120/240V work that usually means Category 2 minimum when working on energized or recently de-energized gear. If you do not have an arc-rated face shield in the truck, get one before the next job.

If your tester reads zero, test it on a known live circuit, then retest the dead conductors. A dead meter reads dead on everything.

Cutting, drilling, and mounting

Most subpanel installs involve cutting drywall, boring studs or joists for the feeder, and mounting the can plumb. A right-angle drill with self-feed bits earns its keep fast if the feeder has to cross more than two bays. For finished walls, an oscillating multi-tool cuts cleaner than a rotary and will not chew wires you forgot were there.

NEC 300.4(A)(1) requires 1-1/4 inch from the nearest edge of a stud or joist, or a steel nail plate. Keep a stack of 16 gauge plates in the truck. They are cheap, and inspectors look for them.

• Right-angle drill with 3/4, 7/8, and 1 inch auger or self-feed bits
• Hole saw kit (1-1/8 through 2 inch) for KO enlargements
• Step bit for sheet metal knockouts up to 1-3/8 inch
• Oscillating multi-tool with wood and bi-metal blades
• Reciprocating saw
• Stud finder, preferably one that flags live wiring
• Hammer drill and masonry bits if anchoring to block or concrete

Terminating the feeder and branch circuits

Torque matters and is now enforceable. NEC 110.14(D) requires listed torque tools where manufacturer instructions specify a torque value, which is effectively every modern breaker and lug. A click-style torque screwdriver covering 10 to 50 in-lb handles most branch breakers and neutral bars. For the feeder lugs on a 100A or 125A sub, you need a torque wrench going up to around 275 in-lb.

Use the antioxidant compound the lug manufacturer calls for when you land aluminum feeders. Wire brush the strands, apply compound, insert fully, torque to spec, then mark the screw with a paint pen so you can confirm it later. Retorquing a week in is good practice but not code required.

• Click torque screwdriver, 10-50 in-lb
• Torque wrench or adapter, 50-275 in-lb
• Anti-oxidant compound (Noalox or equivalent)
• Mechanical lugs and listed splice connectors as needed
• Wire brush for aluminum prep

Grounding and bonding

This is where subpanel installs go sideways. A subpanel fed from the same structure must have the neutral and ground kept separate, with the bonding screw removed or not installed per NEC 250.24(A)(5) and 408.40. Bring a 4-wire feeder: two hots, insulated neutral, and a separate equipment grounding conductor sized per NEC 250.122.

If the subpanel is in a detached structure, refer to NEC 250.32. Since the 2008 NEC, a separate EGC is required, and the neutral is not bonded at the second building unless very specific legacy conditions are met. When in doubt, pull the EGC.

Pull the bonding screw out of the can and tape it to the inside of the door with a label. Next electrician in that panel will thank you.

Documentation and closeout

A clean panel schedule is part of the install, not a nice to have. NEC 408.4(A) requires every circuit to be legibly identified as to its clear, evident, and specific purpose. "Bedroom" is not specific. "Bed 2 recep and light" is.

Before you call it done, walk the circuits with a plug-in tester or a tone and probe, update the directory, photograph the inside of the panel with the deadfront off, and note torque values on a sticker inside the door. The photo saves you on a callback six months out when the homeowner swears nothing has changed.

1. Verify deadfront fits flush, no pinched conductors
2. Energize, meter each breaker output
3. Test AFCI and GFCI functions per NEC 210.8 and 210.12
4. Fill directory, photograph panel, log torque values
5. Walk the homeowner through the main and sub shutoff