Field guide: installing a subpanel, tool list (edition 5)

Why the tool list matters

A subpanel job lives or dies by what you bring to the truck. Forget one torque screwdriver and you are guessing at lug specs, or worse, stripping a neutral bar. The NEC does not list tools, but several articles effectively require them: 110.14(D) on terminal torque, 250.8 on bonding connections, 408.3 on panel arrangement. If you cannot measure, cut, and terminate to spec, you cannot pass inspection.

This list is what working hands carry for a standard 100A or 125A residential subpanel feed off a main panel, same structure or detached. Commercial adds a few items, noted at the end. Every tool here earns its ride.

Layout, measure, and cut-in

Before anything gets mounted, you mark. A good stud finder, a 4 foot level, and a chalk line set the panel square to the framing. Subpanels mounted crooked read as sloppy work even when the terminations are clean. Inspectors notice. Keep working clearance per NEC 110.26, 36 inches deep and 30 inches wide minimum, and mark the floor with tape before you commit.

For cut-in on finished walls, an oscillating multi-tool beats a drywall saw for clean edges around the panel can. Rough-in is simpler: lag the can to a stud or plywood backer, plumb and level.

• Stud finder (magnetic backup for metal studs)
• 4 foot level, torpedo level
• Chalk line, pencil, Sharpie
• Tape measure, 25 foot minimum
• Oscillating tool with bimetal blade
• Impact driver with 5/16 and 3/8 nut drivers

Raceway and cable work

Feeder choice drives the tool choice. SER cable to a subpanel in the same structure needs a good cable stripper and a pair of 10 inch Kleins for the neutral. EMT or PVC between structures means you are cutting, reaming, and bending. A hand bender sees more action than most guys admit, even when a Chicago is on the truck.

For knockouts, a Greenlee slug-buster set pays for itself on the first detached-garage feed. Do not hammer knockouts on an energized main panel, ever. Kill the main, verify dead, then punch.

Tip from a 30 year service guy: keep a short piece of 1 inch PVC in your pouch. Slide it over the SER jacket before you strip, and the braided neutral combs out straight every time instead of exploding in your hand.

• Klein 10 inch high-leverage side cutters
• Cable stripper rated for SER/SEU
• Knockout punch set, 1/2 to 2 inch
• Hand bender, 1/2 and 3/4 EMT
• Reamer or deburring tool
• PVC cutter (ratcheting, up to 1-1/2 inch)
• Fish tape, 50 foot steel or fiberglass

Termination and torque

This is where jobs fail inspection. NEC 110.14(D) requires terminations be tightened to the manufacturer's torque spec, and since the 2017 cycle inspectors are actively checking. A calibrated torque screwdriver and a torque wrench for the main lugs are not optional. The label inside the panel door lists the values, usually 45 to 50 in-lb for branch breakers and 250 to 275 in-lb for 100A lugs.

Mark every torqued screw with a paint pen. It tells the inspector you hit spec and tells the next guy the lug has not backed off. Anti-oxidant compound on aluminum conductors per 110.14, every time, no exceptions.

• Wiha or Klein torque screwdriver, 10 to 50 in-lb
• Torque wrench, 100 to 300 in-lb for feeder lugs
• Paint pen (yellow or white)
• Noalox or equivalent anti-ox
• Linesman pliers, 9 inch
• Wire strippers, 10 to 18 AWG and 6 to 14 AWG

Bonding, grounding, and testing

The bonding screw comes out of a subpanel. Every time. If the subpanel is in a separate structure, you run four wires and drive a ground rod or two per NEC 250.32. A slide hammer or rotary hammer with a ground rod driver bit saves your shoulder. Clamp meters with a low-impedance setting verify the neutral and ground are actually separated once energized.

Test before you close the cover. Voltage between hots, hot to neutral, hot to ground, neutral to ground. Neutral-to-ground should read near zero volts and show no continuity back at the main if you wired it right.

Quick check: after energizing, put the clamp meter around the EGC feeding the subpanel. Any current there under normal load means you missed pulling the bonding screw or a neutral is landed on the ground bar.

Safety and the closeout kit

Arc-rated PPE is the law on any panel work where the main is hot upstream. Category 2 at minimum for residential service equipment, balaclava and face shield included. A non-contact tester lies often enough that a proper meter with a known-good source, test, verify, test again sequence is the only reliable method.

Closeout is half the job's professionalism. Typed circuit directory, not scribbled. Panel schedule matches the directory. Cover screws driven evenly, cover flush.

1. Cat 2 arc-rated shirt, pants, gloves, face shield
2. Voltage tester (Fluke T5 or T6) plus proving unit
3. Clamp meter with low-Z function
4. Insulation resistance tester for commercial feeders
5. Label maker for directory and conductor IDs
6. Vacuum or magnetic sweeper for the work area

Commercial subpanels add a megger, larger hydraulic crimpers for compression lugs, and usually a hot stick for any work near energized bus. Everything else on this list still applies. Carry it once, carry it right, and the 125A subpanel goes in before lunch.