Field guide: installing a subpanel, inspector tips (edition 5)

Plan the feeder before you touch a knockout

Subpanel jobs fail at the feeder sizing stage more often than anywhere else. Before you pull a permit, know your load calc, your distance, and your grounding path. NEC 220 governs the calculation; NEC 215 covers feeders. A 100A subpanel fed from a 200A main is the common residential ask, but the conductor ampacity has to match the breaker at the source, not the panel rating stamped on the can.

Distance kills voltage. Anything over 100 feet, run the voltage drop numbers. NEC 210.19(A) Informational Note 4 recommends 3% on branch circuits and 5% total. Inspectors in some jurisdictions treat that as enforceable, others do not. Ask before you pull wire.

Common feeder pairings that pass without argument:

• 100A subpanel: 3 AWG copper THWN-2 or 1 AWG aluminum, 75C column, NEC Table 310.16
• 60A subpanel: 6 AWG copper or 4 AWG aluminum
• 125A subpanel: 1 AWG copper or 2/0 aluminum
• Always pull a separate equipment grounding conductor sized per NEC Table 250.122

Four wires, always, since 2008

The biggest callback on subpanel work is still the bonded neutral. A subpanel is a separately derived...no, scratch that, a subpanel is NOT separately derived. It is a downstream panel. Neutral and ground must be isolated. NEC 250.24(A)(5) and 408.40 are the articles inspectors cite.

Pull the green bonding screw. Remove any bonding strap between the neutral bar and the enclosure. Add a separate ground bar, bolted to the can, and land all EGCs there. Neutrals go to the floating neutral bar only. If the panel shipped with the neutral bar bonded to the enclosure, that bond has to be broken.

If you can read continuity between the neutral bar and the panel can with the main off, you failed before the inspector walked in. Check it with a meter before you close the cover.

Grounding electrode conductor: when you need one

Detached structure fed by a feeder? You need a grounding electrode system at the subpanel. NEC 250.32(A) requires it for any separate building or structure, with limited exceptions for single branch circuits. Two ground rods, 6 feet apart, bonded with a 6 AWG copper GEC unless you can prove a single rod hits 25 ohms or less (NEC 250.53(A)(2)).

Same building, subpanel on the other side of a wall? No GEC required. The EGC in the feeder does the work. Do not drive rods you do not need and do not bond neutral to ground at the second panel just because there are rods nearby.

Quick reference on GEC sizing from NEC Table 250.66:

1. Feeder 2 AWG or smaller copper: 8 AWG GEC
2. Feeder 1 or 1/0 copper: 6 AWG GEC
3. Feeder 2/0 or 3/0 copper: 4 AWG GEC
4. Rod, pipe, or plate electrode: 6 AWG is the ceiling regardless of feeder size

Working space and panel location

NEC 110.26 is where inspectors live. 36 inches of depth in front of the panel, 30 inches of width or the width of the equipment (whichever is greater), and 6.5 feet of headroom. The space has to be clear and dedicated. A water heater, washer, or shelving unit in the working space is an automatic fail.

Do not mount in a bathroom (NEC 240.24(E)), a clothes closet (NEC 240.24(D)), or over steps. Garages and unfinished basements are fine. If you are putting it in a finished space, think about access for the homeowner and whether that wall will stay clear for the next twenty years.

Rule of thumb from an inspector in Dayton: if you cannot stand flat-footed and open the dead front without bumping anything, move the panel.

Breakers, labeling, and the stuff that trips inspections

Match breakers to the panel. A CH breaker does not go in a BR panel, no matter what the YouTube comments say. Classified breakers exist (UL listed cross-brand) but confirm the listing on the panel label, not the breaker packaging.

AFCI and GFCI requirements follow the branch circuit, not the panel. NEC 210.12 for AFCI (most habitable rooms) and NEC 210.8 for GFCI (bathrooms, kitchens, garages, outdoors, unfinished basements, laundry, within 6 feet of a sink). If the subpanel feeds a garage workshop, every 120V 15 and 20A receptacle outlet needs GFCI protection per 210.8(A). Dual-function breakers at the panel are cleaner than device-level protection for rough-in.

Final checks before you call for inspection:

• Directory filled out, legible, in pen or printed (NEC 408.4(A))
• Torque every lug to the manufacturer spec, NEC 110.14(D) now requires it
• All unused openings closed with listed knockout seals
• Feeder breaker at the main panel labeled with the subpanel location
• Arc flash or equipment label if required by your AHJ

Before the inspector shows

Walk the job yourself with the cover off. Meter continuity between neutral and ground at the subpanel, should read open. Meter between ground and the can, should read closed. Tug every conductor at every lug. Photograph the panel interior before you close it, you will thank yourself on the next callback.

Keep a copy of the load calc, the permit, and the product listings on site. If the inspector asks a question, having the paperwork ready cuts the visit in half. The inspectors who give you a hard time are usually the ones who have seen too many hacks. Show your work and most of them relax.