Field guide: installing a subpanel, safety checklist (edition 5)

Before you pull the permit

Subpanel jobs look simple on paper and get messy in the field. Before you touch a breaker, confirm the feeder calculation, the grounding method, and whether the structure is a separate building or the same structure as the service. That single distinction changes your grounding electrode requirements under NEC 250.32 and it catches more inspectors' red tags than anything else on this kind of job.

Walk the run. Measure it. Check the ampacity against NEC 310.16 for the conductor type you actually have on the truck, not the one you wish you had. If the feeder passes through a garage, attic, or crawl space, think about physical protection per 300.4 before you start drilling.

Pull the permit. Homeowners who "just want it done quick" are the ones who call the AHJ later.

Sizing the feeder and the panel

Size the subpanel for the load you are serving plus realistic future growth. A 100A subpanel in a detached shop is fine until the owner adds a mini-split and a welder. Do the calculation under NEC Article 220, Part III, and document it. If the service is 200A and the existing load is 140A, you do not have 100A of headroom for a subpanel without running the numbers.

For the feeder conductors, match the overcurrent device at the supply end. Copper or aluminum, both work, but terminations have to be listed for what you are using. Check the breaker and the lug ratings. SER cable is common for residential feeders, but confirm whether it is allowed in your jurisdiction for the routing you have planned, NEC 338.10(B) has the ampacity rules and the restrictions.

• Feeder ampacity sized to the OCPD, not the calculated load
• Neutral sized per 220.61, reduced where permitted
• Equipment grounding conductor per 250.122, sized to the feeder OCPD
• Separate neutral and ground bars in the subpanel, always

Grounding and bonding, the part everyone gets wrong

In a subpanel, the neutral is isolated and the equipment grounding conductor lands on the ground bar. The bonding screw or strap that came with the panel does not get installed. This is NEC 250.24(A)(5) and 408.40. If you leave the bond in place, you have parallel paths on the neutral and current flowing on the EGC and every metal part downstream.

For a separate structure fed by a feeder, you need a grounding electrode system at that structure per NEC 250.32(A). A ground rod, two if you cannot prove 25 ohms or less, bonded to the subpanel ground bar with a properly sized GEC per 250.66. You do not reconnect the neutral to ground at the separate building. That rule changed years ago and some older installations still have the jumper, remove it when you find it.

"Every call-back I have had on a subpanel came down to a bond screw left in or a neutral and ground sharing a bar. Check it twice before you energize, and check it again before the inspector shows up."

Working the panel hot, or not

If you can kill power at the service, kill it. NFPA 70E is clear, and so is your back if you drop a screwdriver across a 200A bus. When you cannot kill it, PPE up to the correct category, use insulated tools, and have a second person present. No phone in your hand, no distractions.

Lockout-tagout is not optional when someone else has access to the service disconnect. Put your lock on it. A homeowner who "just wants to check something" in the basement while you have your hands in the panel is a real scenario, plan for it.

1. Verify deenergized with a known-good meter, test the meter before and after on a live source
2. Cover adjacent live parts before you reach across them
3. Torque every termination to the manufacturer spec, NEC 110.14(D)
4. Label the subpanel with the source, the feeder size, and the date

GFCI, AFCI, and the branch circuits downstream

The subpanel does not change the branch circuit protection requirements. GFCI per 210.8, AFCI per 210.12, and both where the occupancy calls for it. A detached garage subpanel feeding receptacles still needs GFCI on those receptacles per 210.8(A)(2). A basement subpanel feeding bedroom circuits still needs combination AFCI.

Watch for shared neutrals on multiwire branch circuits. A two-pole breaker or handle tie is required per 210.4(B), and AFCI protection on MWBCs means a two-pole AFCI breaker, not two singles. Get this wrong and you are pulling breakers back out at inspection.

Final walkthrough before you close it up

Before the cover goes on, do a physical check. Every conductor landed, every breaker seated, no daylight between the breaker and the bus. Torque wrench on every lug, feeder and branch. Dead-front screws all in. Panel directory filled out, and not with "living room," with actual circuit descriptions the next electrician can use.

Test every circuit for correct polarity, proper GFCI and AFCI function, and voltage to ground on the neutral bar, which should read zero or very close. A reading there means the neutral is bonded somewhere it should not be. Find it now, not on the call-back.

"The panel cover is the last thing that goes on and the first thing the inspector pulls. Make it look like you meant it."