Field guide: installing a subpanel, during the job (edition 5)

Before you pull the cover

You've already sized the feeder, picked the panel location, and set the can. Now you're standing in front of an open main with the HVAC guy asking when he can turn the air back on. This is where the job gets made or lost. Check your feeder conductor ampacity against the panel's main lug or main breaker rating per NEC 408.36, and confirm the feeder OCPD at the source does not exceed the subpanel's bus rating.

Verify the grounding electrode conductor size at the source against NEC 250.66 before you commit. If this subpanel is in a separate structure, you're looking at NEC 250.32 and a grounding electrode system at the second building, not just a ground wire riding along in the feeder.

Four-wire feeder, every time

Since the 2008 cycle there is no reduced-neutral allowance for new subpanel feeds inside the same structure. You need two hots, a full-size neutral, and a separate equipment grounding conductor per NEC 250.32(B)(1). The neutral bar floats. The ground bar bonds to the enclosure. If the panel shipped with a green bonding screw or strap, it comes out and goes in the parts bag, not the trash, in case an inspector wants to see it.

Check the neutral bar isolation with a meter before you energize. Put one probe on the neutral bar, one on the can. You want OL, not continuity. If you read continuity, the bonding screw is still in, a bus mounting screw is grabbing the neutral, or a factory jumper got missed.

• Two ungrounded (hot) conductors, sized to feeder ampacity
• One grounded (neutral) conductor, full size, not reduced
• One equipment grounding conductor sized per NEC 250.122
• Bonding screw removed, neutral bar isolated from the can

If the existing service has a shared neutral/ground from a pre-2008 install feeding the same structure, document it and flag it, but don't rewire somebody else's panel on your ticket. Note it on the invoice and move on.

Landing the feeder

Torque matters and inspectors are starting to ask. NEC 110.14(D) requires you to use a calibrated torque tool per the listing. The lug values are printed inside the can or on a sticker near the main lugs. Don't eyeball it. A loose lug on a 100A feeder is a future callback at best and a melted bus at worst.

Strip length should match the lug window. Too short and the lug bites insulation, too long and you've got bare copper sticking past the lug hunting for a fault. Dress the conductors so bends start after the lug, not inside it. Keep hots and neutral together for the feeder run to minimize induced currents on the EGC.

Bonding and grounding in a separate structure

If this subpanel sits in a detached garage, shop, or outbuilding, NEC 250.32(B) still requires the four-wire feed, and you still need a grounding electrode at the second structure. Two ground rods 6 feet apart, or one rod if you can prove 25 ohms or less, per NEC 250.53(A)(2). Most guys just drive two and skip the resistance test.

The GEC at the second structure bonds to the equipment grounding bar in the subpanel, not the neutral. The neutral stays isolated. This is the single most common violation on detached structures I see on re-inspections.

1. Drive two 8-foot rods, 6 feet minimum apart
2. Run GEC per NEC 250.66 sizing to the ground bar
3. Do not bond neutral to ground at the subpanel
4. Verify continuity from subpanel EGC back to main service ground

Circuit work and labeling

Before you start landing branch circuits, snap a picture of the empty panel with your phone. When the homeowner calls in three years saying a breaker is tripping, you'll thank yourself. Land neutrals and grounds on their own bars, one conductor per terminal hole unless the bar is listed for multiple. NEC 408.41 prohibits more than one grounded conductor under a single terminal.

Fill out the directory legibly, in pen, with room-and-purpose descriptions. "Kitchen SABC 1" beats "kitchen." Per NEC 408.4(A), every circuit needs a clear, specific description. "Spare" is acceptable only if the breaker is actually off and the circuit is not in use.

If you're adding AFCI or GFCI breakers, test them before you button up. Push the button, verify trip, reset. Do it now, not after the drywall goes back.

Energize and verify

Turn the main back on with all subpanel branch breakers off. Verify voltage at the subpanel main lugs: 240V across the hots, 120V hot to neutral on each leg, 120V hot to ground on each leg, and near zero between neutral and ground. If N-G reads more than a volt or two under load later, you've got a bond somewhere downstream.

Bring branches up one at a time, confirming each one lands on the correct leg for any MWBCs. A multiwire branch circuit with both hots on the same phase will cook the neutral fast. Use a handle tie or two-pole breaker per NEC 210.4(B). Last step, torque check the feeder lugs once more after everything is under load for 15 minutes. Heat cycle tightens some lugs and loosens others.