Field guide: installing a subpanel, before you start (edition 1)

Scope the job before you pull a permit

Before you price the work, figure out what the subpanel is actually feeding. A detached garage with a welder and a mini split has a different load calc than a finished basement with a bath and a wet bar. Walk the space, count the circuits you are adding, and write down the nameplate ratings on anything fixed in place.

Run a standard load calc per NEC 220, Part III. If the existing service is already close to its calculated load, you have a service upgrade conversation before you have a subpanel conversation. Do not skip this step because the main looks "big enough." Breakers lie, math does not.

Confirm the feeder path next. Measure the run, note every transition (interior to exterior, through masonry, underground), and decide on wiring method before you quote. A 90 foot run through a crawlspace and up a block wall is not the same bid as 30 feet of EMT in open joists.

Size the feeder and the panel

Match the feeder to the calculated load, not to the main breaker on the existing service. A 100 amp subpanel does not automatically need a 100 amp feeder if the calc says 62 amps. That said, upsize the enclosure and bus rating for future circuits. Electricians who size tight get called back in three years to do it again.

For conductor sizing, use NEC 310.16 for the ampacity table and apply 310.15(B) for ambient and bundling adjustments. For dwelling services and feeders that carry the entire load of the dwelling, 310.12 still allows the reduced conductor sizes, but a subpanel feeding only part of a dwelling does not qualify. Read the article, do not assume.

• 60 amp feeder: typically 6 AWG Cu THHN, 4 AWG Al
• 100 amp feeder: typically 3 AWG Cu THHN, 1 AWG Al
• Always verify against the installed termination temperature rating (75C column for most modern lugs)
• Include the equipment grounding conductor per NEC 250.122, sized to the feeder OCPD

Grounding and bonding, get this right the first time

This is where most subpanel jobs fail inspection. A subpanel in the same structure as the service has the neutral and ground separated. The bonding screw or strap comes out, the neutral bar floats, and the equipment grounding conductor lands on its own bar bonded to the enclosure. NEC 250.24(A)(5) is the rule, and 408.40 covers the equipment ground bar.

A subpanel in a separate structure used to allow a three wire feeder with a local ground. That exception is gone. Since the 2008 cycle, NEC 250.32(B) requires four wires (two hots, neutral, EGC) for feeders to separate buildings, with very narrow exceptions for existing installations. If you are doing new work, run four wires, drive the electrodes at the separate structure, and bond per 250.32(E).

Field tip: take a photo of the neutral bar with the bond screw removed and the EGC bar bonded to the can before you close it up. Inspectors appreciate it, and it saves you a trip back when someone questions the work later.

Plan the physical install

Working clearances are not optional. NEC 110.26 wants 36 inches of depth in front of the panel, 30 inches of width, and 6.5 feet of headroom. If the homeowner built a closet around the old panel location, you are moving the panel or moving the closet. Measure before you promise a location.

Think about the dead front too. A subpanel behind a water heater or below a laundry sink is going to get called out. Keep it out of bathrooms (NEC 240.24(E)), out of clothes closets (240.24(D)), and accessible without having to move stored items.

1. Confirm mounting surface and fire rating (drywall, plywood backer, masonry)
2. Locate the feeder entry point and plan the conduit or cable path
3. Verify working space in front, to the sides, and overhead
4. Check for conflicts with plumbing, HVAC, and gas lines before you cut

Pull permits and coordinate the shutdown

Most AHJs want a permit for any new panel, even a 60 amp subpanel feeding a garage. Pull it, schedule the inspection, and do not energize until you have a green tag. If the feeder taps the main or requires a service disconnect pull, coordinate with the utility or have a generator and transfer plan ready for the customer.

Call the customer 24 hours before the shutdown. Ask about medical equipment, aquariums, sump pumps, anything that dies when the power goes off. Nothing ruins a relationship faster than a flooded basement because the sump breaker was off for six hours.

Pre-job checklist

Run through this before you load the truck. Missing one item on this list is the difference between a half day job and a two day job.

• Load calc completed and filed
• Permit pulled and posted
• Panel, breakers, and feeder conductors on the truck and verified against the spec
• Grounding electrodes and conductors (if separate structure)
• Fire caulk, anti-ox for aluminum, torque screwdriver with calibration in date
• Label maker or pre-printed directory template
• Customer notified, shutdown window confirmed, backup plan for critical loads

Field tip: keep a laminated 110.26 clearance diagram in the truck. When a GC or homeowner pushes back on panel location, you hand them the card instead of arguing about it.