Field guide: installing a subpanel, during the job (edition 4)

Before you pull the cover

You staged the panel, you have the feeder run, and the HO is watching. Stop. Walk the job one more time with a voltmeter and a torque screwdriver in your pouch. Verify the feeder size against the calculated load per NEC 220, not against whatever the last guy said he pulled. A 100A subpanel fed with #2 AL THHN is fine on a 75C terminal, but if the existing lugs are 60C rated, you are already out of compliance.

Confirm the grounding path back to the service. If this sub is in a detached structure, you are under NEC 250.32, which means a supply-side bonding jumper and four wires if there are any parallel paths (water line, phone, coax). Do not assume the HO does not have a buried irrigation controller tied back to the house.

If the feeder conduit has more than two 90s between boxes, stop and add a pull point now. You will not regret the extra LB at rough, you will curse yourself at trim.

Setting the can

Mount height matters. NEC 240.24(A) puts the highest breaker handle at 6 ft 7 in max. If you are installing in a garage or utility room, also check 240.24(D) and (E), no panels in clothes closets or over stairs. In a finished basement with a dropped ceiling, measure from finished floor, not slab.

Knockouts are where sloppy work shows. Use the right size step bit or KO punch, not lineman's and a hammer. Every unused KO gets a filler, NEC 408.7. Every cable entry gets a listed connector, and NM through a KO without a connector is a red tag every time.

• Working space: 36 in deep, 30 in wide, 6.5 ft high, per NEC 110.26(A).
• Dedicated space above: 6 ft clear of foreign piping, per NEC 110.26(E).
• Illumination required at the panel location, 110.26(D).

Landing the feeder

This is where subpanels get killed on inspection. The neutral and ground MUST be separated in a subpanel. Remove the main bonding jumper or bonding screw. The neutral bar floats, the ground bar bonds to the can. If the panel shipped with the green screw installed, back it out and bag it with the paperwork.

Torque to the label. Every listed lug has a torque spec, and NEC 110.14(D) now requires you to use a calibrated torque tool. "Gutentight" is a citation waiting to happen. For AL feeders, brush the strands, apply listed antioxidant if the lug manufacturer calls for it, and re-check torque after 24 hours on larger conductors if the job allows.

1. Strip to the lug's strip gauge, no nicks on the conductor.
2. Insert fully, verify no insulation under the lug face.
3. Torque to spec with a click or beam wrench, mark with a paint pen.
4. Dress conductors to the gutter, maintain bending radius per 312.6.

Branch circuits and balance

Load balance is not optional, it is how you keep the neutral current down and the customer's bill sane. Sketch your circuit assignments before you start landing breakers. Put the big motor loads (dryer, compressor, EV charger) on opposite phases from each other and from the high-draw small appliance circuits.

Handle ties vs two-pole breakers matter. Multiwire branch circuits require simultaneous disconnect per NEC 210.4(B). A handle tie on two singles is legal only if the breaker is listed for it. Check the manufacturer's kit, do not field-fabricate with a nail or a zip tie. I have seen both, and both got red tagged.

Label as you go, not at the end. A sharpie on the inside of the door with circuit numbers and rooms takes five minutes during rough and saves an hour at trim when the HO asks which breaker is the fridge.

GFCI, AFCI, and the 2023 cycle

If the AHJ is on the 2023 NEC, your subpanel in a garage or accessory building is feeding a lot more GFCI-protected territory than it used to. 210.8(A) now covers basements, laundry areas, and within 6 ft of any sink. 210.8(F) covers outdoor outlets on dwellings. And 210.8(B) keeps expanding for non-dwelling spaces.

AFCI per 210.12 covers nearly every 120V 15/20A circuit in a dwelling. If you are feeding bedrooms, living rooms, hallways, or similar from this sub, plan for dual-function breakers and budget the cost. They are not cheap, and mixing brands between the panel and the breaker will void the listing.

Before you energize

Megger the feeder if the run is long or you had any doubt about pulling tension. Ring out each branch circuit for shorts and opens before you flip anything. Verify phase rotation if you are feeding any 3-phase equipment downstream.

Energize the main first, check voltage L-L and L-N at the feeder lugs, then turn branches on one at a time. Watch the panel for heat or the smell of cooked insulation on the first 15 minutes under load. Fill out the directory, button the cover, and take a photo of the inside of the door for your records. The next electrician who opens this panel will thank you, even if it is future-you.