Field guide: installing a subpanel, for master electricians (edition 6)

Plan the feeder before you open a knockout

Subpanel jobs fail at the paperwork stage, not the lug stage. Before you stage anything, confirm the feeder OCPD, conductor ampacity, and the subpanel's MLO or main breaker rating line up per NEC 215.2 and 408.36. The feeder breaker in the service panel sets the subpanel's supply ampacity, and your conductors have to carry it under 310.16 with the usual temperature and bundling corrections.

Walk the run. Measure it. Distance matters because voltage drop gets ugly past 100 feet on a 60A or 100A feeder, and the inspector will not calculate it for you. Aim for 3% on the feeder per 215.2(A)(1) Informational Note No. 2, even though it is not a hard rule, because the branch circuits downstream still need their own 3%.

• Confirm feeder size: 4 AWG Cu or 2 AWG Al for 100A at 75°C, assuming no derates.
• Verify the grounding electrode conductor and equipment grounding conductor sizes against 250.66 and 250.122.
• Check the subpanel bus rating. A 100A feeder into a 125A bus is fine. A 150A feeder into a 100A bus is not.

Four wires, always, on a separate structure or remodel

The 2008 NEC killed the old three-wire feeder exception for separate buildings. Today you pull four wires: two hots, an insulated neutral, and an equipment grounding conductor. Neutral and ground are isolated at the subpanel. Period. Pull the bonding screw or strap, verify the neutral bar floats, and land the EGCs on the ground bar bonded to the enclosure per 250.32(B).

If you are retrofitting and the existing feeder is three-wire, you are not grandfathered the moment you touch it. Treat the work as new and pull a proper EGC, or run it in a code-compliant raceway where the raceway itself qualifies under 250.118.

If the neutral bar is still bonded when you power up, you will energize the EGC and anything metallic tied to it. Test between the neutral bar and the can with a meter before you close the door. Five seconds of diligence beats a callback or a shock.

Grounding on a detached structure

Detached garages, shops, and ADUs need their own grounding electrode system per 250.32(A). That usually means two ground rods 6 feet apart unless you can prove 25 ohms or less with a single rod (fall-of-potential test, not a multimeter guess). Bond the GEC to the subpanel's ground bar, size it from 250.66 based on the feeder.

Do not reuse the main building's grounding electrode as the subpanel's GES. It is a common shortcut and a common failure. The local electrode at the second structure is non-negotiable, and the EGC in the feeder is what ties the two systems together for fault current.

1. Drive two 8-foot copper-clad rods, 6 feet apart minimum.
2. Run 6 AWG Cu GEC (for most residential feeders) continuous, no splices except irreversible.
3. Bond to the first rod with an acorn or listed clamp, lace through to the second.
4. Land the GEC on the ground bar, not the neutral bar.

Load calculations and breaker inventory

Run an Article 220 calc before you buy breakers. The standard method under 220.40 through 220.60 works for most residential subpanels. If you are subfeeding a shop with welders, plasma cutters, or EV charging, the optional method in 220.82 or the new 220.83 for dwelling additions may save you amperage and money, but only if the existing main can carry it.

Stock breakers that match the panel. Classified breakers are UL listed to fit in another manufacturer's panel, but some AHJs still push back, and the panel label is what the inspector reads. If the label says "Square D QO only," you are installing Square D QO.

Label the deadfront before you energize. Write the circuit, the room, and the amperage in pencil first, then hit it with a label maker. Pencil saves you when the homeowner points at "bedroom" and means the other bedroom.

Working space, support, and the stuff inspectors flag

110.26 is the article that gets electricians red-tagged more than any other. 30 inches wide, 36 inches deep, 6.5 feet high of clear working space in front of the panel. No water heaters, no shelving, no stored bikes. If you install a subpanel in a garage, the homeowner will fill that space the week after final, so talk to them.

Support the feeder within 12 inches of the enclosure if it is NM cable (334.30), within 3 feet for most raceways. Use the correct connector for the wiring method. AL-rated lugs with the right torque, checked with a calibrated wrench, not a guess. Anti-oxidant on aluminum feeders where the lug manufacturer requires it.

• Torque all lugs to the label spec. Most 100A main lugs are in the 120 to 275 in-lb range.
• Fill knockouts you did not use. Open knockouts fail inspection under 110.12(A).
• Verify AFCI and GFCI requirements downstream per 210.8 and 210.12, because a subpanel does not exempt branch circuits.

Energize, verify, document

Before you flip the feeder breaker, meter the subpanel dead. Check continuity between neutral and ground (should read open in a properly isolated subpanel), verify bus-to-bus has no short, and confirm all branch breakers are off. Energize the feeder, then verify 240V across the hots and 120V from each hot to neutral and to ground. Any deviation, shut it down and find it.

Leave a legible directory, a photo of the torque settings, and a note of the load calc with the homeowner's file. Next electrician in that panel, probably you in five years, will thank you.