Field guide: installing a subpanel, material list (edition 5)

Scope and prep before the truck leaves the shop

Subpanel adds are bread and butter, but the material list is where jobs get slowed down. A missing lug kit or the wrong breaker interlock means a return trip. Build your list off the load calc and the feeder run, not off memory.

Confirm the feeder size against NEC 215.2 and 310.16 before you pull anything from stock. Verify the subpanel is listed for use as a service or feeder panel, and check if the detached structure rule under NEC 225.30 changes your disconnect count. If the sub is in a separate building, grounding electrode system at the new structure is required per NEC 250.32(A).

Measure the feeder run twice before cutting. A 4/0 aluminum SER reel is expensive to waste because you forgot the 90 degree sweep adds three feet.

Panel, breakers, and the main lug question

Pick the enclosure based on load, spaces, and location. NEMA 1 for dry interior, NEMA 3R for exterior or damp. Size the bus for the calculated feeder, not the future dream load. A 100A sub off a 200A service is the common case; a 125A sub with a 100A feeder breaker gives you headroom without oversizing conductors.

Main breaker versus main lug: if the sub is in the same structure and a disconnect exists at the service, main lug is legal. If the sub is in a detached structure, you need a disconnect at the sub per NEC 225.31 and 225.36. Spec a main breaker panel or add a disconnect ahead of it.

• Subpanel enclosure, correct NEMA rating, correct space count (leave 25% spare spaces minimum)
• Main breaker sized to the feeder, or main lug if allowed
• Branch breakers matched to the panel manufacturer (no UL classified swap unless spec permits)
• Ground bar kit (panels rarely ship with enough lugs)
• Neutral bar isolation kit if the panel came bonded from the factory

Feeder conductors and raceway

Size the feeder per NEC 215.2 with the ampacity from Table 310.16. For a 100A feeder at 75C terminations, 3 AWG copper or 1 AWG aluminum is standard. Go one size up on aluminum if the run is long or the conduit fill is tight. Include the equipment grounding conductor per NEC 250.122, and remember the neutral is sized per the calculated unbalanced load under NEC 220.61.

The three wire feeder with a bonded neutral is gone. NEC 250.32(B) requires four conductors between structures: two hots, a neutral, and a separate EGC. The neutral lands on an isolated neutral bar, and the EGC lands on the ground bar bonded to the enclosure.

• Feeder conductors, correct size, correct length plus 10% for terminations and slack
• Conduit: EMT, PVC Schedule 40 underground, or liquidtight where flex is needed
• Fittings: connectors, couplings, two bushings per raceway entry with conductors over 4 AWG (NEC 300.4(G))
• Anti oxidant compound for aluminum terminations
• Torque screwdriver and the panel torque spec sheet

Grounding and bonding at the sub

This is where installs fail inspection. At a subpanel, the neutral bar must be isolated from the enclosure. The green bonding screw stays in the bag or gets removed if the panel came bonded. Ground bar lands on the enclosure and carries the EGC from the feeder plus all branch circuit grounds.

If the sub serves a separate building, install a grounding electrode system per NEC 250.50 and 250.52. Two ground rods 6 feet apart, or a single rod with a tested resistance under 25 ohms per NEC 250.53(A)(2). Bond the ground rod conductor to the ground bar in the sub, not the neutral.

Pull the bonding screw before you energize. Every journeyman has met a shared neutral that made the EGC carry current back to the service, and it only shows up when someone gets bit off a water line.

Finishing material and the small stuff that kills schedule

The list below is what gets forgotten on the truck and costs an hour at the supply house. Keep a subpanel kit pre built if you do these regularly.

1. Panel labels: directory, arc flash warning per NEC 110.16, available fault current per NEC 110.24
2. Knockout seals for unused openings
3. Cable staples or straps within 12 inches of the panel for NM cable (NEC 334.30)
4. Anti short bushings for MC or AC cable
5. Fire caulk or putty pads if penetrating a rated assembly
6. Wire nuts, Polaris taps if tapping a feeder, heat shrink for outdoor splices
7. Sharpie, phase tape in black, red, blue, white, green

Working clearance per NEC 110.26 is not a material, but it is on the list. Three feet in front, 30 inches wide, 6.5 feet high. If the space does not clear, the panel moves before the conductors get pulled.

Final checks before energizing

Torque every lug to the manufacturer spec. Megger the feeder if the run is long or the conduit got wet during the pull. Verify neutral is isolated, EGC is bonded, and the directory is filled out with circuit numbers and descriptions that match what a future electrician can actually use.

Photograph the panel interior before the cover goes on. Capture the torque marks, the neutral isolation, and the directory. That photo ends a lot of callback arguments six months later when someone swears the panel was wired wrong.