Field guide: installing a subpanel, step-by-step (edition 6)

Plan the load and the feeder before you touch anything

Start with a load calculation per NEC 220. If the subpanel feeds a detached structure, garage shop, ADU, or kitchen remodel, size the feeder for the actual calculated load plus reasonable future capacity. A 100A feeder covers most residential subpanels, but do not guess. Run the numbers.

Confirm the main panel has room in the busbar rating and a breaker slot for the feeder. Check the panel schedule label for the maximum breaker count and total amperage. If the main is already near its 80 percent continuous limit, stop and talk to the owner before cutting drywall.

• Calculated load per NEC 220.40 through 220.87
• Feeder ampacity per NEC 310.16 with correction and adjustment factors applied
• Voltage drop under 3 percent for the feeder, 5 percent total (NEC 210.19 Informational Note)
• Conduit fill per NEC Chapter 9, Table 1

Pick the right conductors and conduit

For a 100A feeder in residential, most shops pull 1 AWG copper or 1/0 AWG aluminum THHN/THWN-2 in 1 inch EMT or PVC. That follows NEC 310.12 for dwelling services and feeders serving the main load. For commercial or non-dwelling feeders, size off the full 75C column in Table 310.16.

Four conductors, always. Two ungrounded, one neutral, one equipment grounding conductor. The neutral and ground stay separated at the subpanel per NEC 250.24(A)(5) and 408.40. This is the single most common failure I see on rough inspections.

Pull an extra 18 inches of each conductor in the subpanel cabinet. You will thank yourself when you are making up lugs at 4pm on a Friday.

Mount, bond, and land the feeder

Mount the enclosure with the top between 6 feet 7 inches and the working space requirements of NEC 110.26 respected. Keep 36 inches of clear depth, 30 inches of width or panel width (whichever is greater), and 6 feet 6 inches of headroom. No storage, no shelving, no water heater crowding the panel.

Remove the main bonding jumper or green bonding screw from the subpanel. This is critical. The neutral bar must float. Land the equipment grounding conductor on the ground bar, which is bonded to the enclosure. Neutrals and grounds terminate on separate bars.

1. Drill or punch the knockout for the feeder raceway
2. Install a listed connector rated for the raceway type
3. Land ungrounded conductors on the main lugs or backfed breaker
4. Land the neutral on the isolated neutral bar
5. Land the EGC on the grounding bar bonded to the can
6. Verify the bonding screw is removed and bagged inside the panel

Overcurrent protection and the backfed breaker question

If you are using a main lug subpanel, the feeder overcurrent device lives in the upstream panel. If you are using a main breaker subpanel, you have overcurrent at the subpanel itself. For detached structures, NEC 225.31 and 225.32 require a disconnecting means at the structure, so a main breaker subpanel is usually the cleaner answer.

When backfeeding a breaker as the main on a subpanel, it must be secured with a listed hold-down kit per NEC 408.36(D). A zip tie is not a hold-down. The breaker must be identified for backfeed by the manufacturer, which most standard residential breakers are, but verify on the label.

Grounding electrodes for detached structures

A subpanel in the same building uses only the EGC from the feeder. No new grounding electrode required. A subpanel in a separate building is a different animal. NEC 250.32(A) requires a grounding electrode system at the detached structure.

Drive two ground rods 6 feet apart, or use one rod with a documented 25 ohm or less resistance to earth (NEC 250.53(A)(2)). Bond the rods to the grounding bar in the subpanel with a 6 AWG copper GEC, sized per NEC Table 250.66. Do not bond the neutral to ground at the detached structure. That rule changed in the 2008 NEC and still trips people up.

If the detached structure has a metal water line or rebar in a new footing, include it in the grounding electrode system per 250.52. One rod plus a concrete-encased electrode beats two rods in dry soil every time.

Label, torque, and close it out

Every breaker gets a legible circuit directory entry per NEC 408.4(A). Not "lights." Write "kitchen small appliance 1" or "garage south wall receptacles." The next electrician is you in three years and you will not remember.

Torque every lug and breaker terminal to the manufacturer spec listed on the panel label, per NEC 110.14(D). Use a calibrated torque screwdriver or wrench. Mark each terminal with a torque seal paint pen so inspectors and future service calls can see the work was done.

• Feeder lugs torqued to spec and marked
• Neutral and ground bar terminals torqued
• Arc fault and ground fault protection installed per NEC 210.8 and 210.12
• Panel cover installed, directory filled, deadfront screws snug
• Working space clear, no storage within 36 inches