Field guide: installing a subpanel, for apprentices (edition 5)

Plan the load and the location before you pull a permit

Start with a load calculation per NEC 220. If the subpanel feeds a garage, shop, or ADU, size the feeder for the calculated load plus future capacity the customer will actually use. Undersized feeders are the number one callback on subpanel jobs.

Location matters. Working space per NEC 110.26 is 30 inches wide, 36 inches deep, and 6.5 feet high. No storage, no water heater crowding the dead front, no shelving in front of it. If the panel goes in a garage or basement, confirm the wall is dry and the breaker handle sits between 4 and 6 feet off the finished floor.

• Calculate load: NEC 220, Part III for feeders
• Confirm working clearance: NEC 110.26(A)
• Verify panel rating matches or exceeds the feeder OCPD
• Check that the main panel has a breaker slot and bus capacity for the feeder breaker

Size the feeder and the overcurrent protection

Pick the feeder breaker first, then the conductors. For a 100 amp subpanel fed from a 200 amp main, a 100 amp double-pole breaker in the main with 3 AWG copper or 1 AWG aluminum THHN in conduit is typical at 75°C terminations per NEC Table 310.16. Always check the termination temperature rating on both ends, the lowest rating governs.

Equipment grounding conductor sizes come from NEC Table 250.122. For a 100 amp feeder, that is 8 AWG copper. The grounded (neutral) conductor is sized per the calculated neutral load, NEC 220.61, but never smaller than the required grounding electrode conductor where applicable.

Field tip: when you run 1 AWG aluminum SER or XHHW feeders, torque the lugs with a calibrated driver and hit them again after the first heat cycle. Loose aluminum terminations are the reason insurance adjusters know our trade by name.

Four-wire feeder, and why the neutral floats

Every subpanel in a separate enclosure gets a 4-wire feeder: two hots, one neutral, one equipment grounding conductor. NEC 250.24(A)(5) and 250.142 are clear. The neutral and ground are bonded only at the service disconnect. At the subpanel, the neutral bar must be isolated from the enclosure, and the equipment grounding conductors land on a separate ground bar bonded to the can.

Pull the bonding screw or strap out of the subpanel neutral bar. Leave it in the main. If you forget this step, neutral current rides the grounding conductors and every metal surface in the building becomes part of the return path. It will also trip a GFCI upstream if one is protecting the feeder.

• Main bonding jumper: installed at service disconnect only, NEC 250.28
• Subpanel neutral bar: isolated from enclosure
• Subpanel ground bar: bonded to enclosure, added if one is not factory installed
• Separate structure? Check NEC 250.32 for grounding electrode requirements

Conductor routing, conduit fill, and the physical install

Support conduit per NEC 358.30 for EMT or 344.30 for RMC. Secure cables per NEC 334.30 for NM. Box fill at the subpanel itself is not an issue, but your pull points and LBs need to follow NEC Chapter 9, Table 1 for fill and Table 4 for conduit area. For a 100 amp feeder with three current-carrying conductors and a ground, 1 inch EMT is usually sufficient for THHN, verify with the fill tables anyway.

Keep bends under the cumulative 360 degree limit between pull points. When you land conductors, strip only what the lug requires, keep the insulation up to the lug body, and route conductors around the gutter, not across the breaker stabs. Neat wiring is not cosmetic, it is required per NEC 312.7 and it makes troubleshooting twenty years from now possible.

Field tip: label the feeder breaker in the main panel with the subpanel location and the subpanel itself with the source panel location. Every inspector notices. Every electrician who comes after you will thank you.

Grounding electrodes at a separate structure

If the subpanel is in a detached garage, shop, or outbuilding, NEC 250.32 requires a grounding electrode system at that structure. Two 8 foot ground rods spaced at least 6 feet apart, bonded to the subpanel ground bar with 6 AWG copper minimum, is the common approach. The equipment grounding conductor still runs with the feeder, the rods do not replace it.

One exception: a single branch circuit to an outbuilding with an equipment grounding conductor does not require a grounding electrode. Anything more than that, install the rods. Drive them flush or below grade and protect the GEC connection with a listed acorn clamp or exothermic weld.

Commissioning and sign-off

Before you energize, torque every lug to the manufacturer's spec, NEC 110.14(D) now requires this to be done with a calibrated tool. Verify the neutral is isolated with an ohmmeter between the neutral bar and the enclosure, you should read open. Confirm the ground bar is bonded, you should read continuity.

Energize the feeder breaker, check voltage: 240 hot to hot, 120 hot to neutral on each leg, and less than 2 volts neutral to ground at no load. Load up the subpanel one circuit at a time and recheck neutral to ground under load. Anything climbing past a few volts means a shared neutral or a bonding problem upstream.

1. Torque all terminations to spec with a calibrated driver
2. Verify neutral isolation and ground bonding with a meter
3. Check voltage at no load, then under load
4. Update the main panel directory and the subpanel directory
5. Photograph the finished install for your records before the cover goes on