Field guide: installing a subpanel, for apprentices (edition 3)

Before you pull a single wire

A subpanel job lives or dies in the prep. Walk the run first. Measure the distance from the main to the subpanel location, check the path for studs, fire blocks, and HVAC, and confirm the customer actually has the service capacity to feed it. Load calc per NEC 220, Part III. If the main is already tight, stop and have the conversation before you cut drywall.

Pull the panel schedule on the main. Identify which circuits are moving, which are staying, and what the new subpanel will actually carry. Apprentices skip this and end up pulling a 60A feeder to a panel that needs 100A by day two.

• Confirm feeder size against NEC 215.2 and Table 310.16.
• Verify available fault current for the new panel (NEC 110.24).
• Check working space per NEC 110.26, 30 inches wide, 36 inches deep, 6.5 feet high.
• Locate the grounding electrode system you will bond to.

Feeder sizing and the neutral

Most residential subpanels land at 60A or 100A. For a 100A feeder in a dwelling, NEC 310.12 lets you use 4 AWG copper or 2 AWG aluminum when the feeder supplies the entire dwelling load. If this subpanel is feeding a garage, shop, or addition, you are back to the standard ampacity tables, usually 3 AWG copper for 100A at 75C.

The neutral is where apprentices get burned. A subpanel feeder always runs four wires when the subpanel is in a separate structure or served from a main panel in the same building: two hots, one neutral, one equipment grounding conductor. The neutral and ground must be isolated at the subpanel. Remove the main bonding jumper. Remove the green bonding screw. If you leave it in, you have parallel neutral paths on the EGC, and that is a callback or worse.

If the panel came with the bond screw installed from the factory, put it in a ziplock and tape it inside the dead front. Next guy will thank you, and the inspector sees you knew the rule.

Grounding at a separate structure

Detached garage, pool house, barn. NEC 250.32 governs. You need a grounding electrode at the separate structure, typically two ground rods 6 feet apart, or a single rod if you can prove 25 ohms or less (good luck). Bond the EGC from the feeder to the grounding electrode conductor at the subpanel. Neutral stays isolated. No exceptions for small buildings anymore, the 2008 code killed the three wire feeder to outbuildings.

If the subpanel is in the same building as the service, you do not drive new rods. The EGC from the feeder is the ground reference. Do not add rods thinking it helps. It does not, and it can create objectionable current paths under NEC 250.6.

Mounting and conductor entry

Set the panel so the top breaker is no higher than 6 feet 7 inches above the finished floor, per NEC 240.24(A). Plumb and level matters more than you think, crooked panels telegraph sloppy work to every inspector and every electrician who opens it after you.

Use the right connectors for your wiring method. SER cable needs a listed SER connector, not a standard NM clamp. EMT or PVC needs proper bushings on the conductor ends, NEC 300.4(G) for 4 AWG and larger. Torque every lug to the label value, use a calibrated torque screwdriver, and document it if the spec calls for it.

1. Mount the enclosure, knockouts down if feeder enters from below.
2. Land the two hots on the main lugs or backfed breaker.
3. Land the neutral on the isolated neutral bar.
4. Land the EGC on the ground bar, which is bonded to the enclosure.
5. Torque to spec, mark each lug with a paint pen when done.

Breakers, circuits, and labeling

Match breakers to the panel. A Square D QO panel takes QO breakers, not Homeline, not classified breakers unless the panel label explicitly allows them. NEC 110.3(B) is not optional. AFCI and GFCI requirements follow the circuit, not the panel, so kitchen small appliance branch circuits still need GFCI per NEC 210.8, and most dwelling circuits need AFCI per NEC 210.12 regardless of whether they originate at the main or the sub.

Label every circuit. Legibly. Typed or printed, not chicken scratch in pencil. NEC 408.4(A) requires it, and a clear directory is the difference between a 10 minute service call and an hour of breaker flipping.

Take a phone photo of the finished panel interior before you put the dead front on. Saves you the next time you need to remember which phase landed where.

Before you energize

Megger the feeder if it is anything longer than a short run, especially underground. Check phase to phase, phase to neutral, phase to ground. Anything under 1 megohm, find the problem before you throw the breaker.

Verify the feeder breaker at the main is sized to protect the feeder conductors, not the subpanel bus. A 100A subpanel can be fed by a 60A breaker if that is all the load needs, NEC 408.36 allows it as long as the feeder OCPD does not exceed the panel rating. Energize, check voltage L-L and L-N on every bus, then load test one circuit at a time.