Field guide: installing a subpanel, before you start (edition 2)

Load calc first, everything else second

Before you pull a single wire, run the load. Article 220 exists for a reason. A subpanel sized off vibes is a callback waiting to happen, or worse, a nuisance trip every time the dryer and the microwave run together. Pull the existing service load per 220.83 if you are adding to an existing dwelling, and confirm the main has headroom for what you are about to hang off it.

Most residential subpanels land at 60A, 100A, or 125A. Detached garages with a welder and a mini-split do not belong on a 60A feeder. Do the math, then add the future EVSE the homeowner swears they will never install but absolutely will.

• Existing service demand load per 220.83
• New loads added at 100% for non-continuous, 125% for continuous (210.19(A), 215.2)
• Verify main panel bus rating and breaker space before you promise anything

Feeder sizing and the neutral conversation

Feeder conductors follow 215.2 and the ampacity tables in 310.16. For a 100A feeder in a dwelling, 334.80 lets you use the 60C column for NM cable, which means #4 copper or #2 aluminum if you go that route. Running in conduit? You get more flexibility, but terminations still cap you at 75C for most equipment.

The neutral is where guys get lazy. Per 220.61, the feeder neutral can be sized for the maximum unbalanced load, but do not undersize it on a panel feeding a lot of 120V circuits. And the EGC is sized per 250.122 based on the feeder overcurrent device, not the conductor. A 100A feeder needs a #8 copper EGC, period.

If you are running aluminum SER to a detached structure, antioxidant paste and torque specs are not optional. Loose aluminum lugs are the number one reason I get called back to burned panels.

Four-wire feeder, always

The three-wire feeder to a detached building is dead. 250.32(B) was revised back in 2008, and since then every new subpanel, attached or detached, gets four wires: two hots, an insulated neutral, and a separate equipment grounding conductor. The neutral and ground must be isolated at the subpanel.

This means pulling the bonding screw or strap from the subpanel neutral bar. Add a separate ground bar, kitted or field-installed, and land every EGC there. Neutrals on the neutral bar, grounds on the ground bar, and never the twain shall meet downstream of the service disconnect.

1. Remove factory bonding screw from neutral bar
2. Install dedicated ground bar, bonded to the enclosure
3. Land feeder neutral on neutral bar, feeder EGC on ground bar
4. Verify continuity from subpanel ground bar back to service grounding electrode system

Grounding electrodes at a detached structure

If the subpanel feeds a separate building or structure, 250.32(A) requires a grounding electrode system at that building. Two ground rods spaced at least six feet apart, unless you can prove a single rod hits 25 ohms or less, which in practice nobody measures, so just drive two.

Bond those rods to the subpanel ground bar with a #6 copper GEC per 250.66. The EGC in the feeder handles fault current back to the source; the local electrodes handle lightning and stray voltage. Both have to be there.

I carry a spare ground rod clamp and a 20 foot coil of bare #6 on every subpanel job. You will need it, and the supply house is always 40 minutes away.

Working space, mounting, and the stuff inspectors actually check

110.26 is the article that fails more rough inspections than any other. Thirty inches wide, three feet deep, six and a half feet high, clear. No shelving, no water heater, no washing machine crammed in front of the panel. If the homeowner wants it in the laundry room, show them 110.26 on your phone and let the code have the argument for you.

Mount height: the operating handle of the highest breaker cannot exceed 6 feet 7 inches per 240.24(A). Dedicated equipment space above the panel per 110.26(E), no plumbing, no HVAC ducts passing through that zone. And label the thing. 408.4 requires every circuit to be legibly identified at the panel, and "lights" is not a circuit description.

• 30" wide x 36" deep x 78" high clear working space (110.26(A))
• Handle height max 6'-7" (240.24(A))
• Dedicated space 6' above panel, no foreign piping (110.26(E))
• Legible circuit directory at completion (408.4)

Before you cut in, walk the job

Five minutes with a flashlight saves five hours with a sawzall. Trace the feeder path. Check joist direction, fireblocking, existing romex runs, and whether that wall you want to surface-mount on is actually a shear wall the GC will lose his mind over. Confirm the main panel has a two-pole space at the correct amperage, not just open slots.

Pull the permit. Yes, even for a 60A garage sub. Unpermitted panel work follows the house to resale and becomes somebody else's problem, usually yours when the realtor calls. Get it inspected, get it signed, move on.

Tools staged, conductors on the truck, torque screwdriver calibrated, labels printed. Then you cut in.