Field guide: installing a subpanel, residential version (edition 5)

Plan the load and the feeder first

Before pulling a permit or picking a panel, run the calc. Residential subpanels usually feed a detached garage, a finished basement, an addition, or a kitchen remodel. Size the feeder to the calculated load under Article 220, not to whatever breaker happens to be in the truck. If the subpanel will serve a dwelling unit or a separate structure with one or more branch circuits, NEC 225.39 and 215.2 set the floor.

Pick the panel with expansion in mind. A 100A subpanel with 20 spaces costs a few bucks more than a 60A with 12, and you will thank yourself the next time the owner wants a hot tub. Confirm the panel is listed as suitable for use as service equipment only if you need it to be, otherwise a standard load center is fine downstream of the main.

Verify the feeder ampacity against 310.16 and the 83% rule in 310.12 if this is the main power feeder to a dwelling. For a true subpanel fed from an existing service, 310.12 does not apply. Use 75C column for terminations on modern panels unless the breaker or lug is marked 60C only.

Four wires, always

Since the 2008 cycle, feeders to a separate structure or a subpanel inside the same structure require four conductors: two ungrounded, one grounded (neutral), one equipment grounding conductor. NEC 250.32(B) killed the old three-wire feeder to detached buildings. If you are working on an older install with a bonded neutral at the detached garage, this is the day it gets corrected.

The neutral bar in the subpanel floats. The ground bar bonds to the enclosure. Remove the green bonding screw or strap from the neutral bar, every time, no exceptions. Miss this and you have parallel neutral current on the EGC and every metal path back to the service.

If you see scorch marks on a ground rod clamp or a warm water pipe bond, check for a missing neutral isolation at a subpanel. Nine times out of ten, that is the cause.

Grounding electrodes at a separate structure

A detached structure fed by a feeder needs its own grounding electrode system per 250.32(A). That is typically two ground rods 6 feet apart, or a single rod if you can prove 25 ohms or less, which nobody actually measures. Just drive two. Bond the EGC from the feeder to the electrode system at the subpanel, not the neutral.

Sizing the grounding electrode conductor comes from Table 250.66. For a 100A feeder with #4 copper ungrounded conductors, a #8 copper GEC to the rods is typical. The EGC itself sizes from Table 250.122 based on the feeder overcurrent device, not the conductor.

• Two rods, 8 feet, 6 feet apart minimum
• Acorn clamps listed for direct burial
• GEC continuous or irreversibly spliced per 250.64(C)
• Protect GEC from physical damage where exposed

Conductor fill, burial, and protection

If the feeder runs underground to a detached structure, Table 300.5 sets minimum cover. PVC at 18 inches, direct-buried UF at 24, rigid or IMC at 6. GFCI protection on the dwelling side does not change burial depth. Sleeve the riser in RMC or Schedule 80 PVC where it comes out of grade, per 300.5(D).

Derate for conduit fill and ambient if you have more than three current-carrying conductors or a hot attic run. The neutral is not counted as current-carrying on a 240/120 single-phase feeder because it only carries unbalance. Count it if you have a 3-wire circuit sharing a neutral on a multiwire setup, though that is rare on a feeder.

Expansion fittings on long PVC runs matter more than people think. Table 352.44 gives the expansion per 100 feet per degree F. A 60 foot exterior PVC run in a climate swinging 100 degrees will move close to an inch.

Breakers, labeling, and the handle tie question

Use breakers listed for the panel. Classified breakers are a liability conversation nobody wants to have on a callback. AFCI is required on most dwelling branch circuits per 210.12, GFCI per 210.8. If the subpanel feeds a kitchen, laundry, bathroom, garage, or outdoor receptacles, plan the GFCI breakers now, not after drywall.

240V loads on a subpanel need a 2-pole breaker or two single-poles with an identified handle tie, per 240.15(B). For multiwire branch circuits, handle ties are required at the origin per 210.4(B). Pigtail the neutrals on MWBCs so removing a device does not open the shared neutral under load.

Label the subpanel feeder breaker at the main with the destination. "SUBPANEL GARAGE" takes five seconds with a label maker and saves an hour on the next service call.

1. Torque every lug and breaker to the panel sticker value
2. Fill the directory with circuit descriptions, not "lights"
3. Install a panel schedule inside the door
4. Verify working clearance per 110.26, 30 inches wide, 36 inches deep, 6.5 feet high

Inspection-ready checklist

Before the inspector shows up, walk the install. Neutral floating, EGC bonded, grounding electrodes landed on the ground bar, feeder breaker correctly sized, knockouts filled, bushings on metal raceways over #4, and AIC rating on the panel label equal to or greater than the available fault current at that point. If you are inside a 10 kAIC panel on a service that can deliver more, you have a problem.

Torque values, handle ties, breaker compatibility, working space, and the label on the feeder breaker are the five things most commonly red-tagged on subpanel installs. Take a photo of the completed panel with the dead front off before you close it up. It is the cheapest documentation you will ever make.