Field guide: installing a subpanel, before you start (edition 5)

Load calc first, everything else second

Before you open a knockout or price a panel, run the load calc. A subpanel that cannot feed what the customer actually plugs in is scrap metal with breakers. Use NEC 220 Part III for standard method or Part IV for optional dwelling. Pull the existing service calc if you have it, add the new loads, and confirm the service can carry the sum before you touch the subpanel sizing.

Remember the feeder ampacity must match the calculated load, not the panel's bus rating. A 100A panel fed with 60A feeders is perfectly legal if the calc supports it. The inspector cares about the numbers, not the label on the box.

For a detached structure, NEC 225.39 sets the minimum feeder disconnect rating. Residential is 100A minimum if the structure has more than two branch circuits. Plan for that floor even on small garages or shops.

Feeder conductors and OCPD sizing

Feeder sizing comes from NEC 215.2 and Table 310.16. For a 100A feeder to a dwelling subpanel, 215.2(A)(1) Exception and 310.12 allow reduced conductors, #4 copper or #2 aluminum at 83% of service rating for the main power feeder. Read 310.12 carefully. It applies to the main feeder supplying the entire dwelling load, not every subpanel in the house.

Size the OCPD at the source. A 100A subpanel gets a 100A breaker or fuse in the main panel unless the panel has a main breaker that serves as the disconnect. Check the panel label for max OCPD rating stamped on the bus.

• 60A feeder: #6 CU or #4 AL, 60A OCPD
• 100A feeder: #3 CU or #1 AL per 310.16, or #4 CU/#2 AL per 310.12 if it qualifies
• Add one size for 3+ current-carrying conductors in conduit per 310.15(C)(1)
• Derate for ambient temperature above 86F per 310.15(B)

Four-wire feeder, always

Since the 2008 cycle, subpanels require a four-wire feeder: two hots, a neutral, and a separate equipment grounding conductor. NEC 250.32(B)(1) is explicit. The neutral and ground bars must be isolated at the subpanel. Remove the bonding screw or strap. This is the single most common callback on a subpanel install.

If you are retrofitting an old three-wire feeder to a detached building, do not rebond the neutral at the sub. Pull a proper grounding conductor back to the service, or run a new feeder. The old 250.32 exception is gone.

Ground bar gets bonded to the enclosure. Neutral bar floats on its plastic standoffs. Verify with a meter before energizing: continuity between neutral bar and enclosure should read open.

Grounding electrodes at separate structures

If the subpanel feeds a separate building or structure, NEC 250.32(A) requires a grounding electrode system at that structure. This is in addition to the EGC in the feeder. Rod, plate, ring, or concrete-encased electrode, whatever the site supports.

Two rods at least six feet apart if you cannot verify 25 ohms or less with a single rod. Bond the electrode conductor to the ground bar in the subpanel, not the neutral. Size the GEC from 250.66 based on the largest feeder conductor.

1. Drive rods before trenching so you know where the GEC exits
2. Use an acorn clamp listed for direct burial
3. Keep the GEC continuous or irreversibly spliced per 250.64(C)
4. Protect the GEC from physical damage where it leaves the ground

Working space and mounting

NEC 110.26 working space is not negotiable. 36 inches deep, 30 inches wide or the width of the panel (whichever is greater), and 6.5 feet high. Measure before you cut drywall or set the box. Water heaters, furnaces, and shelving in front of a panel will fail inspection and create a hazard for whoever opens it at 2 AM.

Mount height: the highest breaker handle cannot exceed 6 feet 7 inches per 240.24(A). In a garage or basement this is rarely an issue, but in a tight mechanical closet it catches people.

Shoot a laser line for the top of the panel before you fasten. A panel mounted crooked reads as sloppy work to every inspector and every electrician who opens it after you.

Permits, labels, and the paper trail

Pull the permit. Unpermitted subpanels surface during real estate transactions and insurance claims, and the remediation cost dwarfs the permit fee. Most jurisdictions require a load calc worksheet, a one-line diagram, and a panel schedule with the application.

Label the feeder breaker in the main panel with the subpanel location. Label the subpanel itself with the source per NEC 408.4(B). Fill out the circuit directory with real descriptions, not "lights" and "outlets." Future you, or the next electrician, will thank present you.

• Arc-fault and ground-fault requirements follow the branch circuit, not the panel location, per 210.8 and 210.12
• Series-rated combinations must be field-labeled per 110.22(C) if used
• Available fault current marking per 110.24 applies to service equipment, verify local amendments for subpanels