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

Plan the load before you cut a single knockout

Before you touch the feeder, run the load calc per NEC 220. Sum continuous loads at 125 percent, add non-continuous at 100 percent, and factor demand per 220.82 or 220.83 if you're working residential. A 100A subpanel on a 200A service sounds generous until the HVAC inrush and EV charger show up on the same leg.

Confirm the feeder conductor ampacity against 310.16 and the terminal rating at both ends, 60C or 75C per 110.14(C). If the existing service panel has 60C lugs, you're derating no matter how fat the copper is. Also verify the feeder OCPD in the main panel is sized for the conductor, not the subpanel bus rating.

Check available fault current at the subpanel location. Series-rated combinations only work if the main panel and sub are specifically listed as a matched set per 110.22(C). Don't assume, read the label.

Feeder conductors, grounding, and the four-wire rule

Since the 2008 cycle, NEC 250.32(B) requires four wires to any subpanel in a separate structure, two hots, a neutral, and an equipment grounding conductor. Inside the same building it's always been four-wire. The neutral and ground must be isolated at the sub, no bonding screw, no bonding strap, no exceptions.

Size the EGC per 250.122 based on the feeder OCPD, not the feeder conductor. A 100A breaker needs a #8 copper EGC minimum. If you upsized the ungrounded conductors for voltage drop, upsize the EGC proportionally per 250.122(B), this one gets missed on inspections constantly.

If the previous installer left a bonding screw in the neutral bar at the sub, pull it before you energize. I've seen nuisance GFCI trips, hum on AV systems, and one case of a metal conduit carrying 14 amps of objectionable current for three years.

Panel mounting and working clearances

NEC 110.26 is not negotiable. 36 inches of depth, 30 inches of width or the width of the equipment if wider, and 6 feet 6 inches of headroom. The working space must be clear and usable, meaning no water heaters, no shelving, no stored paint cans. In a finished basement, plan this before the drywaller shows up.

Mount the panel so the highest breaker handle is no more than 6 feet 7 inches off the floor per 240.24(A). In a garage or unfinished space, follow 312.2 for damp locations if the wall backs up to masonry, and use a panel with a rainproof or damp-location listing where required.

• Verify dedicated equipment space per 110.26(E), 6 feet above the panel or to the structural ceiling
• No piping, ducts, or foreign systems in the dedicated space
• Illumination required at the working space per 110.26(D)
• Label the sub with the source panel location per 408.4(B)

Making up the feeder and branch circuits

Torque every lug to the manufacturer's spec per 110.14(D). A calibrated torque screwdriver is cheap insurance, loose connections are the number one cause of panel fires. Anti-oxidant compound on aluminum feeders is required by most manufacturers, check the panel label.

Keep the neutrals and grounds on separate bars from the start. Landing a neutral on the ground bar is a code violation and a callback waiting to happen. If the panel ships with a single bar, order the accessory ground bar kit and install it before you start making up circuits.

For branch circuits in the sub, AFCI and GFCI requirements follow 210.8 and 210.12 based on the circuit location and type, not the panel location. A subpanel feeding bedrooms still needs combination AFCI on those 15 and 20A circuits. Bathroom, kitchen, laundry, garage, and outdoor receptacles per 210.8(A) need GFCI.

Grounding electrode considerations

If the subpanel is in a separate building, you need a grounding electrode system at that building per 250.32(A). Ground rods, a concrete-encased electrode, or a metal underground water pipe if present, bonded per 250.50. Two rods 6 feet apart unless you can prove 25 ohms or less with a single rod, which nobody actually tests in practice so just drive two.

The EGC from the main building still runs with the feeder. The local electrodes bond the metal parts of the separate structure, they do not replace the EGC. This trips people up, the 1999 and earlier code allowed a three-wire feeder with a local electrode, that's gone.

Document the grounding electrode conductor path with a photo before the drywall goes up. Inspectors love this, and you'll thank yourself when you're troubleshooting stray voltage five years later.

Energize, test, and label

Before you throw the main feeder breaker, megger the feeder conductors phase to phase and phase to ground. Anything under 1 megohm, find the problem before you energize. Check phase rotation if you're feeding any three-phase equipment downstream.

Label every circuit per 408.4(A) with a specific description, "bedroom 2 receptacles" not "lights." Include the AIC rating and source on the panel directory. Verify GFCI and AFCI function with a proper tester, not just the test button, and confirm the neutral-to-ground voltage reads near zero at the sub, anything over 2 volts means you've got a bonding problem somewhere upstream.