Field guide: installing a subpanel, residential version (edition 6)

Sizing the feeder and subpanel before you lift a tool

Start with the load calc, not the panel you have in the truck. Run Article 220 Part III for the structure or the added loads, then pick a feeder that carries the calculated load at the terminal temperature rating on the breaker and lugs (almost always 75C for residential gear, per 110.14(C)(1)(B)). Oversizing the feeder because "copper is cheap" still has to land on lugs that are rated for the wire you bring.

Pick the subpanel enclosure based on spaces needed plus 25% headroom for future circuits, and confirm the bus rating meets or exceeds the overcurrent protection at the source. A 100A feeder breaker feeding a 125A bus panel is fine; a 125A breaker on a 100A bus is not. Check 408.36 for panelboard OCPD requirements.

For dwelling feeders, 310.12 lets you use the 83% rule when the feeder is the main power feeder to a dwelling. A subpanel fed from that main is NOT the main power feeder, so 310.12 does not apply. Size by 310.16 ampacity at 75C.

Four wires, always, to a separate structure or interior subpanel

Since the 2008 cycle, the neutral and equipment ground must be kept separate at the subpanel. Pull four conductors: two ungrounded, one grounded (neutral), one equipment grounding conductor. The EGC is sized per 250.122 to the feeder OCPD, not the feeder conductor ampacity unless the feeder was upsized for voltage drop (then 250.122(B) applies).

At the subpanel, remove the bonding screw or strap. The neutral bar floats. The ground bar bonds to the enclosure. If the panel ships with a main bonding jumper installed, it goes in the scrap bucket for this install.

• Ungrounded (hot) A and B: sized per 310.16 at 75C
• Grounded (neutral): sized per 220.61, never smaller than required by 250.122 if it carries fault current
• Equipment ground: sized per 250.122 Table
• No reduced neutral tricks on short residential feeders. The savings are not worth the callback

Detached structures and the GEC question

Feeding a detached garage, shop, or ADU triggers 250.32. One building, one feeder (or branch circuit), and you need a grounding electrode system at that second structure if it has more than one branch circuit or a feeder. Rod, ring, plate, or structural steel, whatever qualifies under 250.52. Two ground rods 6 feet apart if you use rods and cannot prove 25 ohms or less on a single rod (250.53(A)(2)).

The grounding electrode conductor at the detached structure bonds to the equipment grounding bus in the subpanel, NOT the neutral. The neutral stays isolated. This is the single most common failure on detached structure subpanels and the one the AHJ will red-tag first.

If the existing underground feeder to the garage is only three wires and you are adding circuits or a subpanel, you are pulling a new feeder. The 2008 exception for existing installations is not a license to expand a non-compliant install.

Working space, mounting, and physical install

110.26 is non-negotiable. 30 inches wide, 36 inches deep, 6.5 feet high, clear, with the panel door able to open at least 90 degrees. Do not mount a subpanel in a clothes closet (240.24(D)), bathroom (240.24(E)), or over the stairs. Garages and unfinished basements are fine.

Mount the panel plumb, screw through the back into studs or a plywood backer. Use listed connectors at every knockout, and verify the connector is rated for the cable type (NM, SE, MC, EMT fitting, whatever you are bringing in). Torque the feeder lugs to the nameplate spec. 110.14(D) now explicitly requires a calibrated torque tool; an inspector can ask to see it.

• Feeder landing: phase rotation and color consistent with the main
• Neutral lug: torque to spec, no doubled neutrals on a single hole
• Ground bar: added if the panel shipped without one, bonded to the can
• Knockouts: unused KOs closed with listed filler plates

Grounding, bonding, and the piece everyone forgets

Inside the subpanel: neutrals go to the floating neutral bar, grounds go to the bonded ground bar. If the panel has only one factory bar and the bonding screw is removed, that bar is now the neutral. You must install a separate ground bar kit and bond it to the enclosure. Mixing neutrals and grounds on the same bar at a subpanel is a violation of 250.24(A)(5) by way of 408.40.

Metal raceway or MC armor between the main and subpanel counts as an EGC only if it meets 250.118. If you are running PVC or NM cable, the EGC is the wire in the assembly. When in doubt, pull a dedicated copper EGC and stop worrying.

Final checks before you energize

Label the feeder breaker at the source panel with the subpanel location (408.4(A)). Fill out the circuit directory at the subpanel, not with "lights" and "plugs" but with room and purpose. AFCI and GFCI requirements follow the branch circuit location, not the panel location, so a subpanel in the garage still needs GFCI per 210.8(A) on the garage branch circuits it feeds.

Meg the feeder before you energize if it is longer than 50 feet or got pulled through a rough chase. Two minutes with a megger beats two hours chasing a nicked conductor with the panel live.

Energize with the subpanel main (or feeder breaker) off, verify voltage at the lugs, then bring the sub up under load one branch at a time while you watch for smoke, hum, or a nuisance trip. Sign the sticker, close the cover, take the picture for the file.