Field guide: installing a subpanel, for master electricians (edition 1)

Sizing the feeder before you touch a knockout

Load calc first, conduit second. Article 220 Part III is your friend when the homeowner says "just add a subpanel in the garage" and you know they're planning a 60 amp EV charger, a mini split, and a welder. Pull the main panel schedule, add the existing demand, then stack the new loads per 220.82 or 220.83 depending on whether this is a new service or an addition.

Feeder ampacity comes from 215.2 and 310.16. Remember 215.2(A)(1) requires the feeder to carry the calculated load at 125% for continuous loads. Neutral sizing falls under 220.61, and on a subpanel you do not get to reduce the neutral below the grounded conductor requirement for line-to-neutral loads.

Voltage drop is a recommendation, not a rule, but 210.19 Informational Note 4 and 215.2 Informational Note 2 both point to 3% branch, 5% combined. On a 150 foot run to a detached garage at 100 amps, 1/0 copper or 2/0 aluminum is usually where you land. Do the math, do not guess.

Four wires to a subpanel, always

This is the one that still trips guys who learned the trade before the 2008 cycle. A subpanel, whether in the same building or a separate structure, gets two hots, an insulated neutral, and an equipment grounding conductor. The neutral bar floats. The ground bar bonds to the enclosure. 250.32(B)(1) made the old three-wire feed to a detached building obsolete for new installs.

Pull the bonding screw or strap out of the neutral bar and bag it with the panel paperwork. If the inspector opens the dead front and sees that screw installed, you are coming back. Add a separate ground bar kit if the panel did not ship with one, and land every EGC there.

"I keep a sharpie in my bag and write NEUTRAL FLOATED on the inside of the dead front the day I trim it. Saves arguments two years later when somebody else opens it up." , journeyman in Phoenix

Grounding electrodes at a separate structure

If the subpanel feeds a separate building or structure, 250.32(A) requires a grounding electrode system at that structure. One ground rod is almost never enough on its own. 250.53(A)(2) requires a supplemental electrode unless a single rod tests at 25 ohms or less, and nobody is pulling out a fall-of-potential tester on a garage job. Drive two rods, six feet apart, and move on.

Bond the EGC from the feeder to the grounding electrode conductor at the subpanel. Do not bond the neutral to ground at the separate structure, that is still the same mistake as running three wires. The GEC sizing comes from 250.66, usually #6 copper for residential electrode conductors to rods.

• Two driven rods minimum, 250.53(A)(2), unless you test
• #6 copper GEC to rods per 250.66(A)
• Acorn clamp listed for direct burial, 250.70
• Irreversible crimp or exothermic weld if you splice the GEC

Conductor fill, protection, and the panel itself

Feeder in conduit gets 310.16 ampacity at the 75 degree column for most terminations. Check the panel label. If the lugs say 60/75C and the breaker on the supply end says 75C, you are good at 75C. Derate for conduit fill per Table 310.15(C)(1) once you get past three current-carrying conductors, which on a feeder you usually do not.

If the feeder passes through or under the building and enters a detached structure, you need a disconnect at the structure per 225.31 and 225.32. A main breaker panel satisfies this. A main lug panel does not, unless you add a disconnect ahead of it within sight.

Physical protection matters. 300.5 for direct burial depths, 24 inches for residential feeders under driveways without concrete cover, 18 inches under a yard with rigid PVC. Sleeve the riser in Schedule 80 where it comes out of grade, 352.10(F).

Trim, label, and the inspector conversation

Label the subpanel per 408.4(A) with circuit directory that actually describes the load, not "lights." Label the feeder disconnect at the main panel as the source, and label the subpanel with the source location per 408.4(B). Arc flash labeling under 110.16 if it is over 1200 amps, which it will not be on residential, but know the rule.

AFCI and GFCI requirements follow the branch circuits, not the panel. 210.8 and 210.12 apply downstream. If you are adding circuits during the subpanel install, the new circuits need current-cycle protection even if the rest of the house predates it.

"The inspector does not care how clean your wire dress is if the panel schedule is wrong. Fill it out in pen, not pencil, before you call for inspection." , master electrician in upstate New York

Common callbacks and how to avoid them

Three failure modes show up over and over on subpanel installs. Neutral-ground bond left in place at the subpanel, missing second ground rod at a detached structure, and undersized EGC in the feeder because somebody used the old three-wire assumption. Walk the job once before you leave.

1. Dead front off, verify neutral bar is isolated from the can
2. Ohm from neutral bar to ground bar at the subpanel, should read open
3. Confirm EGC size against 250.122 and the feeder OCPD
4. Torque every lug to the label spec, mark with a torque stripe
5. Check the grounding electrode connections are tight and listed

A subpanel is not complicated work, but it is unforgiving work. The code has been consistent on four-wire feeders and separate grounding electrodes for almost two decades now. If you learned it before then, relearn it. The inspectors have.