Field guide: installing a subpanel, during the job (edition 2)

Before you pull the cover

You already surveyed the main, mapped the feeder path, and sized the conductors. Now you are standing in front of an energized panel with a subpanel box in the truck. This is the phase where small mistakes cost hours, so slow down for the first ten minutes and verify what you assumed on paper still matches what is in the wall.

Check the main breaker rating against your feeder calc. If the service is 200A and you planned a 100A subpanel, confirm the existing load calc in NEC 220 still has room. Also confirm the directory. Half the panels you open will have labels from two owners ago, and that two-pole on the left marked "pool" is actually the range.

Lock out, tag out, test. Every time. The panel you worked in yesterday is not the panel you are in today.

Mounting and knockouts

Set the can before you touch a single conductor. Dead-front height should land the top breaker no higher than 6 ft 7 in to the operating handle per NEC 240.24(A). In a finished space, shim the box flush with the drywall face. If it sits proud, your cover will cam and the deadfront screws will strip.

Pick your knockout based on feeder entry direction and bend radius. A 1 inch EMT feeder with THHN needs room to sweep, and a top-entry KO on a tight ceiling will fight you. For NM, use the correct cable connector, not a romex clamp jammed into a conduit KO. Concentric knockouts need a bonding bushing or bonding locknut on anything over 250V to ground, per NEC 250.97, and on service equipment regardless of voltage.

• Ream every conduit end. THHN jacket splits on burrs.
• Use reducing washers only when you cannot find the right KO size, and never stack more than two.
• Seal outdoor or damp-location KOs with listed fittings, not duct seal as an afterthought.

Landing the feeder

Strip, torque, document. The feeder is the one termination that will bite you hardest if it loosens, so use a calibrated torque screwdriver and hit the value printed on the lug or inside the door. NEC 110.14(D) now requires it, and inspectors are actually checking.

For a 4-wire feeder to a separate structure or a subpanel inside the same building, the neutral and the equipment ground land on separate bars. Remove the main bonding jumper, the green screw, or the bonding strap, whatever the manufacturer ships. This is the single most common failure point on a subpanel install, and it will put objectionable current on the ground path per NEC 250.6 if you miss it.

If you can read a voltage between the neutral bar and the ground bar with the main on and a load running, the bond is still in. Pop the deadfront and find it before the inspector does.

Grounding and bonding at the subpanel

Inside the same building, you pull an equipment grounding conductor with the feeder, sized per NEC 250.122 based on the feeder overcurrent device. No local ground rod required, and no re-bonding. The EGC lands on the ground bar, which is bonded to the enclosure.

If the subpanel feeds a detached structure, the rules shift. Under the 2008 and later NEC, you run 4 wires and drive grounding electrodes at the separate building per NEC 250.32(B). Two rods 6 ft apart unless you can prove 25 ohms with a single rod, which you cannot, so just drive two. Bond the electrode conductor to the ground bar, keep the neutral isolated.

• EGC sized from NEC Table 250.122, not the feeder ampacity table.
• GEC at a detached structure sized from NEC Table 250.66 when using a rod, minimum #6 Cu.
• Intersystem bonding termination required at the detached structure per NEC 250.94.

Branch circuits and balancing

Load the subpanel so the legs balance within roughly 20 percent. Big inductive loads, HVAC, well pumps, welders, go near the top where the bus is coolest and the lugs are beefiest. Keep AFCI and GFCI breakers grouped if the panel layout allows, so the pigtail neutrals are not running the full height of the can.

Watch your neutral terminations on multiwire branch circuits. Each ungrounded conductor of an MWBC needs a means of simultaneous disconnect per NEC 210.4(B), either a two-pole breaker or an identified handle tie. A wire nut in the panel is not a handle tie.

Label every MWBC at the breaker and at the first box. The next electrician in this panel is you in three years, and you will not remember.

Close-out and the walk

Before the deadfront goes back on, do a visual. Every conductor under a lug, no copper showing above the strip line, no insulation under the screw. Neutrals one per hole on the bar, grounds can share per manufacturer listing but check the label inside the door.

Fill out the panel directory in pen, not pencil. List circuits by room and function, not by breaker number. Snap a photo of the open panel before you button it up, it saves a trip back when the GC calls you about a tripped breaker at 7pm.

1. Torque verified and marked.
2. Bonding jumper removed, neutral isolated.
3. EGC landed, enclosure bonded.
4. Directory complete, cover square, screws snug.
5. Megger or at minimum an insulation check before energizing anything exotic.

Energize from the main first, verify voltage at the feeder lugs with the subpanel main off, then close the sub main and check each leg to neutral and leg to ground. 120 and 240 where you expect them, zero between neutral and ground. Then start turning on circuits.