Field guide: installing a subpanel, industrial version (edition 3)

Plan the feeder before you touch a knockout

Industrial subpanels live or die on feeder math. Start with the calculated load per NEC 220, then size the feeder conductors per 215.2 with the ampacity tables in 310.16. Voltage drop is not code, but if your run pushes past 100 feet at full load, upsize one trade size and document it. Informational Note No. 2 to 215.2(A)(1) recommends 3% on feeders, 5% total.

Pull the one-line and confirm the upstream OCPD. If the existing service is 480Y/277V and you are landing a 208Y/120V subpanel, you need a transformer and a separately derived system, not just a feeder. That changes grounding entirely under 250.30.

Confirm available fault current at the point of connection. The new panel must have a series or fully-rated SCCR that meets or exceeds it. 110.24 requires the calculated AFC to be field-marked.

Sizing the panel and OCPD

Industrial loads rarely match a residential rule of thumb. Run the actual load calc using Article 220 Part III, apply demand factors where they legitimately exist (welders per 630, motors per 430), and add 25% of the largest motor per 430.24. Do not lean on a 100A "should be fine" number. Inspectors and insurance carriers want the worksheet.

Pick a panel with at least 25% spare capacity in both amps and physical spaces. Future tenant fit-outs are not your problem until they are. A 42-circuit panel with eight open spaces buys you nothing if the bus is already loaded to 95%.

• Match panel bus rating to feeder OCPD, not feeder ampacity
• Verify breaker brand compatibility, classified breakers void some UL listings
• Confirm AIC rating against the AFC study
• Check ambient temperature derating for the room, mechanical spaces run hot

Grounding and bonding, get this right or redo it

This is where most subpanel jobs go sideways. A subpanel fed from another panel in the same building is NOT a separately derived system. The neutral and ground must be isolated. Remove the main bonding jumper, install a separate equipment grounding bar, and land the EGC there. The grounded conductor lands on an isolated neutral bar with no contact to the enclosure. NEC 250.142(B) is explicit.

If you see a green screw driven into the neutral bar of a subpanel, pull it. I have chased ground loops for hours that turned out to be a forgotten bonding screw from the manufacturer.

For separately derived systems (transformer-fed), bond the X0 to the nearest effective grounding electrode per 250.30(A). The system bonding jumper goes at the source or the first disconnect, not both. Pick one and document it.

Run a properly sized EGC with the feeder per 250.122. Do not rely on conduit alone for industrial work, even where 250.118 allows it. A loose locknut on a long EMT run is a fault path waiting to fail.

Working space, mounting, and access

110.26 is not negotiable. 36 inches of depth in front of the panel, 30 inches of width or panel width (whichever is greater), and 6.5 feet of headroom. In industrial spaces this gets compromised by conveyors, racking, and forklift traffic. Walk the area at shift change before you mount anything.

For panels rated 1200A or more, or where the AFC requires arc-flash labeling per 110.16, you also need personnel doors that swing out with panic hardware if working space is more than 25 feet from an exit. 110.26(C)(2) covers it.

1. Confirm dedicated electrical space per 110.26(E), nothing foreign above the panel to the structural ceiling or 6 feet, whichever is lower
2. Mount so the highest breaker handle is no more than 6 feet 7 inches above the floor (240.24)
3. Leave room for future conduit entries on top and bottom
4. Use unistrut and standoffs on block or tilt-up walls, never anchor a panel directly to unfinished CMU

Conductor terminations and torque

110.14(D) requires torque values from the listing or manufacturer to be applied with a calibrated tool. "Snug plus a quarter turn" is not a defense in a failure investigation. Carry a calibrated torque screwdriver and torque wrench, and re-verify after the conductors have been energized for a few load cycles.

For aluminum feeders, use listed antioxidant compound, wire-brush the strands, and torque per the lug manufacturer. AL9CU lugs are required for aluminum, do not assume the panel ships with them.

Mark every torqued lug with a paint pen. When you come back in six months for the addition, you know what has been touched and what has not.

Labeling, documentation, and turnover

408.4(A) requires every circuit to be legibly identified at the panel. "Spare," "lights," and "receptacles" are not identifications. Use room numbers, equipment tags, or grid coordinates that match the as-built drawings.

110.22 requires the disconnect to be marked with its purpose unless obvious. 110.24 wants the available fault current and the date of the calculation. 110.16 wants the arc-flash warning, and if you have done an incident-energy analysis, the label per 130.5(H) of NFPA 70E.

Hand the customer a folder: load calc, AFC calc, panel schedule, torque log, megger readings on the feeder, and the inspection sign-off. The next electrician on this panel will either thank you or curse you, and that folder decides which.