Field guide: installing a subpanel, industrial version (edition 1)

Load calc and feeder sizing

Industrial subpanels get hit with motor loads, welding receptacles, and continuous process equipment. Run the calc per NEC Article 220 Part III or IV. Part IV (optional) works well when you have demand data on existing equipment, otherwise stick with Part III. Size the feeder conductor at 125% of continuous plus 100% of non-continuous per 215.2(A)(1), then verify against the largest motor rule in 430.24.

Don't round down because the panel label says 200A. The feeder is sized to the calculated load, then the OCPD protects the conductor per 240.4. If the calc lands at 168A, you still pull 3/0 copper at 75C and protect at 200A, assuming next-standard-size rule in 240.4(B) applies.

• Continuous loads: 125% multiplier (215.2, 210.19)
• Largest motor: add 25% of FLC to the feeder calc (430.24)
• Welders: apply 630.11 duty cycle factors, not nameplate
• Neutral: size per 220.61, but don't reduce below grounding requirements in 250.102(C)

Grounding and bonding, separate structure vs same building

This is where jobs fail inspection. If the subpanel is in the same building as the service, you run four wires (two hots or three for 3-phase, neutral, equipment ground), keep the neutral and ground isolated at the subpanel, and remove the main bonding jumper. 250.142(B) is explicit: no neutral-to-case bond downstream of the service disconnect.

Separate building or structure fed by a feeder? 250.32 governs. Since the 2008 cycle, you install an equipment grounding conductor with the feeder and treat the remote panel like any other subpanel, neutral isolated. You still drive a grounding electrode at the separate structure per 250.32(A) and bond it to the EGC with a grounding electrode conductor sized from 250.66.

If you find a three-wire feeder to a detached shop from 1995, it was legal then under the old 250.32 exception. Once you touch it for a major modification, you're bringing it up to current code. Pull the fourth wire or run a new feeder, don't try to save it.

Working clearance and mounting

110.26 is non-negotiable and inspectors measure it. For 600V or less and equipment likely to be worked on energized, you need 3 ft depth minimum (Condition 1), 36 in width or panel width (whichever is greater), and 6.5 ft headroom. Condition 2 or 3 bumps the depth to 3.5 or 4 ft if there's grounded or energized stuff on the opposite side.

Industrial spaces get cluttered fast. Rack shelving, compressed air lines, and conduit runs creep into the working space over time. Document the clearance on the as-built and, if you have any pull with the owner, paint a yellow floor boundary. Also: dedicated equipment space per 110.26(E), 6 ft above the panel or to the structural ceiling, no foreign piping or ducts.

Conduit, conductors, and terminations

For industrial feeders, rigid steel or IMC is the default where physical damage is a concern. EMT is fine in most conditioned spaces. Check the environment: wet location, corrosive atmosphere (plating shops, wastewater), or classified areas drive material selection hard. Article 300.5 for underground, 344/358 for the raceway itself, and Chapter 5 if you're anywhere near a classified location.

Ampacity: use the 75C column of 310.16 for terminations unless the equipment is rated 90C both ends, which is rare in panel gear. Derate for conduit fill and ambient before you pick the wire, not after. Parallel sets over 1/0 per 310.10(H) are common on 400A and 600A subpanels, keep the phases separated equally in each raceway.

1. Verify conductor temp rating on the panel label (typically 75C)
2. Apply ambient correction from 310.15(B)(1) if the room runs hot
3. Apply adjustment for more than 3 CCCs per 310.15(C)(1)
4. Torque every lug to manufacturer spec, document it (110.14(D))

OCPD, SCCR, and available fault current

Available fault current at the subpanel must be calculated, not guessed. 110.24 requires it to be field-marked on service equipment, and industrial AHJs often extend the expectation to subpanels. The panel's SCCR (short circuit current rating) has to equal or exceed the available fault current at its line-side terminals. A 10kAIC panel fed from a 45kVA transformer 20 ft away will fail.

Selective coordination isn't required for general feeders, but it's required for emergency systems (700.32), legally required standby (701.32), and critical operations (708.54). If this subpanel touches any of those, coordination study is mandatory and you'll want time-current curves from the OCPD manufacturer.

Before you energize, get the utility transformer data and run an infinite-bus calc as a worst case. If that number exceeds your gear's SCCR, stop and call the engineer. Energizing underrated gear is how panels blow up.

Commissioning and labeling

408.4(A) requires every circuit legibly identified as to purpose. "Spare" and "lights" don't cut it on an industrial panel. Use equipment tags, location, and function. 408.4(B) requires source identification on the panel itself, so whoever opens it knows where the feeder comes from.

Megger the feeder before energizing, phase-to-phase and phase-to-ground, 1000V DC for 600V class conductors. Verify rotation on 3-phase loads before connecting motors. Torque check all terminations with a calibrated wrench and mark them. Record the as-found available fault current with a meter if you have one, it's cheap insurance and gives the next electrician real data to work from.

• Panel directory: typed, laminated, mounted inside the door
• Arc flash label per 110.16, incident energy if a study was done
• Source ID per 408.4(B) with upstream OCPD location
• As-built redlines back to the engineer within a week