Field guide: installing a subpanel, industrial version (edition 4)

Sizing the feeder before you touch a knockout

Industrial subpanels fail at the feeder calc, not the install. Before you pick conductors, pull the connected load, apply demand factors per NEC 220 Part III or IV, and confirm the upstream OCPD can actually protect what you're running. Continuous loads get the 125 percent bump per NEC 215.2(A)(1). Motors get the largest motor plus the sum of the rest per NEC 430.24.

Voltage drop is not a code rule under 210.19 Informational Note, but on a 400 foot run to a machine shop subpanel it will cook your production. Target 3 percent on the feeder, 5 percent total. Do the math on the actual run length, not the blueprint straight line.

Temperature termination ratings kill more installs than ampacity. A 90C conductor terminated on a 75C lug is a 75C conductor. Check the panel label, check the breaker lug, then size from the 75C column in NEC Table 310.16 unless the whole assembly is listed otherwise.

Grounding and bonding, the part that fails inspection

A subpanel is not a service. The neutral bar floats. The equipment ground bar bonds to the enclosure. You pull four wires to a separate structure or a downstream panel: two ungrounded, one grounded (neutral), one equipment grounding conductor. No bonding screw, no bonding jumper between neutral and ground bars. NEC 250.24(A)(5) and 408.40 are non-negotiable here.

Size the EGC from NEC Table 250.122 based on the feeder OCPD, not the conductor. If you upsized the ungrounded conductors for voltage drop, upsize the EGC proportionally per NEC 250.122(B). Inspectors check this.

If you find a bonding screw still installed in a subpanel someone else wired, stop. Pull the cover on the main, check for parallel neutral current on the EGC with a clamp meter. Nine times out of ten the ground is carrying load and something upstream is warm.

Enclosure, mounting, and working space

NEC 110.26 working space is the first thing an inspector measures. 3 feet depth for up to 150V to ground, 3.5 feet for 151 to 600V to ground with grounded parts on the other side, 4 feet if there are exposed live parts on both sides. 30 inch width or panel width, whichever is greater. 6.5 feet headroom. This is clear, unobstructed, dedicated space.

In industrial environments, pay attention to NEC 110.26(E) dedicated equipment space. The 6 foot zone above the panel (or to the structural ceiling if lower) cannot have foreign piping, ducts, or process equipment. Sprinkler protection is allowed. Steam lines are not.

For wet, dusty, or corrosive areas, match the enclosure type to NEC Table 110.28. A NEMA 1 panel in a food plant washdown zone will rust through the dead front in two years. Spec NEMA 4X stainless from day one.

Conductor entry, supports, and fill

Industrial feeders are usually parallel sets in two or more raceways. Each raceway must contain all phase conductors, the neutral, and the EGC per NEC 300.3(B) and 300.20(A). Split your phases across raceways and you will induce heating in the steel and trip on phantom faults.

Conduit fill per NEC Chapter 9 Table 1. For more than two conductors, 40 percent fill maximum. Run the calc, do not eyeball it. 4/0 THHN in a 2 inch EMT looks fine until you pull it and strip jacket off the corners.

1. Mount the enclosure plumb, torque the mounting hardware to the manufacturer spec.
2. Knockout or hole saw from the top or sides, never the back unless the panel is listed for it.
3. Use listed connectors rated for the raceway and location. Liquidtight in washdown, rigid in Class I Division 2.
4. Support raceways within 3 feet of the enclosure per NEC 358.30 (EMT) or the applicable article.
5. Bond all metal raceways with listed bonding bushings on concentric or eccentric knockouts where the feeder exceeds 250V per NEC 250.97.

Breakers, torque, and labeling

Every breaker in an industrial subpanel gets torqued to the manufacturer value. Not "tight." Not "German tight." The number on the label, with a calibrated torque screwdriver. NEC 110.14(D) made this explicit in the 2017 cycle and inspectors are enforcing it. Document the torque with a sticker or a log.

Series ratings only apply if the combination is tested and marked. If your upstream breaker is 65kAIC and your subpanel breakers are 22kAIC, you need a series-rated combination listed by the manufacturer, and you mark the panel per NEC 240.86(C). Do not assume it works because the numbers feel right.

Keep the manufacturer torque chart taped inside the dead front. Next guy in there will thank you, and so will the inspector doing a three year re-cert.

Panel directory per NEC 408.4(A) is legibly filled out, circuit by circuit, with enough detail to identify the load. "Spare" is acceptable for spares. "Lights" is not acceptable for a 60 amp three-phase branch feeding a CNC coolant pump. Inspectors read these.

Before you energize

Meg the feeder phase to phase, phase to neutral, phase to ground. 500V DC on 480V systems, 1000V DC on higher. Anything under 1 megohm, find it before you close the breaker. Insulation resistance testing is not in the NEC, but it is in every serious industrial spec and every insurance carrier's eyes.

Verify phase rotation with a rotation meter on three phase subpanels feeding motors. Pull the main disconnect, lock it out, land the conductors, torque, close up, then energize from the top down. Measure voltage at the bus before closing any branch breakers.