Quick Hits

EPISODE 13 — Supporting Conductors in Vertical Raceways & Induced Currents (300.21, 300.22)

Slide 1 — Hook "Today: how heavy vertical conductor runs get supported so their own weight doesn't rip them out of the terminals, and how to avoid heating up the metal raceway around them."

Slide 2 — 300.21 Vertical raceway support

• Conductors in vertical raceways need support once the rise exceeds the values in Table 300.21(A) — spacing varies by conductor size and material (copper vs. aluminum/copper-clad aluminum), generally getting tighter as conductor size increases (e.g., copper 18 AWG-8 AWG: max 100 ft; over 750 kcmil copper: max 35 ft — aluminum gets roughly double those distances at each size band).

• At least one support at (or as close as practical to) the TOP of the vertical run, with intermediate supports added as needed to stay within the table's limits.

• Support methods: insulating wedge clamping devices (with conductor clamping too, if insulation clamping alone isn't enough), inserted boxes with insulating supports rated for the conductor weight, 90°-deflection junction-box routing (cables carried horizontally at least 2× their diameter on 2+ insulating supports, at intervals no more than 20% of the Table 300.21(A) spacing), or other approved means.

• Fire-resistive cable/conductor systems follow their own listing restrictions, but never exceed the Table 300.21(A) values either way.

Slide 3 — 300.22 Induced currents in ferrous metal

• (A) AC conductors in ferrous (magnetic) metal enclosures/raceways: group ALL phase conductors (plus grounded conductor and EGCs, where used) together to avoid inductively heating the surrounding metal. Narrow exceptions exist for certain legacy EGC installations and single-conductor skin-effect heating applications.

• (B) A single AC conductor passing through ferrous metal: minimize induction by cutting slots between the individual conductor holes, OR passing ALL circuit conductors through one large insulating-walled opening together. Exception: very low-current circuits (vacuum/electric-discharge lighting, signs, X-ray apparatus) can ignore this — the inductive heating is negligible. Aluminum isn't magnetic, so it doesn't need this treatment either (though induced currents still exist, just not enough to matter).

Slide 4 — Quiz + CTA Q: "Why must all phase conductors (plus grounded/EGC conductors) of an AC circuit be grouped together when run in a ferrous metal raceway?" A) To reduce material costs B) To avoid inductively heating the surrounding ferrous metal C) It's purely a labeling convention D) There's no technical reason

✅ Answer: B — 300.22(A): grouping prevents dangerous inductive heating of ferrous (magnetic) metal enclosures/raceways.

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