Quick Hits
Quick Hits
Embedded Files
EPISODE 12 — Ampacity Adjustment: Shield Grounding, Burial Depth & Duct Spacing (315.60(D))
EPISODE 12 — Ampacity Adjustment: Shield Grounding, Burial Depth & Duct Spacing (315.60(D))
Slide 1 — Hook "Today: the corrections that modify the base table ampacity — multi-point-grounded shields, deeper burial, and duct entry spacing."
Slide 2 — 315.60(D)(1) & (D)(2)
(D)(1) Grounded shields: several ampacity tables assume the cable shield is grounded at just ONE point. If a shield is grounded at MORE than one point, you must apply a shield current correction factor (calculated via a formula factoring conductor AC resistance, shield resistance, and mutual reactance) to account for the extra heating those circulating shield currents cause. Alternative calculation methods are allowed under engineering supervision.
(D)(2) Burial depth: decreasing burial depth from the table's assumed value never requires an ampacity reduction. Increasing depth: if the deeper section covers LESS than 25% of the total duct run length, no derating is needed; otherwise, apply a 6% ampacity derating for every 1 ft of additional depth.
Slide 3 — 315.60(D)(3) & (D)(4)
(D)(3) Where electrical ducts enter equipment enclosures from underground, the spacing between those ducts CAN be reduced (per Figure 315.60(D)(3)) without requiring an ampacity reduction — a practical allowance for the tight quarters right at equipment entry points.
(D)(4) Ambient temperature correction: same concept as Article 310 — Table 315.60(D)(4) gives multiplier factors for ambient temperatures other than the table's baseline of 40°C, split by MV-90 vs. MV-105 rating. Hotter ambient = lower multiplier (reduces ampacity); colder = higher multiplier (increases it, within limits).
Slide 4 — Quiz + CTA Q: "If underground duct burial depth is increased beyond the table's assumed depth, and that deeper section covers MORE than 25% of the total run, what ampacity derating applies per 1 ft of added depth?" A) 2% B) 4% C) 6% D) 10%
✅ Answer: C — 315.60(D)(2)(b): a 6% derating factor per additional foot of depth.
CTA: Follow for more daily quick-hit study notes 📚⚡
⚠️ Educational NoticeQuick Hits are educational explanations based on my understanding of the NEC. Always verify requirements in the current adopted NEC, applicable local amendments, manufacturer instructions, and with the AHJ.
Page updated
Google Sites
Report abuse