Quick Hits

PART IV — AMPACITIES

EPISODE 10 — Ampacity Basics: Tables vs. Engineering Supervision (315.60(A)–(B))

Slide 1 — Hook "New Part! Ampacities — the section with TWENTY tables. We're covering it conceptually across 4 episodes rather than trying to read entire tables on a slide."

Slide 2 — 315.60(A) General approach

• Ampacity for solid-dielectric-insulated conductors comes from tables OR engineering supervision calculation — same two-path structure you learned in Article 310.

• Cable installed in cable TRAY specifically follows 392.80(B) instead.

• Where more than one ampacity could apply along a circuit, use the LOWEST — unless the lower-ampacity portion is capped at the lesser of 10 ft or 10% of the total circuit (same exception structure as low-voltage Article 310).

Slide 3 — 315.60(B) Engineering supervision

• Under engineering supervision, ampacity can be CALCULATED using a formula factoring in: conductor temperature, ambient temperature, DIELECTRIC LOSS temperature rise (a factor unique to medium-voltage cable — insulation itself generates heat at these voltages), DC resistance, AC resistance from skin/proximity effects, and thermal resistance between conductor and ambient.

• Good detail to know: dielectric loss temperature rise is considered NEGLIGIBLE for single-circuit extruded dielectric cables rated below 46kV — meaning for most typical Article 315 applications, this extra factor barely matters in practice.

Slide 4 — Quiz + CTA Q: "The engineering-supervision ampacity formula in 315.60(B) includes a factor NOT present in the low-voltage Article 310 formula. What is it?" A) Ambient temperature B) Dielectric loss temperature rise C) DC resistance D) Skin effect

✅ Answer: B — dielectric loss temperature rise (ΔTd) reflects heat generated by the insulation itself at medium voltage, an effect not significant at lower voltages.

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