Quick Hits

EPISODE 1 — Scope & Minimum Feeder Sizing (266.1, 266.4)

Slide 1 — Hook "New article! Article 266 is the medium/high-voltage version of Article 215 — Feeders over 1000V AC / 1500V DC. Short article, so we'll clear it in just 3 episodes." Text overlay: ARTICLE 266 — FEEDERS OVER 1000V ⚡ Part 1 of 3

Slide 2 — 266.1 Scope

• Covers installation requirements, overcurrent protection, minimum size, and ampacity for feeders over 1000V AC or 1500V DC.

• This is what was left of old Article 235 after branch circuits (265), outside branch circuits/feeders (267), and services (268) got their own dedicated medium-voltage articles for 2026.

Slide 3 — 266.4 Minimum ampacity & sizing

• Ampacity follows 310.14 and 315.60 as applicable; the grounded conductor (if present) can't be smaller than what 250.122 requires for an equipment grounding conductor (with a parallel-conductor exception).

• Feeders supplying ONLY transformers: ampacity ≥ sum of the transformers' nameplate ratings.

• Feeders supplying transformers + utilization equipment: ampacity ≥ transformer nameplate sum + 100% of the utilization equipment's simultaneous designed load.

• Supervised installations: qualified persons under engineering supervision can determine sizing themselves — the same "supervised installation" concept (engineering-supervised design + documented, experienced qualified persons for install/maintenance) you've seen throughout this medium-voltage series.

Slide 4 — Quiz + CTA Q: "A feeder supplies both transformers and utilization equipment. How is its minimum ampacity determined?" A) Transformer nameplate sum only B) Transformer nameplate sum + 100% of the simultaneous utilization equipment load C) 80% of the total connected load D) Whichever is smaller of the two loads

✅ Answer: B — 266.4(B): transformer nameplates PLUS the full simultaneous utilization load.

CTA: Follow for more daily quick-hit study notes 📚⚡

← View Chapter 2 Home

← Back To Article 265 Home

← Back To Quick Hits Home