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EPISODE 11 — The 20 Ampacity Tables: How They're Organized (315.60(C))
EPISODE 11 — The 20 Ampacity Tables: How They're Organized (315.60(C))
Slide 1 — Hook "Today: making sense of TWENTY ampacity tables by understanding how they're organized, rather than trying to memorize them."
Slide 2 — The organizing logic
Tables 315.60(C)(1)-(10): conductors IN AIR — split by configuration (triplexed vs. isolated single conductor vs. three-conductor cable vs. in isolated conduit) and material (copper vs. aluminum). Each table splits ampacity by voltage tier (2001-5000V vs. 5001-35,000V, sometimes further split) and by MV-90 vs. MV-105 temperature rating.
Tables 315.60(C)(11)-(20): conductors UNDERGROUND — split between electrical duct installations (11-14) and direct burial (15-20), again by material and configuration (single conductors vs. three-conductor cable, one circuit vs. multiple circuits sharing a duct/trench).
Pattern that holds everywhere: copper ampacities run higher than aluminum at the same size; MV-105 ampacities run higher than MV-90 at the same size (since it tolerates more heat); and ampacity generally DROPS as more circuits share the same duct bank or trench (heat has nowhere to dissipate as efficiently).
Slide 3 — A representative example to anchor the pattern
Table 315.60(C)(1) (copper, triplexed in air, 2001-5000V): a 4/0 AWG conductor runs 335A at MV-90, 375A at MV-105.
Table 315.60(C)(11) (copper, three single conductors, ONE circuit per duct, 2001-5000V): that same 4/0 AWG runs 290A at MV-90 — noticeably LOWER than the in-air number, because underground duct installation dissipates heat less efficiently than open air. Bump to THREE circuits sharing ducts and it drops further to 235A; SIX circuits drops it to 195A.
Slide 4 — Quiz + CTA Q: "As more circuits share the same underground duct bank, what generally happens to each conductor's ampacity?" A) It increases B) It decreases C) It stays exactly the same D) There's no relationship between circuit count and ampacity
✅ Answer: B — more circuits sharing ducts means less efficient heat dissipation per conductor, so ampacity drops.
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⚠️ 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.
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