Table 310.15(B)(16) for Underground Conduits & Cables

[beanland]

I find it interesting that Figure 310.60 applies to underground conduits and conductors 2001-35000V but there is no such figure associated with Table 310.15(B)(16) for conduits and conductors up to 2000V. I can find no guidance in 310.15 that describes any relavant spacing for underground conduits. Annex B provides copius information but is informative only, not regulatory. In fact, 310.15(A)(1) states "Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15(B) -OR- under engineering supervision, as provided in 310.15(C)." Since 310.15(B) provides tables but no guidance on conduit spacing, there seems to be a big hole in the NEC relating to underground conduits.

I have seen multiple cases where underground conduits are crammed together, conductors are sized per Table 310.15(B)(16), and the result is overheating cables.

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Does anyone see any guidance in the NEC for conduit spacing for underground conduits containing conductors rated below 2000V?