GoldDigger,
As I try to collect my premise does this line of thought work --
310.15(B)(3) refers to the adjustments needed for 0 - 2000v conductor ampacities for based upon the sub sections of 310.15(B)(3)(a), 310.15(B)(3)(b), and 310.15(B)(3)(c).
310.15(B)(3)(a) basically deals with more than three conductors in raceways and cable separations with 5 additional specific conditions.
310.15(B)(3)(b) refers to conduit/raceway separation without any conditions on number of conductors in either free air or underground duct configurations.
310.15(B)(3)(c) defines it's self with raceways exposed to sunlight on a roof condition.
There are tables to refer to for adjustments within the 310.15(B)(3)(a), & 310.15(B)(3)(c) requirements.
Information regarding 310.15(B)(3)(b) was first inserted in the NEC 1984 code to be applicable 1/1/87. This table moved to Annex B in the 1990 NEC edition and is in the current edition. Tables in Annex B sections purpose is informative and factual for parallel type installations. A Neher-MGgrath calculation is an option to the Annex B information in order to be precise on the adjustments for in those tables.
The Annex B tables and the Neher-McGrath calculation are both reliable and code applicable information regarding a parallel installation per NEC 310.15(B)(3)(b).
Looks like a well organized line of thought, but nothing proceeds in a straight line in the real world.
So here are some digressive comments to round out the picture (in no particular order, but numbered just the same):
1. The calculated adjustments for more than three conductors in a raceway found in Annex B build in a diversity factor of 50%. The main body of the Code provides tables of adjustment factors which DO NOT allow for diversity. To use the Annex B tables for adjustment you would need to justify that diversity factor, under engineering supervision.
2. The ampacity tables in Annex B give ampacities directly, rather than as an adjustment factor. To apply these in combination with other derating factors you must either use these ampacities as your starting values and then apply the other factors or convert these values to factors and apply the various factors in any order.
3. There is a fundamental difference between the temperature correction table in the body of the code for circular raceways on or above roofs and the duct tables in Annex B: The table in the main body does not require engineering supervision, while any use of the Annex B information is only applicable as part of engineering supervision, not as a replacement for it.
I am not sure what you mean by "factual" in "informative and factual". It certainly does not allow you to bypass engineering supervision, although the engineer involved can rely on them as he will, based on his informed judgement.
4. The Annex values for duct banks do not take into consideration that the ambient correction for each of the different ducts within one Detail # configuration will be different. Instead it assumes that parallel conductors are involved and so, given that the conductors must be identical, the worst case duct in the bank will determine the necessary correction.
5. Just for fun, consider what would happen if the provision for paralleling cables, 310.10(H)(2), required the parallel wires in one group to have the same calculated (adjusted and corrected) ampacity instead of the same physical characteristics. :blink: