Re: Conductor Adjustment Factors
Proposal 6-89 submitted by CMP 6 for the 1987 code cycle resulted in the "50% diversity factor" note being added to the ampacity adjustment factor table in Note 8 to the ampacity tables. The substantiation pointed out that the ampacities in Article 310 were extracted from IPCEA (Insulated Power Cable Engineers Association) document #P-46-426. This document was originally published in 1962. The document is now published by IEEE and is standard #835-1994. The original document included the ampacity adjustment factors as shown in the 1984 and previous codes, however the IPCEA document included the "50% diversity factor" wording for wire fills greater than 9. The substantiation did not include reports of field problems caused by the omission of the "diversity factor" rule. The panel was just making the code technically correct based on the original document. Calculations were shown that showed cables could overheat when there were more than 9 current carrying conductors in a raceway without the "50% diversity factor". This same proposal also added an additional adjustment factor table that was to be 8(b) for derating when horizontal banks of conduit were installed with a conduit spacing less than the conduit diameter. This table would have required additional adjustment factors of up to 68% for closely spaced conduit. The requirement for derating based on conduit spacing is also found in the original IPCEA document. In the comment stage, panel action on comment 6-98 deleted the conduit spacing adjustment factor and in its place inserted new wording that read: "8(b) Spacing between conduits, tubing or raceways shall be maintained". The code has never given any clear guidance as to what spacing is required. The IPCEA document requires additional derating whenever the conduit spacing is less than one conduit diameter, but the code does not require this spacing. There were proposals submitted for the '90, '93 and '99 code cycles to clarify the spacing that is required between conduits, but they were all rejected. The panel did state that the spacing is required to dissipate the heat, but would not specify the amount of spacing that is required.
Proposal 6-98 for the 1990 code resulted in a second ampacity adjustment factor table being placed into Note 8. This second table was for raceway fills where the conductors did not have a "50% diversity factor". This second table in the 1990 code is the same as the current ampacity adjustment factor table that is now Table 310.15.(B)(2)(a) with one exception. In the '90 code the 35% factor applied for wire fills of 41 to 60 conductors, but in the current table the 35% factor applies to all wire fills above 41. The substantiation for this new table said that this change is required: "... due to the missunderstanding of many inspectors and contractors on the use of Note 8(a) and the load diversity of 50% for 10 or more conductors ... ".
In the '93 code cycle, CMP 6 submitted proposal 6-110a which moved the ampacity adjustment factor table that included the "50% diversity" to Appendix B. The panel stated that this move was made to clear up the confusion that had resulted from the addition of the "50% diversity factor" wording to Note 8(a). In the panel comment on comment 6-51, the panel said: "... the panel reaffirms its decision to remove the 50% diversity column from Article 310 because of the misapplication in the field".
Don
[ December 26, 2003, 10:08 PM: Message edited by: don_resqcapt19 ]
