Prior to and including the 1956 National Electrical Code (NEC) there was a limit of nine conductors in a raceway, except when used for control, stage lighting, elevators, and similar uses.
In the 1959 NEC, a proposal submitted by a Technical Sub Committee on Raceway Fill and Heating of Conductors, and based on a UL Fact Finding Report, was accepted, and the limit of nine conductors in a raceway was removed. The 70PR-1958 document (forerunner to today's Technical Committee Report) included this statement by the Technical Sub Committee: "That the reduction factors-for more than nine wires in a steel raceway-give recognition to diversity and be 70% for 10 to 24 wires, 60% for 25 to 42 wires, and 50% for 43 and more wires."
A Table following this recommendation appeared in the 1959 NEC in Note 8 to the ampacity tables, but with no mention of diversity.
In Note 8 in the 1984 NEC, "Reduction of load current" was changed to "reduction of conductor ampacity."
This Table limiting the load current, or the ampacity, on more than three conductors in a raceway or cable, and with no mention of diversity, served the electrical construction community well, with no problems, for 27 years (nine three-year code cycles).
Then, nearly 30 years later in the 1987 NEC, an asterisk was added with a note that for 10 wires and more, the factors include the effects of a load diversity of 50 percent. The supporting comment for this change in the NFPA's 1986 TCR was: "The *note is added to call attention to the reader that the derating factors for 10 and more conductors are based on a diversity effect of 50 percent."
In the next edition, the 1990 NEC, the Table was divided into two columns, A and B. Column A was for 50 percent diversity, and Column B was for no diversity. Column A was the same as the Table introduced in the 1959 NEC that was used with no problems for 27 years without regard for diversity.
The NEC itself does not contain a definition of "diversity," but the proposal on page 249 in the 1989 TCR defines diversity as "50% of the conductors (when 10 or more are considered) are energized and 50% are NOT energized."
What application requires that half the conductors in a raceway are not energized?
That does not make any sense.
As far back as the 1923 NEC and up to the 1959 edition, as many as nine conductors in a raceway were allowed for power and lighting. The number of conductors greater than nine in a raceway were for elevator controls, sign flashers, motor controls, and the like, where the loads were either light, intermittent, or short-time. This balance of loaded and lightly loaded conductors might make sense today, but the idea of half the conductors not being energized at all is bewildering. Why would anyone install de-energized conductors in a raceway?
Other interpretations of "diversity" have included "all conductors loaded to 50%", but calculations have shown that this arrangement would produce more heat than where half the conductors were energized and half were not.
In response to a large number of proposals to change Note 8, the 1993 NEC dropped the undefined term "diversity" (except that it was retained in the FPN), retained Column A, with factors down to 35 percent for 41 conductors and above, and relegated Column B (no diversity) to Appendix B as Table B-310-11. Appendix B is headed: "This appendix is not part of the requirements of this Code, but is included for information purposes only." Appendix Table B-310-11 can be used only under engineering supervision [See 310-15(a)(1)].
In the 1993 NEC, the heading of Note 8(a) and the Table heading were revised to read "current-carrying conductors." This change was made to Appendix B, Table B-310-11. So that means the unenergized conductors are not counted. Most confusing.
So how do we apply the derating factors to a conductor that is not energized and therefore carries no current? How does one derate the ampacity of a conductor below zero amperes, which it carries when de-energized?
Due to the extensive editorial work done for the 1999 NEC, Note 8 to the Ampacity Tables became Section 310-15(b)(2)(a).
So now we have this ampacity restriction, which makes it impractical to run more than nine current-carrying conductors in a raceway. With the permission for more than nine conductors-if they are control wires, elevator controls, sign flashers, etc.-that would put us back to the pre-1959 Code, and wasn't that much simpler?
It is unfortunate that in the blind zeal to be technically correct we lost a procedure which had worked with no trouble for almost 30 years. What happened to "If it ain't broke, don't fix it?"
SCHWAN is an electrical code consultant in Hayward, Calif. He can be reached at BevSchwan@aol.com.