Just under the heading "Annex B" it states that this is NOT part of the N.E.C. but is used for information only. Use 310.16. There is no need to derate unless your neutral is considered a current carrying conductor or you have a large harmonic load. I assume you have 4 conduits
each with 500kcm conductors.
[ March 11, 2003, 04:05 PM: Message edited by: bob ]
Annex B should only be used by a PE who can show the calculations. Stick with 310.16 or TABLE 310.15(B)(2)(a). If you have 3 current carrying conductors in each conduit, 310.16. As stated if the neutral is counted 310.15.
This is a topic that has concerned me for some time. I agree with Bob that Annex B does not contain code requirements. I agree with Dereck that a PE needs to be involved when you use formal calculations. Clearly, with only three current-carrying conductors in a conduit, the derating factors of 310.15(B)(2)(a) don’t apply. But I have a fundamental problem with the way the title block to Table 310.16 is worded. Here’s the problem:
Table 310.16 applies to “not more than three current-carrying conductors in raceway, cable, or earth.” So how do we address the fact each conduit is not alone? In JLR’s situation, there are three other conduits nearby, each having the same set of three current carrying conductors, each generating heat, and therefore each making it harder for the others to reject their heat to the surrounding dirt. The title block to 310.16 does not apply to this situation, because there are more than “… three current-carrying conductors in … earth” (in this case, there are 12). The existence of Figure 310.60 is proof that 310.16 doesn’t apply. But 310.15(B)(2)(a) doesn’t apply either, because any single conduit has only three current-carrying conductors. So what is left? There is nothing left! The code leaves us with no clear direction.
What I think we need is a clarification of the intent of 310.16. We need a clear statement from the NFPA that this table applies to multiple underground runs if, and only if, … (and let them fill in the if statements).
if you have the handbook( i know the comments are not part of the code) then it makes it clearer that the neher-mcgrath formula should be used for underground duct banks.
John M Caloggero wrote a article "how to use the neher-mcgrath method to calculate ampacity of underground conductors" nfpa fire journal pp 17-18 may/june 1988
I read that article about a year ago, along with several others. It mentions that several different computer software packages are available that can calculate ampacity using the Neher-McGrath methodology. I have used two such packages, and I have formally issued such calculations under my own PE seal.
My basic concern is that the NEC does not provide clear guidance to the electricians and inspectors who work with this issue on a daily basis. In other words, the NEC has no clear answer to JLR's original question: "SHOULD ANNEX B OR 310.16 BE USED?"
This is some info about the table 310-16.
The reason we get by is the caculation done in accordance with article 220 adds considerable safety margins.
Samuel Rosch – 1938.
He developed an ampacity table for 50 degrees C. insulations. When converted to 60 degrees C and rounded off to the nearest 5 amperes we get Table 310.16.
Where Did Table 310-16 Come From?
Since 1889, many individuals and organizations have attempted to find the correct ampacity for conductors so they would not overheat and ruin the insulations. In 1889 Kennelly published one of the first tables listing 46 amperes as the ampacity of a number 10 conductor. In 1890 Fisher listed 19.1 amperes, and in 1894 the insurance industry listed 20 amperes as the ampacity for the same conductor. But that was not the end of it. By 1937 there were 16 ampacities discovered for the same size conductor. In 1938 Samuel J. Rosch, an associate member of the American Institute of Electrical Engineers and the manager of insulated products development for the Anaconda Wire and Cable Company, conducted a thorough investigation to find the correct ampacities for all the standard size conductors used at that time. To establish the maximum prolonged operating temperature for insulations, he performed aging and elongation tests in environmental ovens. He built a structure, wired it, embedded thermocouples in the conductors, and applied voltages and measured the ampacities and temperatures. He published his findings in a paper titled, "The Current-Carrying Capacity of Rubber-Insulated Conductors" delineating the results of his experiments. His work resulted in a table XI that became Table 310-16 of the National Electrical Code. Rosch's original table was based on an ambient temperature of 30 degrees centigrade and a conductor temperature of 50 degrees centigrade for code grade rubber, the type of insulation used in those days. If we convert the ampacities in table XI to 60 degrees centigrade using the formula given in note 1 to tables 310-69 through 310-84, setting delta TD equal to 0 (delta TD is for high voltages: we are only concerned with 600 volts and under), and rounding off to the nearest 5 amperes, we can calculate the ampacities for 60 degree insulations as found in the first column of table 310-16. Likewise, the same calculation can determine the ampacities for the 75 degree and 90 degree columns in table 310- 16.
Faults with Table 310-16
There are three very important deficiencies in Rosch's paper. First, he did not investigate the effects of proximal heating from adjacent conduits, ducts, and duct banks. Secondly, his experiments were only for above ground installations. Thirdly, the heat produced by high voltages was not investigated. But for most applications when load calculations are performed according to Article 220, there is enough safety margin built in to preclude any problems. To explain this, a fine print note was added to section 310-15(a) in the 1990 NEC stating that Tables 310-16 through 310-19 are application tables that are for use in determining conductor sizes on loads calculated in accordance with Article 220. When calculating loads per article 220 a substantial safety margin is included as opposed to some engineering calculations that calculate the "actual" load.
The deficiencies to Table 310- 16 became a problem in the 1950's when Americans began installing very large air conditioning systems in the larger buildings, using underground service laterals run in massive underground duct banks. In cases where engineers performed load calculations using engineering methods in place of Article 220, and used Table 310-16 to determine the size of conductors, conductors overheated and burned open, especially the conductors located near the center of the duct banks. Rosch used a basic heat transfer equation with the addition of a term "n" for the number of conductors in the same cable or raceway. But there were no terms in his equation to adjust the ampacity for heat that came from adjacent ducts and duct banks, or for the differences for heat dissipation in an underground installation. Later calculations using the Neher-McGrath equation found in 310-15(b) of the NEC would determine that the center conductors in a 3 by 3 duct banks must be derated to almost 60 percent because of the proximal heating effect from adjacent ducts and duct banks.
To develop a more accurate method of finding the ampacity of conductors in underground installations two cable engineers, in 1957, developed the Neher-McGrath equation found in 310-15(c) of the 1999 NEC.
Perhaps I can provide some information regarding Table 310.16 versus the Neher-McGrath calculation and Appendix B of the NEC. When sizing conductors based on Table 310.16, the load calculation must be based on Article 220. When calculating ampacity
by the Neher-McGrath method, Article 220 is not used. There is a load factor that is calculated and used in the N-M formula. When applying the N-M
formula, there are some assumptions made pertaining to the temperature of the soil, temperature in the conduit, soil resistivity etc. Since the soil is not homogeneous and temperature not consistent, it is at best an educated guess. I hope this will offer some help.
John M. Caloggero, (retired 12/31/03)