History

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 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 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 develope 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.