Underground Cable Ampacities and the NEC Code

The NEC Code is a very familiar document since it is the law of the land with regard to safe electrical installation practices.  However, the NEC’s ampacities have always applied to only a very narrow range of defined installations parameters. A proposed expansion on the treatment of 0-2000 volt cable ampacities was printed in the 1984 code, and a revised version was issued in the 1987 NEC Code. In the 1987 Code ampacity tables, the configurations in NEC Figure 310-1 were utilized and ampacities calculated in accordance with Neher-McGrath. In addition to the ductbanks illustrated in NEC Figure 310-1, the 1987 NEC provided a method to arrive at ampacities for single row ductbanks with either two or four ducts. In versions of the NEC prior to 1987, 2000-35000 volt cable ampacities had already been based on Neher-McGrath calculations.

The 1987 Code generated a great deal of controversy because of the additional complexity introduced into the sizing of 0-2000 volt cables. The controversy was resolved by elimination of the new, (as first proposed in 1984), ampacity tables for 0-2000 volt cables from Article 310 of the 1990 Code. However, these tables were moved to Appendix B of the Code along with some additional materials on cable ampacity. (The Appendix indicates that it, “… is not part of the requirements of this Code, but is included for information purposes only.”) The additional materials in Appendix B from the 1987 Code include derate interpolation curves to arrive at ampacities for different values of earth thermal resistivity and circuit load factor, as well as three additional “High ampacity” duct bank configurations. The attempts of the Code to address the issue of cable ampacity illustrate the complexity of the task, and the impossibility of reducing it to tables which can even begin to meet the needs of all installations. While the expanded treatment of alternative underground configurations is useful, it is still very limited. In the “real world” implementation of underground cable systems, there can be totally different layouts and design parameters than those which must be assumed in the NEC Code.

Just some of the restrictions of the Code are as follows:

  • The ductbank or directly burial installation must essentially conform to those depicted in NEC Figure 310-1 or the “High ampacity” configurations in Appendix B. These configurations fix variables such as maximum depth in the earth, ductbank width and height, and quantity and spacing of cables. (There are some derates given which allow for variations in some of these parameters.)
  • The NEC simplification of the Neher-McGrath calculations assumes that all cables in the installation have the same current rating. Thus, the ampacity of the installation is determined by the current which causes the hottest cable to run at rated temperature. There is no easy way to account for a mixture of circuits of various ampacities and different cable sizes and ratings within the same underground system. All cables must be identical in size and all other characteristics.
  • The NEC tables are built around 20 degree C earth ambient temperature, 100% load factor, and an earth thermal resistivity of 90. Although multiplying factors and/or curves are provided in Appendix B, a combination of a number of derates and deviances from NEC configurations can make ampacity calculations based on the Code difficult to apply.
  • Cable rated temperatures of 60, 75, or 90 degrees C depending on the cable being used.
  • For shielded cables single point grounding is assumed.
  • There is no provision for determining the derating effects of multiple ductbank and/or direct buried cables in close proximity.

While the variation of some of the above parameters will not have a large effect on calculated ampacities, others can be major factors. Also, an accumulation of differences in a number of minor parameters can result in major changes in the calculated ampacities. The NEC itself recognizes it’s limitations and in article 100 defines ampacity as, “The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.” Thus it is recognized that the same cable will have a different ampacity depending on the manner in which it is installed.

Where an underground installation defers from a standard NEC arrangement, the code indicates that calculations can be done to determine actual rating.  General formulas are provided for that purpose, Equation 310.15(C) for cables 0 to 2000 Volts and Equation 310.60(B) for Conductors rated 2001 to 35,000 Volts. Complex although they may be, these formulas are really not adequate since they do not include the effect of mutual heating between cables, nor is it indicated to the reader the complexity of deriving the “RCA” term.

The only alternative for unique configurations is to seek out the Neher-McGrath calculations. However, a problem arises in that the Neher-McGrath calculations are not a simple “cookbook” method, but consist of scores of equations which require thorough analysis and much data development which can consume an inordinate amount of time. Small errors in the many calculations can lead to large deviations in the results. Finally, to properly distinguish ampacities for the various cables in various positions and perhaps various sizes, it is necessary to construct and solve a set of simultaneous equations for each case to be considered, with the number of equations equal to the number of cable positions.

By using AmpCalc, any underground configuration can be input and ampacities and/or cable temperatures calculated with none of the NEC limitations. The parameters that are fixed in the NEC and S-135 are variable in AmpCalc. The data sets representing the basic NEC arrangements are provided with the program and they can be used or altered at will. In every case a system of equations is set up by AmpCalc per the Neher-McGrath procedures and a matrix solution technique is utilized.