Cable Ampacities, The NEC and Computerized Applications

The cable ampacities in Table 310-16 of the 1984 National Electrical Code (NEC) for low voltage underground cables are based upon a method developed by S J Rosch, in 1938. The NEC is proposing new tables which will become effective in 1987. The new tables are based on a paper written by J H Neher and M H McGrath, in 1957. This method is used In the 1984 NEC for high voltage cables. The Neher/McGrath calculations are complex, requiring a large amount of data and the solution of numerous equations. This paper will compare the Rosch and Neher/McGrath theories and note their differences. Limitations to the proposed NEC ampacity tables will also be discussed. Finally, a new microcomputer program will be discussed which calculates underground cable ampacities for any set of given conditions and which provides an efficient cost-effective means of optimizing cable sizes.