Aerial Ampacity Calculations and the NEC Code[Last] Page 2 of 6 [Next]
Calculation of Higher Ampacities for 0-2000 Volt Cables
Using the industry accepted Neher-McGrath calculation procedures, higher ampacities can be determined than provided in the Code for 0-2000 Volt cables. The calculations procedures used to produce the 0-2000 Volt NEC Code ampacity tables were developed in the 1930s and documented in a 1938 AIEE paper by S. J. Rosch1. In its time, this work was excellent and greatly advanced the state of the art of ampacity calculation procedures. However, the subsequent Neher-McGrath2 calculations further advanced the state of the art and form the basis of all subsequent cable ampacity standards, including IEEE Standard S-135, (IPCEA P-46-426), and IEEE Standard 835-1994.
The NEC Code itself contains a number of ampacity tables which are based on the Neher-McGrath calculations as implemented in the S-135 standard. A summary of the NEC tables shows that all ampacities for cables greater than 2000 Volts are derived directly from Standard S-135, which in turn was based on the Neher-McGrath calculation procedures. This results in some internal inconsistencies in the NEC Code between 0 – 2000 Volt cable ampacities, (referred herein as low voltage), and 2001 – 35,000 Volt ampacities, (referred herein as high voltage.) The net result as demonstrated below, is that the Code specifies significantly higher ampacities for high voltage cable applications than for low voltage applications.
Refer to Tables 1 which compare various ratings for three 500 kcmil cables in conduit in air. Note that for various cable voltages from 2001 through 35,000 Volts for the NEC Code, and from 1 to 15 kV for Standard S-135, all ampacities for this installation stay in a relatively tight band of values from a minimum of 473 amps to a maximum of 481 amps. In contrast, the NEC Table 310.16 rating, at 430 amps, is significantly lower.
Comparative Ampacities for Three 500 kcmil Single Conductor Cables in Conduit in Air
Copper Conductors at 90 Deg C in 40 Deg C Ambient
However, even this is not an equivalent comparison since the Table 310.16 values are based on a 30 Degree C ambient air temperature, whereas the other ratings are at a 40 Degree C ambient. Converting to a 30 Degree C ambient yields a Table 310.16 ampacity of only 391 amps, which is 82% of the S-135 value for 1 kV class cable.
Comparative Ampacities for One Single Conductor 500 kcmil Cable Isolated in Air
Copper Conductor at 90 Deg C in 40 Deg C Ambient
In like manner, Table 2 summaries the various ratings for a single conductor 500 kcmil cable isolated in air. Again, all ampacity ratings are in a relatively tight band from 678 to 695 amps, except for the NEC Table 310.17 value which at 637 amps, is 92% of the equivalent S-135 rating.
1"The Current-Carrying Capacity of Rubber-Insulated Conductors", S. J. Rosch, AIEE Transactions, Vol. 57, April 1938, p. 155-167.
2"The Calculation of the Temperature Rise and Load Capability of Cable Systems", J. H. Neher, M. H. McGrath, AIEE Transactions, October 1957, p. 752 – 771.