The Neher-McGrath Calculations provide a method for
calculating underground cable temperatures or ampacity ratings and are derived
from the following technical paper:
J. H. Neher and M. H. McGrath,"The Calculation of the
Temperature Rise and Load Capability of Cable Systems", AIEE Transactions,
Part III, Volume 76, pp 752-772, October, 1957.
This paper considers the complicated heat transfer issues
associated with the determination of underground system ampacities. The paper
cites the following basic equation for calculation of a cable ampacity:
However, this single equation masks the great complexity
involved in these procedures. There are scores of complicated equations involved
in developing the terms in this equation and those required for temperature
calculations. (The paper defines over 80 variables and contains in excess of 70
formulas excluding appendices.) To solve for unique ampacities or temperatures
at each cable position, a multiple set of equations must be developed to take
into account interference heating from every position in the system, and a
matrix solution technique for simultaneous equations utilized.
AmpCalc handles all the complexity and allows the user to
quickly and easily determine ampacities for virtually any underground ductbank
or direct burial arrangement.