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Z-COLO DUCTBANK CALCULATION
W.J. Griffin Electric
Project scope was to provide an Engineering 3rd Party Supervision per NEC 310.15(C) as it pertained to four Duct Banks given duct bank feeder information from WJGEI. The most recognized way to accomplish this supervision in the industry today is to use the calculation method known as the Neher–McGrath formula. It is for calculation of conductor ampacity in underground electrical duct banks. The results for the duct banks within the scope of the study were calculated using the CalcWare Software AmpCalc Version 4.0. for calculation of Underground Cable Ampacity.
The Neher-McGrath Calculations provide a method for calculating underground cable temperatures or ampacity ratings and are derived from the 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. Per the Calculation formula ampacity: The results of the study show the four conduit ductbanks in question passed the required Calculations for both the Normal and Generator sources to the ATS-A and ATS-B respectfully at 76% of the 3000A load maintaining the 75 Degree C termination temperature. The values were based on the average of the shallowest and deepest elevations along the length of the ductbanks as a worst-case scenario.
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