Determining the allowable ampacity for underground conductors is a complicated process involving numerous parameters and complicated algebra. In their seminal work “The Calculation of the Temperature Rise and Load Capability of Cable Systems”, Neher and McGrath established the methodology used to create the ampacity tables in NFPA-70, The National Electrical Code. This work was explained and expounded upon by George Anders in his books, Rating of Electric Power Cables and Rating of Electric Power Cables in Unfavorable Thermal Environments.
This calculation demonstrates the application of the Neher-McGrath ampacity calculation on three directed buried cables.
