As with voltage drop, underground duct bank calculations are not a code requirement. Although local authorities may require these calculations.
Neher-McGrath calculations are important, especially if the design sizing will ever be fully loaded. A prime example would be a data center application.
There are many expensive software tools available and may be required to fit your exact installation. The National Electrical Code (NEC) provides some guidance on the subject; although, applying all of the considerations can be tedious.
This application looks-up the data within the NEC. If you have ever looked through Annex B, you know the fun double chart you have to remember how to use. This app also pulls through some of the written text to achieve two cell duct banks, for instance.
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