What is the cable size required for a 390 amp load?

The required cable size for a 390 amp load depends on several factors including the type of insulation, temperature rating, length of the run, and whether the installation is in a conduit or exposed. Generally, for copper conductors, a cable size of at least 250 kcmil is often recommended; for aluminum conductors, a larger size may be necessary, typically around 350 kcmil. It's crucial to consult the National Electrical Code (NEC) or a local electrical code for precise calculations and to account for factors like voltage drop and ambient temperature. Always consult with a qualified electrician or engineer for specific applications.