What size wire for 3 phase 400 amp underground?

For a 3-phase 400 amp underground installation, you typically need to use a minimum of 250 kcmil copper wire or 350 kcmil aluminum wire, according to the National Electrical Code (NEC). However, factors such as the length of the run, ambient temperature, and installation conditions can affect the size requirements, so it's important to consult local codes and regulations or an electrician to ensure compliance and safety. Additionally, consider using larger wire sizes for voltage drop purposes if the distance is significant.