What size must the conductors between the transformer and the main panelboard be?

The conductors between the transformer and the main panelboard must be sized based on the load they will carry, which is determined by the amperage and the type of transformer installed. In this case, selecting 250 kcmil indicates that it is sufficient to handle the expected load while also considering the factors such as the distance between the transformer and the panelboard, conductivity, and the ampacity of the conductor material being used.

It's important to ensure that the conductor size meets the NEC (National Electrical Code) requirements for ampacity, and choosing 250 kcmil aligns with typical load calculations for larger installations. This size would adequately ensure that the conductors can carry the necessary current without overheating or causing loss of voltage over longer runs, thus ensuring the electrical system operates safely and efficiently.

This sizing helps to prevent issues such as excessive voltage drop and potential overload conditions, maintaining the integrity and performance of the entire electrical system.