Can we build one building over the top of existing underground main service feeders (either medium voltage or 480V) that currently serves another building?
Not if it is owned by the utilities otherwise there is no problem as long as all the rules are followed. When you say service feeder it is confusing. Service or a feeder?
Can we build one building over the top of existing underground main service feeders (either medium voltage or 480V) that currently serves another building?