We have a project where there is an existing pump house building with set of 3/0 CU 480V 3-phase service conductors that enter a trough in a building from the meter outside (overhead service drop). In the trough, the 3/0 are then tapped to 3 enclosed circuit breakers. The tap conductors are sized for the enclosed breaker and not the 3/0. This all seems acceptable as the tap rules don't apply to service conductors. The loads are basically 2 large motors and then a small transformer for house loads. We have been asked to design for a new underground power feed and a new outside service disconnect then reconnect the trough and existing 3/0 and demo the overhead.
First, can the new disconnect outside be non-fused? If we fuse it, we plan to make it 200A to match the rating of the existing conductors but owner prefers to not have fuses. It is then my interpretation that by adding the disconnect, either fused or non-fused, the 3/0 in the trough now becomes a feeder and the tap rules now apply for the enclosed breakers. However the conductors are less than 10' so as long as the taps are rated for the enclosed breaker there should not be an issue. Next, if we fuse the new disconnect, I believe the code allows the fuse size to be larger than the conductor rating due to the motor load, correct?
First, can the new disconnect outside be non-fused? If we fuse it, we plan to make it 200A to match the rating of the existing conductors but owner prefers to not have fuses. It is then my interpretation that by adding the disconnect, either fused or non-fused, the 3/0 in the trough now becomes a feeder and the tap rules now apply for the enclosed breakers. However the conductors are less than 10' so as long as the taps are rated for the enclosed breaker there should not be an issue. Next, if we fuse the new disconnect, I believe the code allows the fuse size to be larger than the conductor rating due to the motor load, correct?
