46 CFR 111.10-9: -9 Ship's service supply transformers; two required.

If transformers are used to supply the ship's service distribution system required by this subpart for ships and mobile offshore drilling units, there must be at least two installed, independent power transformers. With the largest transformer out of service, the capacity of the remaining units must be sufficient to supply the ship service loads.

Note to § 111.10-9:

A ship's service supply system would consist of transformers, overcurrent protection devices, and cables, and would normally be located in the system between a medium voltage bus and a low voltage ship's service switchboard. It is not the intent, nor is it required, that transformers fed by the ship's service switchboard, such as 480/120 transformers, be duplicated.