46 CFR 111.10-9: Two ship's service supply transformers required
Last reviewed: October 14, 2026
This section covers transformers that supply the ship's service distribution system on ships and mobile offshore drilling units. If transformers are used, at least two independent power transformers must be installed. The remaining units must carry the ship service loads with the largest one out of service.
Applies toShips and mobile offshore drilling units that use transformers to supply the ship's service distribution system required by this subpart.
What it says
If transformers supply the ship's service distribution system required by this subpart on ships and mobile offshore drilling units, at least two independent power transformers must be installed.
“there must be at least two installed, independent power transformers.”
With the largest transformer out of service, the remaining transformers must have enough capacity to supply the ship service loads.
“With the largest transformer out of service, the capacity of the remaining units must be sufficient to supply the ship service loads.”
The note says transformers fed by the ship's service switchboard, such as 480/120 transformers, are not intended or required to be duplicated.
“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.”
Common questions
How many ship's service transformers are required?
If transformers supply the ship's service distribution system, at least two installed, independent power transformers are required.
“there must be at least two installed, independent power transformers.”
How much capacity must the remaining ship's service transformers have?
With the largest transformer out of service, the remaining units must have enough capacity to supply the ship service loads.
“With the largest transformer out of service, the capacity of the remaining units must be sufficient to supply the ship service loads.”
What does a ship's service supply system consist of?
The note says it would consist of transformers, overcurrent protection devices, and cables. It would normally sit between a medium voltage bus and a low voltage ship's service switchboard.
“A ship's service supply system would consist of transformers, overcurrent protection devices, and cables”
Do 480/120 transformers fed by the ship's service switchboard have to be duplicated?
No. The note says duplicating transformers fed by the ship's service switchboard, such as 480/120 transformers, is not the intent and is not required.
“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.”
The text of 46 CFR 111.10-9
Electronic Code of Federal Regulations (eCFR), current as of April 16, 2026 · Read it on eCFR
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.
The plain-English explanation on this page was written with AI from the regulation text above and checked automatically before publishing: every quote appears in the text, every number comes from it, and a second model confirmed each statement adds nothing the text doesn't say. Regulation text current as of April 16, 2026. The regulation text governs. RegKnot is not affiliated with the U.S. Coast Guard.