This is a 3-phase Y 100A 120/208V female connector designed for use at the end of a dockside shore power adapter or extension cord. Typically, such an adapter or extension cord would be custom built from components.
This connector has a weatherproof cover for those times when the extension cord may be left on the dock while your vessel is away, or simply to keep the contacts clean and dry when the cord or adapter is not in use.
The dock end male plug on your 3-phase Y 100A 120/208V cordset mates with this connector.

Yellow Zytel, a super tough nylon, watertight connector body
Spring-loaded cover closes automatically when not in use, and is watertight (IP67 - withstands temporary immersion) when securely fastened
Cord grip range is 1.00" - 1.94" (25.4 - 49.3 mm)

NOTE: This device cannot be mated to a boatside shore power inlet. You must use your cordset, with its M5100C9R female connector to connect to your vessel's shore power inlet.


Heavy on the technical side...

Why the Wye in 3-Phase Y (3Y) Power?*

Three-phase power allows utilities to deliver more power over smaller, less expensive wires.

Most homes are wired with single-phase power, which uses one AC voltage delivered over two hot wires and one neutral wire.
The voltage across the two hot wires measures 240V AC (for your oven, dryer, or A/C ), and across either hot to the neutral measures 120V AC (for everything else).

Most commercial businesses are wired with 3-phase power, which consists of three single-phase AC voltages separated from each other by 120 electrical degrees (a third of a cycle).
Three-phase power is commonly available in 208 volts and 480 volts (typically used by larger commercial facilities).
These systems deliver power over three hot wires, where the voltage across any two hot wires measures 208V AC or 480V AC.

There are two types of circuits used to maintain an equal load across the three hot wires in a 3-phase system Delta and Wye, referring to the shape of the wiring diagrams describing them:
The Delta configuration shows the three phases connected like a triangle (the letter "Delta"), whereas the Wye (or "star") configuration shows all three loads connected at a central neutral point - so the wiring diagram looks like the letter "Y".

Delta 3-phase systems have four wires - three hot and one ground.
Wye 3-phase systems have five wires - three hot, one neutral and one ground.

While both Delta and Wye systems measure 208V or 480V AC (3-phase) between any two hot wires, Wye systems also measure 120V or 277V AC (single-phase) between any single hot wire and the neutral.
In other words, while Delta systems can only provide one voltage, the extra neutral wire of the Wye system allows for providing two different voltages, and the powering of both 3-phase and single-phase devices.
As examples, many commercial facilities run lighting on single phase 120 or 277 volt current, and larger HVAC loads on the higher 208 or 480 volt current, both available from the same Wye connected electrical service.

Delta is used in long distance power transmission because its expensive to run an extra neutral wire all those miles.
So power stations receive their power in a Delta configuration, but commonly distribute it to their customers in a Wye configuration.
Thats why power station distribution transformers are commonly wired as Delta Wye.
The Wye configuration creates the neutral wire that allows customers to operate single- as well as 3-phase loads from the same electrical service.
Note that delta-wired devices, such as large motors or heaters that don't need a neutral, can be fed from a Wye source by simply omitting the neutral.

* Much of this information courtesy of Michael Salvador on Belden Inc.'s "The Right Signals Blog", 12/31/13.