Before I explain about Y adaptors I want the readers to be aware that this is just a simplified explanation. Under real conditions at the dock and boat amounts of voltage and amps that can be drawn may fluctuate due to other factors such as tolerances of the circuit breakers.

Let us start with the 30 amp 125 dock receptacle and a boat with two 30 amp 125 volt inlets

Think of the dock as a pool of amps. A 30 amp 125 volt receptacle on the dock has a 30 amp pool available. The limit to what can be pulled from the dock is the circuit breaker on the dock. Normally in a 30amp 125 volt service it has a 30 amp circuit breaker. Now we do have one more limiting factor. The boat should have a main circuit breaker panel with a main circuit breaker per inlet . These breakers would matching the inlet example would be a 30 amp 125 volt inlet will have a 30 amp circuit breaker limiting the amount of amps the boat can draw from the dock. What a Y cord allows you do to is to pull from this 30 amp pool to your boat. You can have many combinations of amps drawn all adding up to a total of 30 amps or less. So 10 amps to one inlet and 20 to the other inlet or 15 to one and 15 to the other … If the boat tries to draw more than 30 amps from either inlet the 30 amp circuit breakers in the boats will trip. If the boat tries to draw more than 30 amps total between the two inlets the dock 30 amps circuit breaker will trip. Example 25 amps from one inlet and 10 amps from the other inlet 25 amps + 10 amps = 35 amps. The 30 amp circuit breaker will trip at 30 amps.

Next lets look at a 50 amp 125 volt dock receptacle and a boat with two 30 amp 125 volt inlets.

In this scenario the dock now has a pool of 50 amps with the docks limiting factor being 50 amp circuit breaker on the dock. The boat still has only two 30 amp inlets with the boats limiting factor being the 30 amp circuit breakers in the boat. The Y again will allow you to pull a combination of amps totaling now 50 amps from the pool; However the circuit breakers on the boat will only allow you to pull 30 amps per inlet. So you can have many combinations of amps all adding up to 50 amps or less So you could end up pulling 30 amps to one inlet and 20 to the other or 25 and 25 etc.…… If the boat tries to draw more than 30 amps from either inlet the 30 amp circuit breakers in the boats will trip. If the boat tries to draw more than 50 amps total between the two inlets the dock breaker will trip. Example 30 amps to one inlet and 30 to the other inlet. 30 amps +30 amps = 60 amps the 50 amp circuit breaker on the dock will trip at 50 amps .

Another scenario is the 50 amp 125/250 dock receptacle on the dock and a boat with two 30 amp 125 inlets on the boat.

This scenario is a little different. The 50 amp 125/250 contains two hot wires each containing 50 amp 125 volts. So in essence two pools of 50 amp. The dock will have a dual circuit breaker each rated 50 amp 125 volts. Again the boat still has only two 30 amp 125 inlets with the boats limiting factor being the 30 amp circuit breakers in the boat. In this case you have two pools of 50 amps . The Y adaptor will allow each 30 amp inlet to pull from one of the pools. The limiting factor being the 30 amp circuit breakers on the boat. . So in this case you can draw 30 amps per inlet or less, but no more than 30 amps as the 30 amp circuit breakers on the boat will trip. With the 2 breakers on the dock being rated 50 amps the main limiting factor falls on to the boats breakers. This set up will allow for the most power to your boat.