Dual 30A Dock to 50A Boat Adapter

[RobertAB]

Hi folks,

So I bought a boat that has a single 50A power port. It came with a converter that would allow me to plug my 50A cable into a 30A dock. It is a bit of a pain to manage power though because if the heater is on at the same time that the battery chargers kick on we will trip the dock (or two room heaters + freezer will trip it).

I bought a reverse Y adapter that will combine two 30A 125V males to a single 50A 125/250V female (like this one: https://www.marinco.com/en/ry504-2-30) but I am super afraid to plug it in. I don't understand how it can be both 125/250V on the boat side, I would think my boat would either be 125V or 250V and if I plug this thing in it could fry the boat. I've plugged computers in a data center into 240V before and they gave off the magic blue smoke, I really don't want my boat to give off that smoke!

There is a 30A/50A selector switch aboard. If I swap the boat over to 50A and plug in am I safe or is there something else I can check to make sure things are compatible before I plug it in.

[Dsanduril]

US power is 120V from hot leg to neutral/ground. If you have opposite phases you get 240V. This is very typical, nearly every house in the US has 240V (or possibly 208v) but most appliances only use one side of the circuit or 125V. You balance the load on each side (at least that's the plan).

This adapter requires that the two 30A receptacles be on opposite phases. Only your marina electrician can tell you that (or trying the adapter, it has lights to tell you). Basically it is really using two 25A circuits to make one 50A one. It also requires your boat to be wired to use power that way. Some are, some aren't. The manual is pretty clear:

Read the following instructions before using the Reverse Y.
The Marinco® Reverse Y provides 30A of current from the dock into a 50A 125/250V boat electrical system.
Boats with 50A 125V inlets cannot use the Reverse Y.

Take a look at the plug on the boat. What markings does it have? Should tell you if the plug is 120(5)V or 125/250. If you can access the back of the plug on the boat there should be 4 wires for a 125/250 - white, green (or green/yellow), and two colors (probably red and black). If there are only three wires (white, green (or green/yellow), and black) then it is a 120V only plug and the adapter won't help you. (Those wire colors are for US built/wired. If the boat was built in Europe or elsewhere post back with the wire colors you have).

That's the easiest way, look for the wire count on the back of the boat plug and see what you've got.

Where are you in the SFBay area? I happen to be in Oakland at the moment....

[rslifkin]

Some of those adapters are smart and will confirm the 30A plugs on are different phases before providing power. However, if the boat has a 50A 125/250 inlet and only uses it as 2x 125V legs (and has no 250V stuff on board), then it doesn't actually matter if they're out of phase or not, things will work fine regardless.

[RobertAB]

Looking at the back side of the plug is great idea. I'll look and report back. It's a Canadian built boat from 1973. I see other boats on the dock with the adapter, so I'm pretty sure the dock side will be compatible. I'm on the boat Saturday and will report back.

The boat is in Channel Islands Harbor down in Oxnard, I'm planning to leave it down there for at least the winter and might bring it north in the spring, we'll see if I get enough time anchoring at the islands by then...

[Dsanduril]

Yes, the adapter will actually work if both 30A are the same phase and you don’t need 240V. The reason the adapter is 125/250 is specifically so you get two legs. If they sold a dual 30A to 50A single phase (125V only), and you plugged your two 30A into different phases, that are connected to the same wire on the 50A side, well....

[Dsanduril]

One more thought, is the main breaker on your panel two-pole or one-pole? Two-pole may indicate 120/240 (but is not definitive), one-pole indicates a single 50A, 120V circuit.

Is there an electric water heater? If yes, and it also has a two-pole breaker then almost certainly 240V.

[smac999]

if it's a 240v 50a plug then the dual 30 y adapter will give you 30a 240. (twice of power of a single 30a, but still only half the power of a true 50a 240v plug)

if it's a 50a 120v boat then all you can do is plug it into a single 30a 120v

I have only ever seen 1 120v 50a boat. it must have been a thing in the 60's or something. certainly not something you see today.

[rslifkin]

Originally Posted by smac999

if it's a 240v 50a plug then the dual 30 y adapter will give you 30a 240. (twice of power of a single 30a, but still only half the power of a true 50a 240v plug)

if it's a 50a 120v boat then all you can do is plug it into a single 30a 120v

I have only ever seen 1 120v 50a boat. it must have been a thing in the 60's or something. certainly not something you see today.

50A/125V is definitely an older, less common standard. My 80s build boat has 2x 50A/125V inlets. I could easily re-wire to a single 50A 125/250 inlet, but I figure the 2 inlets and 2 cords is a bit more flexible.

[ikanode]

I've heard that depending on how your boat is wired that you can end up with one of the male plugs being "hot" when you plug in the other one. It wouldn't be an issue if you followed good practice and didn't turn on the dock breakers until you have everything hooked up.

Any comments on this possibility from the experts here?

[Cadence]

Stick a meter probe into both hots at the dock and see if you read 0 or 220/240. I suspect you want 0 and they are single phase a Y will just tie them together for the additional amperage.

The more I read the more I believe boaters should take a course in AC and DC fundamentals.

[waterman46]

Are you in USA? In the US, I don't think we have 240V on the dock except for the superyachts, and a power cord for that will have 4 pins and not be compatible with the 120V 50A outlet. We generally have 50A 120V single phase only on the dock, and that supplies a 50A breaker and a 30A breaker in parallel at the pedestal.
I assume your 50A receptacle on the boat has three pins. Go to the dock outlet with your multimeter and check between the hot side of each of those 30A outlets. If I am right, you will get close to zero volts difference between the hot sides of each 30A outlet on the dock. If so just plug in your two 30A cords.

[waterman46]

Originally Posted by Dsanduril

One more thought, is the main breaker on your panel two-pole or one-pole? Two-pole may indicate 120/240 (but is not definitive), one-pole indicates a single 50A, 120V circuit.

Is there an electric water heater? If yes, and it also has a two-pole breaker then almost certainly 240V.

On my old boat the 120V breakers are both 2 pole. The neutral and hot are both disconnected when breaker is open. This may be by code I don't know.

[smac999]

Originally Posted by waterman46

Are you in USA? In the US, I don't think we have 240V on the dock except for the superyachts, and a power cord for that will have 4 pins and not be compatible with the 120V 50A outlet. We generally have 50A 120V single phase only on the dock, and that supplies a 50A breaker and a 30A breaker in parallel at the pedestal.
I assume your 50A receptacle on the boat has three pins. Go to the dock outlet with your multimeter and check between the hot side of each of those 30A outlets. If I am right, you will get close to zero volts difference between the hot sides of each 30A outlet on the dock. If so just plug in your two 30A cords.

Every boat in the 40-50’ range has a 240v 50a plug nowadays. A 100’ will have a 100a 240v. (or dual 50’s) And bigger will be 3 phase.

120v 50a is non existent around here.

[smac999]

Originally Posted by waterman46

On my old boat the 120V breakers are both 2 pole. The neutral and hot are both disconnected when breaker is open. This may be by code I don't know.

Yes 120v. Needs 2 pole main and 240 needs 3 pole.

But 2 poles does not mean 120. It could just be 240 done wrong.

[smac999]

Originally Posted by ikanode

I've heard that depending on how your boat is wired that you can end up with one of the male plugs being "hot" when you plug in the other one. It wouldn't be an issue if you followed good practice and didn't turn on the dock breakers until you have everything hooked up.

Any comments on this possibility from the experts here?

Not if it’s wired right.