50 amp to 30 amp.

Please advise!
I have a 2005 39’ Sea Ray motor yacht. Aft cabin. It runs on a 50 amp plug or split two 30 amps into separate plugs. I have a splitter.
However, I can’t run 120 volt systems on 30 amp. I have an adapter to the so the 50 amp can fit into a 30 amp plug.
I know you can’t run the A/C without 50 amps but shouldn’t everything else be OK? Am I missing something?

This is somewhat of a guess because your post just isn't very clear. But, sounds like you have two 30amp inputs on your boat. And you have a splitter the takes a 50amp connection at the dock pedestal and splits it to the two 30 amp lines at your boat? Is that correct?

The a/c needs 10 amps to run. It plus everything else you have turned on for that line can't add up to more than 30 amp draw total.

Have you verified you have power at the pedestal turned on? Do you have the power switches at the connectors on the boat, turned on? Do the voltmeters at the power panel show 120 volts?

Ok. Sorry for my lack of terminology.
I am trying to take my 50 amp cord— that comes from the boat— and use an adapter that fits a 30 amp plug.
I thought I’d be able to run the 120 volt portion of the boat and not the 240 side which is A/C.

When I have the 30 amp adapter plugged into the 50 amp cord, then plugged it into the 30 amp pedestal (on the dock) nothing happens. The boat is dead except for the 12 volt system running off the batteries.
Everyone I’ve talked to says the same thing you did in your last post. But it doesn’t work on my boat.

All you are doing is putting a 30 amp end on a 50 amp cord and it doesn't work? That would be a first. More likely there is a problem with the 30 amp connection at the power pedestal.

Yes. A 50 amp cord comes from the boat. I have a 50 to 30 amp pigtail.
When I plug into the 30 pedestal- nothing happens. The pedestal has power. I am not sure if I am missing something or the boat isn’t designed for 30 amp service. This hasn’t worked in several slips.

Oh... and by the way it all works great in when plugged into 50 amp. (Or running the generator.

Sounds like your 30amp adapter might be bad. Have you tested it?

The adapter is brand new. I guess I could borrow one and try it. I wonder if it was designed not to accept 30 amp.

If anybody is still interested... after looking through the owners manual it implies that the 30 amp converter is an option?

I read your boats spec sheet and it says it needs 240v shore power. If that's true than a 30amp source won't work as those are 120v at marinas.

Here
a 30 amp is a 3 wire, a 50 amp is a 4 wire

It sounds like you need a reverse Y with 2 30 amp plugs

https://www.marinco.com/en/ry504-2-30

Ted Christensen said:

Ok. Sorry for my lack of terminology.
I am trying to take my 50 amp cord— that comes from the boat— and use an adapter that fits a 30 amp plug.
I thought I’d be able to run the 120 volt portion of the boat and not the 240 side which is A/C.

Click to expand...

Sounds like what you need is a "Smart Y" adapter. It connects two 30 amp supplies from the dock to your 50 amp cable; however, it will not work if the two 30 amp supplies are the same phase.

The boat shore power has to be 50 amp. I have used a y smart adapter (that splits into two 30 amp plugs). As long as the pedestal has two 30 amp plugs on separate circuits... It’s fine
I just trying to figure out if there is a way I can run part of the boat on 30 amps. Apparently the boat isn’t set up for that. Many boats are.

Ted
I have the same boat. When you select run on the start switches, do you get a low oil pressure alarm. If so does it come out of the smart craft or the system monitor?

No. When I switch to the run position I don’t get a low oil pressure alarm at either station.

Ted - lots of information but much incorrect - here are a couple of hard facts that need consideration -

50 AMP power at marinas (NMEA L14-50 twist-lock) - The configuration is three blades and a conductive outer shell (2 blades are 120 volt the third is neutral, and outer shell is ground). The power to the receptacle is two 120 volt sources (hence 125/250 volt designation) that are 180 degrees out of phase; consequently a voltage measurement across the two 120 volt sources will be 240 volts and how that 240 volt is derived. Your boat's 120 volt accessories are from one or the other 120 volt lines and the neutral; however the exception is, if your boat has an isolation transformer then you must provide the 240 volt source to that isolation transformer or nothing will work as your 120 volt accessories on the boat are end taps from that 240 volt isolation transformer's secondary windings. As an edit - if you have an isolation transformer, the neutral from the dock power is not used and not connected at the boat; the isolation transformer derives the neutral from the boat's ground network connected to a center tap on the transformer's secondary windings so, there is no way to get a single 120 volt supply from the dock power.


30 AMP power at marinas (NMEA L5-30R twist-lock) - The configuration is three blades (one 120 volt, one neutral, and one ground). All US marina 30 amp power is 120 volt. Sea Ray boats with two 30 AMP cords are simply providing enough current (amperage) at 120 volts to support the boat's demand. Now an interesting fact is most all marina's provide 240 volt to the power pedestals and one leg of that 220 volt provides one of the 120 volt 30 amp receptacles while the other leg provides another 120 volt 30 amp receptacle. Should you measure across those two 30 amp feeds you will get 240 volts.

Also, some marinas have 50 amp 125 volt capabilities but those are rare and to my knowledge, Sea Ray has never optioned for this.

If you are using a correct 50 amp 125/250 to 30 amp 125 volt to volt adapter and it's not working then you probably have an isolation transformer. Look for a rather large white box in the incoming wiring on the boat; my boat has an isolation transformer.
So if you do not have an isolation transformer you would need that specialty "Y" adapter such as a Marinco P/N 153AY then you could select which 120 volt side to supply from the single 30 amp dock power.

Tom, the spec sheet for the 2005 390 aft cabin implies it has the isolation transformer.

As previously mentioned, If you have an isolation transformer then you need a reverse y adapter. For example, Marineco
Reverse Y Adapter, (2) 30A 125V Males to 50A 125/250V FemaleRY504-2-30
Thanks
Mike