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Old 17-09-2010, 09:36   #1
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50 Amp Boat with 30 Amp Shore Power ?

Hi, my new boat has a single 50 amp cable but my dock has 2x 30 amp outlets on the post. Is there a way I can safely plug in?
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Old 17-09-2010, 09:40   #2
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Is your boat 50a 240 volt? If so you want to use a Smart Wye. This device has relays that sense that there is 240 volts available via the two 30 a connections, and will click on if so.

If you have a 120 volt boat a simple adapter will do, and you will just use one of the two 30's.
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Old 17-09-2010, 16:52   #3
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Also if your boat has two separate AC systems - each being 30 amp, then the simple Y-adaptor also works. It consists of two 30 amp plugs wired into a single 50 amp companion to your shore power cord's plug. Some modern Catamarans have two 30 amp power receptacles on the boat. One is for normal boat household AC needs and the other for heavy AC loads like air conditioners - all of them being 120VAC equipment.
- - You can also use a simple voltmeter to test shore power receptacles to see if the hot's will give you 220-240VAC. Normally the shore power pedestal will have 30 amp connectors on opposite sides of the pedestal. Some have two 30 amp connectors on the same side of the pedestal. The same side connectors are normally on the same pedestal power supply for 120VAC. The receptacles on the "other" side of the pedestal are on the "other leg" and if you connect the hot from one side of the pedestal to the hot on the other side of the pedestal you will see 220-240 VAC on the meter.
- - So you need to know if your boat's AC system includes 220-240 VAC loads (e.g. washer-dryer) before you can know which shore power 30 amp receptacles on the pedestal you should connect the adapter Y-cable.
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Old 17-09-2010, 19:13   #4
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Well if he has a 50 amp plug then it is a given that it is a 240 hook-up. 2 -120v legs. That said I find it interesting that they wire the pedestals different , here in the good old USA one side has your all of your circuit breakers , 120-120(2-30 amps on seperate legs)' just use 1 if you are a single 120 boat,or your 50 amp 240 plug in. Sounds like fun figuring out how the marina is wired. On a side note you get more bang with 2-30 amps (total 60 amps) versus the single 50 amp.
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Old 17-09-2010, 19:27   #5
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My marina bought the adapter for me with their wholesale discount (a 50 to two 30s Y adapter) , saved me $100. Maybe your marina will do the same.
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Old 17-09-2010, 19:44   #6
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I Just had my boat rewired...

My electrician split the boats amp draw to utilize the existing twin 30 amp shore power receptacles on the boat..( The PO had " Bodged" around it...( Thanks Dave)....He told me to just buy a Y and go ahead and hook into a single 30 amp power supply at the dock as that is most common...he said the chances of actually drawing 30 amps in any one instance is slim to none in actuality.....For my boat anyway.
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Old 17-09-2010, 20:59   #7
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Originally Posted by Mark Zarley View Post
Well if he has a 50 amp plug then it is a given that it is a 240 hook-up. 2 -120v legs. That said I find it interesting that they wire the pedestals different , here in the good old USA one side has your all of your circuit breakers , 120-120(2-30 amps on seperate legs)' just use 1 if you are a single 120 boat,or your 50 amp 240 plug in. Sounds like fun figuring out how the marina is wired. On a side note you get more bang with 2-30 amps (total 60 amps) versus the single 50 amp.
- - With electrical systems on sailboats there are few if any "givens." As Stillraining mentioned many previous owners think they are "improving" the system when actually they are screwing everything up such that you need to hire a professional to put it back to what the manufacturer originally designed. I have seen some "scary" electrical modifications done by previous owners who thought they were fixing things.
- - And also in many marinas, especially older ones and 3rd world ones there are some pretty scary electrical dock pedestals/boxes/connections. You really need to personally use your own voltmeter to find out how or if their receptacles work and how they are wired. It is not uncommon for the dock electrical receptacles to be wire backwards (reversed polarity).
- - I carry an assortment of various size plugs and receptacles that I can mix and match on an adaptor cable to get the proper electricity to the boat.
- - One important hint, get some "dielectric silicon" in a tube. West Marine sells a very small size tube and electrical outlet stores sell bigger tubes for a fraction of the West Marine price. And then smear some on the blades of your shore power cord connectors. This helps prevent the arcing that turns receptacles and plugs black and eventually melts away the contacts.
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Old 19-09-2010, 10:51   #8
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Quote:
Well if he has a 50 amp plug then it is a given that it is a 240 hook-up. 2 -120v legs.
Not so fast...50 amp/125 VAC shore pedestals are also prevalent in some parts of the USA.

I concur with Post #2; that the Smart Wye is the safest way to bring 2 x 30 amp service into a 50 A /250 VAC vessel. The reasoning can get a bit technical very fast and has to do with how the pedestal power is derived from the shore mains; either delta or wye wound transformer and whether the 2 x 30 amp services are in or out of phase.


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Old 19-09-2010, 11:50   #9
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Originally Posted by Mark Zarley View Post
On a side note you get more bang with 2-30 amps (total 60 amps) versus the single 50 amp.
Actually if it is a 240-volt boat the math goes like this: Power = Volts X Amps. The two 30's give you 30 amps at 240 volts = 7.2 KVA. The 50 amp service at 240 volts yields 12 KVA.

If the boat is a 120 volt system then two totally separate 30 amp services installed in the boat gives 7.2 KVA. The 120 volt 50 amp connection would provide 6.0 KVA. The emphasis is placed on separate because the possibility of supplying any loads on a boat from two shore power circuits combined in parallel is a very serious hazard.
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Old 19-09-2010, 13:17   #10
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The size of your cable doesn't necessarily have to be the actual current you draw. It might be over dimensioned. It indicates only the maximum power you can safely transport through that cable.

You need to know the actual maximum power consumption of your onboard systems.

Unless you have an airconditioner (might be around 10 amps at 240V or 20 amps at 115 V) or an electric heater (might also be 10 amps at 240V) or electric cooker (might be up to 30 amps at 240V, double for 115V), you might never be drawing 50 amps from the shore supply. You need to add al possible appliances to come to a safe total, so include appliances connected to an outlet, that is connected to shore power.

On our 36 ft sailboat we have a 16 amp, 240V cable, but we never use more than 10 amps. But we can safely plug into both 30 and 50 amp receptacles.
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Old 19-09-2010, 13:49   #11
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European AC power systems are significantly different from N&S American AC systems both in wiring and usage.
- - Assuming the OP is wired as a North American boat then he needs to determine if he has 2 Parallel AC systems (normally port and starboard on older boats) with some 240VAC loads such as washer/dryers or other heavy load items like heaters. Or, he has a two separate AC systems with one being a primary ship's AC system and the other being an isolated heavy load buss for such things as air conditioners, dive compressors, etc.
- - After determining what the boat is wired for, then he can determine if a single 30 amp to 50amp adaptor cable will suffice or he needs an adaptor cable with a Y and two 30 amp connectors that must be plugged into a dock box with "split leg" receptacles as described above by myself and others.
- - Only after determining what the boat has and needs can you safely choose the proper adaptor.
- - Some ancient marinas have only the household 20 amp 120VAC 3-prong plug receptacles and you need to make your own adaptors or find a series of store bought adaptors that will combine to fit your 50amp/240VAC cable connector.
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