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Old 04-03-2015, 12:20 PM   #1
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Shore Power - 50/30 amp Question

Hello All.

Just moved to a new slip. My boat is set up with a 30 amp shore-power inlet. The new slip has a 50 amp hookup next to the boat. Down the dock about 60 feet is a 30 amp hookup.

I apologize for my electrical ignorance, but what happens if you plug a 50 amp source into my 30 amp boat using some sort of 50/30 converter pigtail?

Or do I need to buy another power cord?

Thanks!
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Old 04-03-2015, 12:48 PM   #2
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You can certainly use an adaptor, but the real issue is the breaker at the dock probably doesn't protect your cord for it's capacity; check the "ampacity" of the wires within the cord. Marinco EEL Pigtail 30 Amp Female to 50 Amp Male
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Old 04-03-2015, 01:10 PM   #3
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It works fine, until you overload the 30 amp cord. Easy to do too. Run an electric heater in winter, combined with the water heater in the morning for a shower onboard, combined with your battery charger kicking in, your fridge firing up, the coffee maker getting you started for the day, etc... easily over the 30 amp rating of the cable, but if less than 50 the outlet will let it go thru without tripping the breaker at the dock.

We split our electric into 2 "legs" and use a 50 amp shore power with a pigtail that splits it into 2 of the 30 amp legs.. water heater/batt charger on one leg, electric outlets and fridge on the other. you can still over do it if you don't watch it.. but that is true no matter the set up.

If you do not live aboard and only use a little power while the boat sits there, not an issue using the 50 to 30 adapter. Just keep an eye on what is really running and inspect your cord/external plug regularly.

Oh, and remember you really don't want to run your amps much over 80% of the cable rating! You can still smoke a cable without tripping a breaker if you bring it up close to max and let it sit there.. .they get rather warm!
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Old 04-03-2015, 01:16 PM   #4
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Good points tmiller. A very regular routine should be to grab the shore power connecters and feel the first few feet of cable beyond them as well, at both dock and boat and see if they are at all warm. If anything they should feel slightly cool to your hand.
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Old 04-03-2015, 02:20 PM   #5
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So I understand, (I'd rather turn a wrench than chase electrons!)- a 50/250 outlet split will send 30/125 into each leg, or 50/125 per leg?

I'm moving to a new slip at the end of the month and have a 50/250 at the slip- our boat is twin 30A inlets.
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Old 04-03-2015, 02:46 PM   #6
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Originally Posted by caltexflanc View Post
You can certainly use an adaptor, but the real issue is the breaker at the dock probably doesn't protect your cord for it's capacity; check the "ampacity" of the wires within the cord. Marinco EEL Pigtail 30 Amp Female to 50 Amp Male
caltex... is correct your cord will only be protected for the amperage of the breaker at the pedestal. Your boat should be protected by a main breaker of 30 amps which should trip if you overload (more than 30 A). The risk is that you get a short in the cord before it enters the boat - probably a small risk.
I ran the set up you mention for 20+ yrs w/o any incidents as our marina has all 50A outlets and most of our boats are 30A.

For others wanting to run 2 - 30Amp cords from one 50Amp outlet - it's also doable - just be sure whether the 50A is 125V or 250V - there are different adapters for each and you need the correct one... don't ask how I know... the hard way.

Caution to all - be sure to check the temp of the cord at the boat entrance and the condition of the terminals - if any corrosion / burning is evident - common condition on many boats - both the cord or cord end female plug and male boat inlet need replacement. The attached issue of Seaworthy is worth a read if your boat has a power cord.

This NEMA TWIST LOCK PLUG CHART may also be helpful
Attached Files
File Type: pdf SeaworthyJul10.pdf (1.19 MB, 59 views)
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Old 04-03-2015, 02:48 PM   #7
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So I understand, (I'd rather turn a wrench than chase electrons!)- a 50/250 outlet split will send 30/125 into each leg, or 50/125 per leg?
50/125 into each. And 250v potential between the hots of the two legs, since it is alternating current and those two legs should be exactly 180 degrees out of phase. (It may help to visualize two sine waves, representing the AC of the two legs, super imposed on top of each other -- the result is a sine wave with twice the amplitude, which represents voltage.) Sometimes they are not 180 degrees out of phase, so you can read 125 on each leg, but the reading between them may be much lower than double, like only 210 (or some other, much lower than desirable voltage).
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Old 04-03-2015, 02:51 PM   #8
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Hello All.

Just moved to a new slip. My boat is set up with a 30 amp shore-power inlet. The new slip has a 50 amp hookup next to the boat. Down the dock about 60 feet is a 30 amp hookup.

I apologize for my electrical ignorance, but what happens if you plug a 50 amp source into my 30 amp boat using some sort of 50/30 converter pigtail?

Or do I need to buy another power cord?

Thanks!
50 amp service comes two ways....single phase and split phase....since you have not identified which is available we cannot help you with the some sort of converter you will need. Nevertheless, once you get the right converter you should have no problem. You only have one more thing to check. It is electrical law that your shore power have over-current protection for both positive and neutral conductors as soon as the shore power enters the boat. You need to be sure that your boat is in compliance. If it is in compliance then you should have 30 amp protection of your shore cord and it won't matter that you are hooked up to 50 amps.
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Old 04-03-2015, 02:54 PM   #9
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Doesn't the marina tell the slip user what you need for the hookups on the dock?
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Old 04-03-2015, 02:59 PM   #10
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Originally Posted by Pau Hana View Post
So I understand, (I'd rather turn a wrench than chase electrons!)- a 50/250 outlet split will send 30/125 into each leg, or 50/125 per leg?

I'm moving to a new slip at the end of the month and have a 50/250 at the slip- our boat is twin 30A inlets.
50/250 will split into 50/125 each leg, with the two legs you get 100 amps total.

You will only use 60 because your boat is set up for 30 amps each leg.
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Old 04-03-2015, 03:01 PM   #11
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Doesn't the marina tell the slip user what you need for the hookups on the dock?

HeHe HeHe
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Old 04-03-2015, 03:08 PM   #12
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Originally Posted by Pau Hana View Post
So I understand, (I'd rather turn a wrench than chase electrons!)- a 50/250 outlet split will send 30/125 into each leg, or 50/125 per leg?

I'm moving to a new slip at the end of the month and have a 50/250 at the slip- our boat is twin 30A inlets.
Each leg of 50/250 is protected by half of a dual pole 50A breaker. So if you load up one leg, it will trip around 50A, not 30. The breaker is sensing amps on both legs, either going over 50A gets a trip.

But most boats with 30A services have a breaker on the panel set at 30A. So you should be protected there. Check that.

I would not have any problem using a splitter to go from 50/250 to two 30/125's. Just recognize that the pedestal breaker is going to trip at 50 and not 30. As long as your panels have 30A breakers you should be ok.

But when leaving boat, it is safer to minimize electric load so shore connections are not loaded up. And do check for warmth on cables near the plugs/receptacles.

As you know from the ins business, damage from crappy shore cord connections is common.

I assist in investigating these things and one common bit I find on shore cord fires is a high amp load. Like leaving block heater on in winter or leaving a bunch of AC's on in the summer.
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Old 04-03-2015, 03:43 PM   #13
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I am officially electrically educated! Many thanks!
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Old 04-03-2015, 03:51 PM   #14
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Hello All.

Just moved to a new slip. My boat is set up with a 30 amp shore-power inlet. The new slip has a 50 amp hookup next to the boat. Down the dock about 60 feet is a 30 amp hookup.

Or do I need to buy another power cord?

When we moved to our current slip, the pedestal was set up on our side for twin 30s. The marina had their electrician re-do the pedestal to a single 50A/250V outlet. I didn't even have to ask.

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Old 04-03-2015, 03:55 PM   #15
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I just got a call from the marina owner (it isn't your typical shiny new big marina, it's just one long old wooden floating dock) and he said he would change the breaker so that it will trip at 30 amps. So I'll be all set with a plug adapter.
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Old 04-03-2015, 04:24 PM   #16
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As others have noted the two take different adaptors, so find out from him if its 50amp 250v or 50amp 125 at the pedestal. 125 is very rare now, but exists, assume nothing!

Quote:
But most boats with 30A services have a breaker on the panel set at 30A. So you should be protected there. Check that.

I would not have any problem using a splitter to go from 50/250 to two 30/125's. Just recognize that the pedestal breaker is going to trip at 50 and not 30. As long as your panels have 30A breakers you should be ok.
I and others here strongly differ with you Ski, the pedestal breaker protects the shore power cord, and a bad connection or short in the cable can drive the amps into the cord up. This is how fires happen even when the boat itself is not demanding current.
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Old 04-03-2015, 04:30 PM   #17
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I think the splitter cables are fine for temporary use, like visiting a dock for a few days. The exposure is in your 30A cord and the power connectors are each end, all of which will be able to "draw" up to 50A before the shore side breaker trips. If the overload is in your boat, your boat's breaker will protect against that.

But for longer term use, which is what it sounds like you are after, I think the correct way to do it is with a splitter box. I've never seen one commercially available, but it wouldn't be too hard to make one.

Basically you would have a short 50A cord that goes from the splitter box to the shore power outlet. In the splitter box would be two 30A breakers, then two 30A outlets. That would give you two 30A outlets, protected to 30A, that you can 100% safely plug your 30A power cord(s) into.
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Old 04-03-2015, 04:36 PM   #18
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Well...a bazillion y adapters have been sold and used by every major marina I have been in. They split 50 amp 125/250 service to 2 30 anp plugs and all have 50-60 amp breakers at the pole.


I think the theory is...the pole breaker would pop based on a fault whether a 30A cord or a 50A cord....how many faults are going to be exactly between 30-50A?


The amount of draw is limited by the breakers on the boat and should pop before the shore power cords are overloaded. Now if they fail and you draw more than 30...sure...but the designers, manufacturers, users and marinas allowing/supplying those Y cords don't seem to think it is worth worrying about.


I do agree that for long term use it would be better to have the correct setup...but I haven't ever heard or read of the use of a y cord causing an issue. Sure...maybe the ends or connection or something other than the lighter cord itself.
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Old 04-03-2015, 05:43 PM   #19
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The amount of draw is limited by the breakers on the boat and should pop before the shore power cords are overloaded.
That's the mistake people make, ignoring what can cause high amp draw even with the boat's demand minimal. And that's the kind of mistake that causes electrical fires on a few of those bazillion installations without good practices and very regular checkups.
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Old 04-03-2015, 05:58 PM   #20
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I agree I was a bit flippant on the lower protection offered by a 30 vs 50A breaker. But as PSN noted, and I agree, if a fault occurs, the difference in the breakers is a rather narrow window.

Also, in my investigations regarding shore cord fires, almost never was it an overload. The fault was a high resistance connection at the boat connection that was carrying below trip level current. This created enough heat to ignite something. Current draw likely never went up above normal. Never did the breakers trip until the fire caused conductors to short. At this point the fire was already started and the trip level of the breakers was irrelevant.

That's where it is a big deal to check for warm cable ends. If you are drawing amps and it is cold, it is good. Warm is BAD, next is hot and next is fire.

Also why it's a big deal to leave the boat with minimal AC amps being drawn. It's hard for a few amps of draw to start a fire through a high resistance connection. 25A is a different story.
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