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Old 07-02-2022, 09:15 PM   #1
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GB42C at a 30 amp dock?

Our new to us GB42C 1986 looks like it is wired for 50amp shore power since both forward and aft outlets are rated at 50amps.

Our dock only has 30amps which is just what it is. There isn’t another 30amp pedestal option so we can’t pigtail two 30amps into a 50amp supply.

When we arrived at this new dock, we noticed there was on board our new to us boat a 50amp (boatside) to 30amp (shoreside) single shore-power cable… no adapters… just 50 on one end and 30 on the other.

Well, not surprisingly in the winter we exceeded 30 amps (actually less would be the ABYC threshold), and I noticed the 30amp plug at the pedestal was burnt on one of the terminals last week.

I would have expected the pedestal breaker to have tripped, but it didn’t.

Possibly the sealed Marino plug was ‘strained’ in a previous life and ‘cooked’ because of a poor internal connection…. So my ire against the pedestal breaker may be misplaced.

We will encounter other cases where only 30amps is available, so has anyone heard of an in-line marine grade 30amp breaker box to trip if the boat draws more than the limit?

What do other Grand Banks owners do when facing this scenario?
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Old 07-02-2022, 11:12 PM   #2
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Sounds like your 50 amps is 120 volts, not the 240 volts you would normally see with 50 amp service. You rarely, if ever see marinas offering 120/50 service. It is either 120/30 or 240/50.
I used to have a boat that came from the factory wired for 120/50. Apparently, some years ago, back in the eighties, that service could be found.
Like you, the previous owner of our boat had power cords made up with 50 amp female end and 30 amp male end. With energy management, we managed just fine on 30 amp service. We had air conditioning on one cord and everything else on the second cord. With 3 air conditioning units pulling 29 amps, it was close to throwing breakers, but seemed to work ok.

BTW, my boat was a 1988, so both built around the same time with the same electrical service. It’s not a GB issue, but rather an issue with boats built back in the 80’s.
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Old 07-02-2022, 11:25 PM   #3
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I might add, stick with what you have as most likely you will never see 120 volt 50 amp service anywhere. If your cords are cooked, replace with the same 30/50. You might want to have the pedestal plug replaced as well as the pedestal breaker. Sounds like you had a bad breaker from the get go.
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Old 07-03-2022, 12:09 AM   #4
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Also if you pull close to 30 amps then you should look at a Smart Plug cordset. They have much more contact area than an L5-30 plu.
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Old 07-03-2022, 12:13 AM   #5
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First, you should have a master circuit breaker on the boat. What is it’s amp rating?

Second, you can change the master circuit breaker to 30 amps to solve your problem.

Third, your power cord could easily burn up with out ever exceeding 30 amps. A poor connection between your male end and the docks female end could result in extreme heat from as little as 15 amps. It is the heat that does the damage not the amps. Although more amps will generate more heat.
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Old 07-03-2022, 09:20 AM   #6
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I'm curious how you split the 30A to a 50. Just make two legs off the one hot 30?
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Old 07-03-2022, 01:48 PM   #7
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Ha - you guys nailed it. Looking at the electrical diagram it shows input is 50amp at 125v. The boat breaker is 50amp.

There is no split other than there is a fore/aft input plug selector. Once selected, power feeds to the central load distribution on the AC panel.

Since there aren’t too many 50amp / 125v docks out there, is it a consideration to look for a 240v to 125v step down transformer?
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Old 07-03-2022, 02:13 PM   #8
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You can use one of the 50A 120/240 to dual 50A 120 splitters and just plug in 1 leg.
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Old 07-03-2022, 02:42 PM   #9
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Excellent.. that’s a whole lot cheaper than buying a new gizmo. We have a Y splitter like that on board.

Appreciate all the contributions… learnt something fundamental about our boat today.

Leaves one key question; where the 30 amp dock breaker is flaky, and with the older marinas this is quite possible, has anyone seen a 30amp breaker box unit to be put in-line to save the power cable and reduce the risk of fire?
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Old 07-03-2022, 02:47 PM   #10
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What I would do is replace the boat socket from 50a 125v to 50a 125/250v. Cut the end off your 50a cord (do not use a 30a cord) and add a 50a 125/250v socket on both ends of your power cord. Your power cord will be black white green wire. The sockets will have hook ups for 4 wire but you won’t use the red.

Now you can plug into a 50a 125/250v socket all will work fine.

Next buy a 30a 125v to 50a 125/250v adapter. You will need to manage power when using the adapter or you will trip the dock breaker.

A two 30a to one 50a 125/25v Y adapter won’t be of any use with this set up.
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Old 07-03-2022, 07:10 PM   #11
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We are on our 4th Grand Banks. I was told by our long-time GB broker that almost all older GB's were made with 50A 120 volt shore power inlets. One boat still had the 50A inlets but came with a bin full of 30 amp cords and a short 30A male x 50A female pigtail. Other boats had already been converted to 30A inlets. I am only aware of one dock in the PNW that has a 50A 120 volt service although there are no doubt more out there, but definitely uncommon. The current boat came with a newer 30A inlet on the bow and the factory 50A inlet at the stern. I changed them both out for 30A SmartPlug receptacles. We don't have AC and really have no need for 50A service, but YMMV. Good luck!
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Old 07-03-2022, 08:22 PM   #12
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Thanks Scott, seems like good advice.

Curious to know which Marina around the PNW still runs 50amp at 125v?
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Old 07-03-2022, 11:08 PM   #13
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The one spot that I am aware of is the hadicapped-accesible guest dock (aka guest dock 2) at the Port of Everett marina, near Lombardi's restaraunt. Not the whole marina. That's the only one I can remember!
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Old 07-04-2022, 01:51 PM   #14
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Originally Posted by tiltrider1 View Post
What I would do is replace the boat socket from 50a 125v to 50a 125/250v. Cut the end off your 50a cord (do not use a 30a cord) and add a 50a 125/250v socket on both ends of your power cord. Your power cord will be black white green wire. The sockets will have hook ups for 4 wire but you won’t use the red.

Now you can plug into a 50a 125/250v socket all will work fine.
Seconded, this is the setup I use at my home dock. I use the 240v 50 amp smartplug on the boat end. In the future, this also lets you add a second 120v leg or 240v service to the boat, however that is a much bigger and more complex change. Having 50 amps versus 30 makes a big difference for me and the connectors are much more robust, I would never go back to 30 amp.

In addition, I also have a standard 30 amp cord with the boat end cut off an replaced with the 240v 50 amp smartplug to have a lighter cord to drag around at transient docks with 30 amp service. It isn't ideal relying on the 30 amp dock breaker with the boat's breaker at 50 amps, but is still better than a lot of options.

If you never care about 50 amp service, you could switch out to the 120v 30 amp standard setup and change out the main breaker on the boat to 30 amps, however on my boat that same breaker is used running off the generator which can deliver >50 amps in my case so that is a non-starter.

You certainly could rig up an inline breaker but I'd want it permanently wired into the cord or boat, wouldn't want to add two more plug in connections in the middle. You could also rig up something on the boat using a couple of breakers and a source selector lockout (eg. https://www.bluesea.com/products/803...ource_Selector) but I'd probably consider that overkill.


If this is a long term dock you could consider having an electrician out to swap the breaker and dock side outlet for new ones (120v 30 amp), as both do wear out, especially the 120v 30 amp plugs.
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Old 07-10-2022, 08:58 AM   #15
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Ha - you guys nailed it. Looking at the electrical diagram it shows input is 50amp at 125v. The boat breaker is 50amp.

There is no split other than there is a fore/aft input plug selector. Once selected, power feeds to the central load distribution on the AC panel.

Since there aren’t too many 50amp / 125v docks out there, is it a consideration to look for a 240v to 125v step down transformer?
Why not Change your master breaker to 30 amp and replace fore and aft inlets with Smart plug 30 amp ends, now you are 120V 30amp with great connectivity, totally support changing from Marinco to smart plug we did it 3 years ago, and have not had an issue since.
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Old 07-10-2022, 09:22 AM   #16
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Why not Change your master breaker to 30 amp and replace fore and aft inlets with Smart plug 30 amp ends, now you are 120V 30amp with great connectivity, totally support changing from Marinco to smart plug we did it 3 years ago, and have not had an issue since.

If his boat is designed for 50A/125V service, a 30A supply may not be enough to power all his systems simultaneously.

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Old 07-10-2022, 09:27 AM   #17
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If OP's GB is a 50A/125V boat...

Given a 50A/125V boat and a 50A/125V shorepower cordset... for the times when a 50A/250V service is available... I'd think this adapter would work easily enough:

https://marinco.com/en/p/P504-503/EE...-125-250V-Male

A 4-wire 50A/250V cordset is fairly heavy for schlepping around, heavier than necessary for a 125V boat... so the adapter would get around that issue.

There's also an adapter from 30A/125V to 50A/125V, and I suspect it only allows 30A to pass through... which I think assumes a 50A/125V cordset... but which might speak to OP's question about breakers:

https://marinco.com/en/p/P30-503/EEL...A-125V-Male-to

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Old 07-10-2022, 01:58 PM   #18
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If his boat is designed for 50A/125V service, a 30A supply may not be enough to power all his systems simultaneously.

-Chris
But his home base is a 30-Amp dock, which means he'll never have enough to support a 50-Amp system and will continue to have burnt ends and the attendant fire hazard. He must change to onboard breaker to 30 Amp or he's gambling every day.
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Old 07-10-2022, 06:14 PM   #19
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But his home base is a 30-Amp dock, which means he'll never have enough to support a 50-Amp system and will continue to have burnt ends and the attendant fire hazard. He must change to onboard breaker to 30 Amp or he's gambling every day.
This option was already offered. OP responded with he had access to 50a 125/250v. It was then recommended that he change the ends on his power cord to take advantage of 50a power. This solves his problem.
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Old 07-10-2022, 06:32 PM   #20
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But his home base is a 30-Amp dock, which means he'll never have enough to support a 50-Amp system and will continue to have burnt ends and the attendant fire hazard. He must change to onboard breaker to 30 Amp or he's gambling every day.
Absolutely wrong. If the dock pedestal is properly breakered at 30 amps then it's safe to plug in a 50 amp boat with an adapter and you won't be able to overload the 30 amp components.
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