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Old 19-04-2010, 10:59   #1
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Shore Power Cord Problems

The ground on my shore power cord (at the boat) showed signs of getting hot (plastic burned). I was told by my dockmaster that it was probably because it was probably loose and not screwed in tight. So, I bought a new power cord and made sure it was screwed in tight. Yesterday, it also showed signs of getting hot. Whats going on here??? Any thoughts??????????

Bob
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Old 19-04-2010, 12:03   #2
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This is a very common occurrence with typical 30A shore cord connections.

Whenever you see a burned lug, it's very likely that BOTH the receptacle and the plug have been damaged and will need to be replaced.

Burning can occur because of a loose connection, or because of a dirty connection or because too much load is being put on the connection or because of all three.

Fact is, 30A connections cannot carry 30A continuous. They're WAY overrated. IF they're brand new and have very tight, clean connections, they can approach the 30A mark, but it's best not to count on more than about 24-25A current in normal operation.

Bottom line: the ubiquitous 30A connector is a terrible design which lends itself well to the meltdowns and burned connections we so often see. There's a new 30A connector out which shows promise of reducing these problems. Meanwhile, replace any connectors -- including power post connectors -- which show signs of burning with brand new clean ones, and keep the connections very tight. Also, be very careful not to place a continuous load on these cords over about 25A.

Note also that if you're using a "splitter" the situation is even worse, because you could draw 50A thru EACH side before the 50A shoreside breaker would trip. That would ensure that your shore cords do some burning, and possibly your boat.
BEWARE.

One more thing: these connectors use compression posts to secure the wires. Because of "copper creep", it's necessary to periodically tighten these connections inside both the male and female connectors. Failure to do so will allow high-resistance connections to occur and that, of course, equals the potential for excessive heat and burning.

Bill
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Old 19-04-2010, 12:04   #3
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Since it's still happening with the new plug, the problem is in the socket. Most likely the contacts are corroded and pitted. Less likely but still possible is that the wires to the socket are loose. Either way the current running through the high resistance is generating heat.

Get the dockmaster to replace the socket. In the meantime, you might try *carefully* spraying some contact cleaner into the socket and plugging/unplugging a few times to try and scrape off the corrosion. At best this is a short-term fix.
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Old 19-04-2010, 12:18   #4
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Also, since it's the ground that's burned, you may have dropped a neutral somewhere and are pulling current thru the ground wire. Or, maybe somewhere on board you have both neutral and ground connected together. This shouldn't be the case....the only ground for an AC system should be at the source (shorepower, generator, inverter, etc.).
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Old 19-04-2010, 12:30   #5
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... One more thing: these connectors use compression posts to secure the wires. Because of "copper creep", it's necessary to periodically tighten these connections inside both the male and female connectors. Failure to do so will allow high-resistance connections to occur and that, of course, equals the potential for excessive heat and burning.
Bill
Under normal operating conditions, Copper wire doesn’t suffer from creep or cold flow.
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Old 19-04-2010, 13:04   #6
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Under normal operating conditions, Copper wire doesn’t suffer from creep or cold flow.
Gordon,

Well, I suppose that depends on what the definition of "normal operating conditions" is :-)

What I can tell you, from both personal experience and in learning from one of the nations best marine engineers, is that copper dock wiring connections DO loosen over time due to compression. When I first learned about this, I checked my own boat which has two 30A lines (six compression fittings at each end). What I found was that more than half of them were loose, and this despite the fact that I had installed them some time before and I tend to tighten things really tight.

Gremlins? Extreme temperature changes (we go from zero to 100 degrees Fahrenheit here)?

Bill
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Old 19-04-2010, 13:15   #7
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William:
Copper will begin to exhibit “thermal” creep at about 135o F, which might be plausible on a continuously “fully” loaded circuit in 100o ambient conditions.
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Old 19-04-2010, 13:31   #8
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Since you are going to change the inlet, I suggest you look at a much improved 30 amp/120 V model by SmartPlug: SmartPlug Systems - Shore Power Products and Accessories and change the inlet and the vessel end of your shore power cord.

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Old 19-04-2010, 13:32   #9
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shore power cord problems

I'm dealing with the same problem, waiting for the dockmaster to replace the socket so I now have 40' of cable running to an unused pylon down the dock. Part of the problem is the weight of the cable on the connection tends to pull the plug out. Even though they are twist connections, and the socket on the pylon is angled slightly downward the topmost connector tends to get loose in the socket. Unfortunately, at our marina, they don't use the better quality sockets with the threaded connection. I jury rigged a bunge cord to support the weight of the cable, seems to help in the short term.
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Old 19-04-2010, 13:46   #10
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I think one needs stress relifef on both ends With the tides and wave action the plugs will work loose in time
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Old 19-04-2010, 13:55   #11
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motion30 - Agreed, we're on a floating dock so not so much problem here but something to keep in mind when away from "home". Plus the boat connections have threaded ring connectors and, still I loop the cable over a lifeline stanchion so there's very little stress on that end. If the marina would use that type of connector on the power pylon it would help.
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Old 19-04-2010, 14:26   #12
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In addition to the foregoing suggestions, you might also want to ensure you are turning off the power before removing the plugs from their respective recepticals. I have been given to understand that, while powered, as one withdraws a plug the contact area of the prongs decreases causing current to flow through a progressively smaller surface area which gives rise to heating, so much so that the prong ends can develop indications of burning and the plastic at the edges of the slots will char or discolor.

FWIW...
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Old 19-04-2010, 14:41   #13
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In addition to the foregoing suggestions, you might also want to ensure you are turning off the power before removing the plugs from their respective receptacles ...
Indeed - always connect or disconnect under No-Load conditions.
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Old 21-04-2010, 22:48   #14
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In regard to tightening copper wire lugs, I can guarantee that the wires do compress and need to be tightened. I have 25 HVAC service technicians who do this task daily. Bill is right about cord ends not being able to handle continuous power. If you think about it, the connections are really pretty poor.
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Old 23-05-2010, 17:28   #15
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I'll be cruising the Great Lakes shortly and am wondering what length cord to buy. I've just bought a 50' 30 amp cable and am discouraged by the weight of the thing. (I have a weight sensitive 32' cat). Could I get away with a 25' cable. In other words, how often am I likely to find marinas where the power pedestal is more than 25' from my cockpit.

Thanks in advance.
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