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Old 06-28-2019, 11:14 PM   #1
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Shore Power

I have a 50 amp shore power plug. Right after the plug on the boat, it splits into two circuits. One circuit powers the 3 AirCon units. The other powers everything else.

Right at the plug on the boat, there are two 50 amp fuses: one for each circuit.

From the fuses, each circuit runs to the breaker panel where each has a circuit breaker. There is a light above each breaker that illuminates when there is power to that breaker.

The other night the AirCons suddenly lost power. All the other AC power on the boat still worked. The light above the AirCon circuit was off, the other was on. I switched to generator power and both lights illuminated and the AirCons worked again.

My theory was the shore power cord only had one circuit. Since one circuit on the boat still worked and one did not, the problem had to be after the shore power plug on the boat, but before the breaker panel. This pretty much left the fuses at the shore power plug. I pulled both and both had continuity. I replaced both with new fuses anyway since I was in there, but had the same result.

My next thought was the only possibility was the wire between the shore power plug on the boat and the breaker panel.

I thought since there was only one circuit from the dock pedestal to the boat, and since one of the AC circuits on the boat still worked, the problem could not possibly be the pedestal or the shore power cord.

My wife convinced me to try it anyway. We moved the dock side of the shore power cord to another pedestal, and everything suddenly worked.

What I take from this is shore power doesn't work the way I thought it did. Either my 50 am cord somehow has 2 circuits or my whole theory of how circuits work is incorrect.

Can someone explain the theory to me that explains how a single 50 amp shore power cord can bring down one of the circuits on the boat, but not the other? The 50 AMP shore power cord has three prongs. I always thought once was hot, one neutral, and one ground.
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Old 06-29-2019, 12:53 AM   #2
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You have a 50a 120v/240v circuit. This gives you 4 wires, two black ones that are both positive but out of phase with each other, one neutral and one ground. One of the positives in the pedestal lost power. If you had any true 240v appliances on board they would have stopped working unless they use a step up transformer off just one of the 120v legs. Now I’m probably just confusing you.

You only see three prongs because the ground uses the metal on the side of the plug to complete its circuit.
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Old 06-29-2019, 05:55 AM   #3
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Many 240V 50A outlets have dual circuit breakers, one for each 50A leg of the power.

Resetting the breaker probably would have solved the problem.

For RV's there are devices that will stop 2 air cond from starting at the same time.

There is about a 2 min delay for the second , or third unit to be powered.

My guess as to what popped the CB.
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Old 06-29-2019, 06:22 AM   #4
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If switching power pedestal plugs resolved your problem, that's great. As to whether it's a breaker problem, failed wire between the breaker and the receptacle, or something further upstream is only a guess at this point. Having a multi meter to check continuity of fuses and for voltage at the power pedestal receptacle, makes diagnostics much quicker.

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Old 06-29-2019, 06:23 AM   #5
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Quote:
Originally Posted by Rain Dog View Post

.... The 50 AMP shore power cord has three prongs. I always thought once was hot, one neutral, and one ground.
This is not true for the 240/120 50 amp plug.
There are two hots, one black and one red, one white neutral, and one green ground.
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Old 06-29-2019, 06:29 AM   #6
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What I don't know about electricity would fill several books but I did have a 42 GB with the same set-up. You might want to look at the 2 fuses located at the shore power plug on the boat. The 42 GB has 2 shore power connections, bow and stern, and you could try plugging into the opposite end of the boat. We had a similar issue and replacing the fuse solved the issue.
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Old 06-29-2019, 08:57 AM   #7
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You can lose one of the two hot feeds anywhere almost back to the road and act the same way on your.boat. Because changing power pedestals fixed it, the problem lies there. Either the receptacle or its breaker are bad.
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Old 06-30-2019, 05:24 AM   #8
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The 120/240V 50A plugs/receptacles can be confusing. They only have 3 prongs, yet there are 4 conductors. The ground is the shell of the plug, and the prongs are line 1, line 2, and neutral.
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