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Old 19-12-2012, 09:04   #1
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Shore power voltage fluctuates 90V-120V

I am in Shelter Bay, Panama and have shorepower, my boat requires 30Amp, 125V. The boat is docked bow first so I have two cables connected in order to receive the shore power aft. My AC volt meter shows fluctuations between 90V and 120V and the charger switches on and off. What could be the reason. I had the dock master check the outlet on the dock, is seems OK. We checked the connections of the cable, they seem OK. Any suggestions on what to do or cause?
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Old 19-12-2012, 09:20   #2
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Re: Shore power voltage fluctuates 90V-120V

If you haven't measured the dock voltage to ensure it is constant, that should be the first step. If it is, the only other possible reason for v fluctuation is an intermittent resistance someplace in line.

The one assumption here is that the source of the resistance is NOT your charger which, if shorted intermittently or open intermittently can cause this problem.
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Old 19-12-2012, 09:26   #3
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Re: Shore power voltage fluctuates 90V-120V

I've seen a number of "brownouts" in the BVI on shore power and they can be very bad for electrical equipment, since when the voltage drops the amps used go up accordingly and those can burn out electrics in short order. I would measure voltage at your dock receptable, or better yet, a neighbouring receptable to show it is a supply side problem. I am assuming that your charger is cutting in and out due to low voltage - do you ever see lower voltages when the charger is turned off?
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Old 19-12-2012, 09:45   #4
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Re: Shore power voltage fluctuates 90V-120V

If you have an infrared thermometer check the temp. at all of your connections, also check the temp. along your shore power cords. The most suspect place for high resistance is right at the shore power plug on the boat. If you can't measure the temp. try putting your hand on the connection. If it is hot, or even warm, you have a problem there. This connection point is a big source for fires.

Edit: I am assuming that your power cords a capable of handling the load. If they are not capable of handling 30 amps then they could be your problem.
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Old 19-12-2012, 20:31   #5
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Re: Shore power voltage fluctuates 90V-120V

Thanks everybody. We tested the voltge at the dock, this seems fine. We are testing our cables (yes marine cables to carry 30AMP 125V) for resitance but the readings are not conclusive (0 - 25 Ohm). I guess testing the shore plug on the boat is the next item.
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Old 19-12-2012, 21:08   #6
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Re: Shore power voltage fluctuates 90V-120V

1. All connections need to be clean and tight. If you see ANY sign of discoloration around the pins for the 30A receptacles or plugs, you need to CHANGE them out.

2. Copper creep can set in as well. If your 30A plugs or receptacles have set screws, be sure they are very tight.

3. Typical 30A cords and connectors cannot carry 30A continuous. These connectors are a terrible design which has caused multiple problems and multiple fires and meltdowns for several decades. Be absolutely sure everything is clean and tight, and don't load more than 24-25A on these cords.

4. Don't coil the cords around the shore post or elsewhere; this limits the current-carrying capacity.

Bill
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Old 19-12-2012, 22:44   #7
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Re: Shore power voltage fluctuates 90V-120V

Where were you measuring the 0 - 25 ohms? 25 ohms on a wire in a cable is way to much.
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Old 19-12-2012, 23:32   #8
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Re: Shore power voltage fluctuates 90V-120V

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Originally Posted by btrayfors View Post
4. Don't coil the cords around the shore post or elsewhere; this limits the current-carrying capacity.
Bill, can you explain this? The only possible reason I can think of is that the closely-spaced turns of the coil might restrict airflow and thermal radiation (cooling). For this to be an issue, the cable's got to be *really* tightly coiled (or bunched), and the current has to be *really* high. I think the connector will be an issue long before the cable itself starts to get too hot.
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Old 19-12-2012, 23:38   #9
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Re: Shore power voltage fluctuates 90V-120V

Did you measure the voltage at a second plug on the same circuit on the shore pole while plugged in drawing power? If you measure the voltage with no load you won't discover problems with the shore side of shore power.

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Old 20-12-2012, 03:48   #10
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Re: Shore power voltage fluctuates 90V-120V

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Bill, can you explain this? The only possible reason I can think of is that the closely-spaced turns of the coil might restrict airflow and thermal radiation (cooling).
Running AC through a coil creates a magnetic field. That magnetic field will then resist the flow of AC.
Maxwell's equations - Wikipedia, the free encyclopedia
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Old 20-12-2012, 07:16   #11
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Re: Shore power voltage fluctuates 90V-120V

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Originally Posted by btrayfors View Post

4. Don't coil the cords around the shore post or elsewhere; this limits the current-carrying capacity.

Bill

That doesn't limit the current limiting 'capacity' but might under very limited circumstances change the inductance of the line but that too would not have a very pronounced effect on AC as the op described.
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Old 20-12-2012, 10:24   #12
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Re: Shore power voltage fluctuates 90V-120V

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Running AC through a coil creates a magnetic field. That magnetic field will then resist the flow of AC.
Maxwell's equations - Wikipedia, the free encyclopedia
OK, this is definitely *not* an issue in this case. Here's why:

1) The cable carries both phases of the current -- the currents are balanced (hot and return, or hot and /hot). With balanced currents the magnetic fields cancel out and there is no back-EMF (Electro-Magnetic Field). This is the reason we can take our SSB coax (*) and run it through ferrite cores: the desired signal is not attenuated. What *is* attenuated are the unbalanced (interfering) currents on the cable. This is not going to affect your shore-power connection.

2) The 60Hz (or 50Hz) frequency of the shore power is so low that even if you took the apart the cable and coiled the wires individually, the back-EMF of the coil would be negligible. Here's some technical stuff if anyone is curious:

If you coiled the individual wires in tight one-foot diameter coils of ten turns (30 ft of wire), you would get about 55 microhenrys of inductance, which at 60 Hz has an inductive reactance of 0.021 Ohms. Since we have two coils, there is a total reactance of 0.042 Ohms. With a 30A current in the wires, that gives us a total of 1.2V reactive voltage drop -- pretty insignificant on a 120VAC circuit. And remember, this is with the wires cable broken out of the cable and coiled individually. When the intact cable is coiled, the back-EMF is canceled out.

(*) I am aware that coax cable is called "unbalanced", but this describes the physical geometry. The currents in the center conductor, and in the shield, are equal and opposite, so the *currents* are balanced.
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Old 20-12-2012, 10:52   #13
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Is the voltage drop on both cables? As mentioned previoiusly the 25ohms seems high. Is that on just one conductor? All 3? You could have a partial open in one of the phases and as load is applied resistance increases w heat an you get more V drop. Just speculating.
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Old 20-12-2012, 11:25   #14
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Re: Shore power voltage fluctuates 90V-120V

My guess is your cables are OK, but the transformer is to far away or the wiring is inadequate to the pedestal. We had a similar problem here in the US, Voltage dropped below 105 V with any major appliance use.
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Old 20-12-2012, 11:53   #15
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Re: Shore power voltage fluctuates 90V-120V

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My guess is your cables are OK, but the transformer is to far away or the wiring is inadequate to the pedestal. We had a similar problem here in the US, Voltage dropped below 105 V with any major appliance use.
Agree, assuming that that 25-Ohm resistance measurement isn't real.

If your connectors aren't heating up, they're probably not the problem. Your shore-power cable is probably OK, but you should feel for hot-spots. Cable problems are usually at or near the connectors.

You need to measure the voltage at the dock-side outlet, *under load*. Be careful -- don't electrocute yourself!
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