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Old 29-07-2012, 20:16   #1
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Decreasing A/C Voltage When A/C Comes On

This is going to be a long post, because I want to try to get all of the relevant facts out in front of the experts and, to be honest, I am not really sure which facts are relevant. The short description is that when my air conditioning units come on, the shorepower voltage drops and, if the conditions are right (or wrong, depending on your viewpoint), the air conditioning unit breaker opens and shuts down the air conditioner.

Now for all of the facts. The boat is a 1978 Morgan 452 ketch. We are currently located on the James River, near Richmond, VA. We are about a mile downriver from a coal-fired power plant. When the tide is going out, the outflow from the power plant causes the river water, measured at my speed knot meter, to be between 89.5 F and 90.5 F. When the tide changes, the river temperature runs between 84.5 F and 88.5 F.

We have a forward air conditioning unit - a Dometic 16K BTU unit purchased and professionally installd last summer. We have an after air conditionig unit - a Mermaid 12K BTU unit purchased 5 years ago and professionally reinstalled last summer.

The marina we are in is more than 30 years old and has seen very little maintenance. We are at the T-head furthest from the electrical connection to the main. Our electrical connections appear to have been installed by amateurs, and most of the other connections for the slips between me and the fuse panels and meters appear to be similarly installed. The current owner of the marina is not amenable to making any upgrades and barely covers the cost of maintenance.

Originally, the boat had a single 30 amp shorepower connection. About two years ago, in anticipation of adding the second air conditioning unit forward, I installed a second 30 amp line. I then built a single A/C power control station, using two 7 breaker Blue Sea panels and a Blue Sea digital A/C multimeter. In order to monitor both lines from a single multimeter, there is a switch that transfers from one line to the other. The multi-meter reads A/C voltage, frequency, and amperage and calculates wattage. However, the amperage is measured by using coils that one side of each A/C input is run through and the magnetic flux from one tends to interfere with the other, causing the amperage and the wattage to read inaccurately unless one line is completely shut down.

My original intent was to put both air conditioning units on one power panel and all other A/C loads on the other power panel. This would allow me to run one shorepower cable when I did not need A/C or heat and only run the second line when needed. This worked when I was in the marina where I had the air conditioning units installed and in a marina where I stayed overnight when returning to my home port.

However, as soon as I got to the home port and plugged in the second shorepower line, breakers started tripping. In order to work around the problem, I transfered the aft A/C unit to the "house" panel and left the forward A/C unit on the second line.

Here is where we get to the actual problem. When I connect both shore pwer cables, but before I turn on the air conditioning units, I will tend to have an A/C voltage of, for example, 115 volts on one line and 114 volts on the other line. When I turn on the after unit, the A/C voltage on that line will drop between 5 and 7 volts, depending on things like the water temperature, how clean the raw water strainer is and what the starting temperature on the boat is. When I turn on the forward unit, the A/C voltage will drop between 6 and 10 volts, again depending.

Usually, if the voltage starts high and doesn't drop very far (in the spring and fall), the systems run OK. But, when the temperature is high and the starting voltage is low (in the summer), I can't keep the units running. The individual breakers (15 amp) trip, dropping the units off-line. Sometimes, it will happen once or twice, and then run OK, other times - like today - I can't keep them running for more than five or ten minutes.

So, here are my questions. First off, is it normal for a large internal load to drop the A/C shorepower voltage on the boat, or do I have a problem with my connections, my shorepower lines or something else? I have retraced all of the connections on the boat and they appear to be solid and tight. One shorepower line came with the boat when I bought it five years ago, the other is about four years old. I have used an emery board to clean up the male connections on the pier side end of the cable, but I have not been able to clean the male ends at the boat connection and obviously, can't clean the female ends of the cable or the pier connector. Is there something else I should be checking?

Is the problem that I should be using 20 amp breakers rather than 15? The Dometic spec sheet indicates that the fully loaded cooling amperage should be a little more than 10 amps at 115 volts and the sea water pump is another 2 amps. The Mermaid spec sheet indicates that its cooling amperage is about 9.5 amps, with a pump running about 2 amps. It seems that this would be close to the amperage of the breaker. Does A/C power work the same as D/C power, where a decrease in voltage for a given load would result in an increase in amperage to compensate?

The Mermaid has a high pressure trip on it if the sea water simply cannot cool the unit - flow is too low or temperature is too high. This has tripped occasionally, but the breaker opening occurs much more frequently now. The documentation for the Dometic unit indicates that it will become "less efficient" if the sea water temperature is over 90 F, but it doesn't say anything about it tripping off line.

I am looking at moving my boat to another marina, closer to the Chesapeake Bay, where the water temperature would be cooler for the next month or two, but if the voltage issue continues, I am not sure if it is worth it. The new marina has only a few boats on the electrical branch I would be on, but the wiring is of a similar age to the current marina. The owners at the new marina appear to be much more amenable to upgrades, but they are on their own schedule.

I am pretty good dealing with the D/C power system, but the A/C system sometimes strikes me as PFM. I know that when I lived in a house, I never felt the need to check the incoming voltage, so I don't know if this happens normally, or if it just happens in this situation. I guess I assumed that if there was 115 volts at the power plant, and more loads came on line across the system, they would keep upping the generator output to compensate, but that doesn't seem likely now.

Bottom line - is there anything I can do about this, or do I just have to suck it up and know that the loads will drop the voltage and that's all I can do?
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Old 29-07-2012, 20:25   #2
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

The voltage drop is a direct function of two things: what size cables do you have in your AC lines onboard and for shore power and how long is the run?
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Old 29-07-2012, 21:00   #3
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

What is the compressor motor start surge current? It can be as much as six times the normal running current for the motor. You don't mention if this happens more often when the water is hottest or if it doesn't matter but hot water can put a larger load on the compressor and maybe the pressure of the refrigerant needs to be adjusted.

Check out this blog and scroll down to the video "First boat home in 34 yrs" and read the article directly below that.
(Blog) GSSR No. 14 - The Kids Come Home: Greetings...
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Old 29-07-2012, 22:18   #4
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

In the what else could it be column, check your shore power cord connections. You can get 10-15 volts drop just from a corroded connector. Look for burnt insulator around a plug prong. Discolored means replace...

Cheap fix is install new end plugs. Best is a new cable.
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Old 30-07-2012, 04:26   #5
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

Thanks for your feedback. I thought about the starting amperage issue, but the system doesn't seem to trip on start up or when cycling. It seems that it will be running steady state and then drop off. I definitely believe it is related to the saewater temperature, because the problem did not occur three weeks ago before the river temperature reached its current level. The piece by Ken aboard San Souci is interesting, and I will follow up with a local air conditioning guy to see how difficult it would be to lower the amount of refrigerant and then raise it again. I also think I will run by West Marine and get a new power cord today, just to test the cord. I know at least one of the boat side connectors is new and there is probably very little I can do about the shore side connectors.
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Old 30-07-2012, 04:45   #6
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Quote:
Originally Posted by CaptFrankM
.

This worked when I was in the marina where I had the air conditioning units installed and in a marina where I stayed overnight when returning to my home port.

However, as soon as I got to the home port and plugged in the second shorepower line, breakers started tripping. In order to work around the problem, I transfered the aft A/C unit to the "house" panel and left the forward A/C unit on the second line.
Because it worked at another marina, before assuming that your cable or ends are bad, I would suggest developing a means of testing the voltage in the marina's distribution system. Is the undercurrent condition the same at all hours of the day? If it is worse when other boaters are using power ( daytime/weekends) the it is almost definitely a marina issue.

I would suggest starting the evaluation at the beginning (marina connections) and working from there.

Bill
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Old 30-07-2012, 05:42   #7
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

I have a similar situation at a marina near LAFB. Not metered power, so it is free to me, but not a lot available.
the power with no AC running is 117 volts
Turn on AC and volts drops to anywhere from 100 to 107

I have AC running with a 20 amp breaker for a Cruisair 16K.
The cooling ability drops a lot when the voltage drops.
Run from generator, cooling is much better.
I have read running at 100 volts on the newer Cruisair-Dometic it will turn off the compressor to protect it from burning up.

Compressor running on lower volts then draws more amps so kicks off the breaker.

So your AC power is lousy as is mine, is it free?
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Old 30-07-2012, 06:35   #8
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

If you are trying to use two 30 amp cords and have wired the boat yourself, you should hire an electrician to come and check it out, especially if it is popping breakers. Depending on how your dock is wired, those 30 amp circuits could be in phase or out of phase.
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Old 30-07-2012, 06:56   #9
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

While many of the things mentioned above are important and could be a source of problems (especially bad connectors on the power cord and/or marina's power post), it sounds very much to me like insufficient power from the marina itself.

If the wiring in the marina itself is insufficient and is not well maintained, you would normally see voltage drops as increasing loads are applied, e.g., as A/Cs are turned on during hot spells.

It might be worthwhile moving for a day or two to a marina with good electric power just to see what happens.

Just as a matter of good practice, you should not run the 30A power cords or the circuit breakers anywhere near their ratings on a continuous basis.

The 30A cords and connectors really can't sustain more than about 25A, especially in hot weather, and 22-23A is better.

As was mentioned above, if you see discoloration around the plug connections -- male or female -- the connector should be REPLACED.

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Old 30-07-2012, 07:17   #10
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

IMHO your breaker is way too small. I just looked at the specs of this AC and have installed several myself. It should have a 10AWG wire providing it with power with a 30A breaker. In the event of the compressor seizing the locked rotor amps are listed as 62. If your running amps approach the capacity of your 15A breaker (and they do) you'll pop it all the time, as voltage drops your current increases too.
Yes the shorepower still sucks, but you've not made it better with breakers that are too small.
Check the wire gauge that supplys the AC and increase the breaker if the wire size will allow. If the wire size won't allow, re-wire.
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Old 30-07-2012, 08:34   #11
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

Quote:
Originally Posted by Sailmonkey View Post
IMHO your breaker is way too small. I just looked at the specs of this AC and have installed several myself. It should have a 10AWG wire providing it with power with a 30A breaker. In the event of the compressor seizing the locked rotor amps are listed as 62. If your running amps approach the capacity of your 15A breaker (and they do) you'll pop it all the time, as voltage drops your current increases too.
Yes the shorepower still sucks, but you've not made it better with breakers that are too small.
Check the wire gauge that supplys the AC and increase the breaker if the wire size will allow. If the wire size won't allow, re-wire.
I certainly agree with that. As I said above, "Just as a matter of good practice, you should not run the 30A power cords or the circuit breakers anywhere near their ratings on a continuous basis."

As Sailmonkey said, with a LRA of 60A, a 15A circuit breaker isn't likely to cut it, especially if there are other loads on the circuit.

Bill
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Old 30-07-2012, 09:01   #12
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

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Originally Posted by btrayfors View Post
I certainly agree with that. As I said above, "Just as a matter of good practice, you should not run the 30A power cords or the circuit breakers anywhere near their ratings on a continuous basis."

As Sailmonkey said, with a LRA of 60A, a 15A circuit breaker isn't likely to cut it, especially if there are other loads on the circuit.

Bill

As a matter of observation across a wide sample, I think the max continous current on a 30A shorecord should not exceed 18-22 amps. This is not scientific, just a curiosity I had when replacing customers burnt shore ends.
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Old 30-07-2012, 10:02   #13
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

Quote:
Originally Posted by Sailmonkey View Post
As a matter of observation across a wide sample, I think the max continous current on a 30A shorecord should not exceed 18-22 amps. This is not scientific, just a curiosity I had when replacing customers burnt shore ends.
The NEC limits the current on receptacle* to 80% of nameplate rating for continuous loads. Therefore, continuous current drawn from each 30 Amp receptacle would be limited to 24A.
* General purpose receptacles are not normally considered to be continuously loaded.
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Old 30-07-2012, 12:15   #14
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

OK, some more data to throw into the pile. Thanks, everyone, for comments to date. I have already considered most of them, but since you can't read my mind (except maybe GordMay - he definitely has strange and mystical powers), you don't know what I have already checked.

I opened all breakers and brought loads on one at a time to get a better set of data. They are as follows:

Line 2 - no load. 115.8 volts, 0 amps.
Line 2 - forward A/C running. 110.7 volts, 11.9 amps.

Line 1 - no load. 115.5 volts, 0 amps.
Line 1 - aft A/C running only. 108.6 volts, 13 amps.
Line 2 - aft A/C and Battery Charger running. 105.3 volts, 20.1 amps.

The river water temperature at this point was 89.2 F.

I have picked up, but have not yet installed, a brand new 30 amp, fifty foot shore power cable. I will install that this evening, as well as checking the wire guage from the shore pwer connections on the boat to the A/C power panel and from the power panel to each A/C units.

One of the things that throws me is that the aft A/C unit - the 12K BTU unit - seems to be drawing significantly more amps than it should. If it is supposed to draw about 11.5 fully loaded amps, with the seawater pump running. Of course, that would be at 115 volts.

As noted, by the way, the fully loaded amperage on each line should be well below the 24 amp point - if the voltage was holding steady at about 115 volts. Of course, taking into account starting current for things like the A/C unit, I would exceed that amperage momentarily, but I would think that would be within the normal operating requirements.

Oh, and one last thing - no, my electricity isn't free, at least not where I am now...
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Old 30-07-2012, 12:21   #15
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Re: DECREASING A/C VOLTAGE WHEN A/C COMES ON

Just to be clear, these two shorepower lines are completely separate from each other - the phase between them should not be an issue - but thanks for pointing it out.

I have also made arrangements to move down to Urbanna for the next 30 days, starting this weekend. That should give me a different power setup to see if that helps.
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