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Troubleshooting
A/C Unit runs on generator but not shore power

A/C Unit runs on generator but not shore power
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Kismet

posts 3

8:24 pm July 6, 2011
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Hi Steve,

I have recently purchased an 1989 Silverton 40' Aft Cabin.  It has seen fresh water all it's life in the Great Lakes and St Lawerence Sea Way.  My AC unit is a Rotary Domtec 17000 BTU with Rotary Digital control. 

My problem is that the this unit will run normally on my generator but not shore power. The panel voltage shows 120 volts from the generator and everything runs normally.  When I switch to shore power (private residence), the panel reads 119-120 volts with less than 5 amps, the fan runs normally but when the compressor tries to start I get a loud buzzing if not rapid clicking noise, huge spike in amperage, huge drop in voltage, and the boat electrical panel breaker pops. 

The shore power consistantly shows 119-120 volts on both lines going into the boat.  I inspected the run capacitor and it had a little bit of corrosion around the top edge.  I'm suspecting a bad rely, start or run capacitor?   

The previous owner and the marine survey both said it operated normally on shore power at the marina.  I believe this is the original unit that came with the boat. 

I was able to run it on the generator and switch the power source to shore power with out turning off the unit at the panel.  It would run on the shore power but the voltage at the boat panel was barely at 115 volts with normal amps.  Within a few minutes the same symptoms from above would happen and the panel breaker would pop. 

Thanks for your help in advance.

 

Erik
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Steve Pooler

posts 1127

9:14 am July 7, 2011
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Hi Eric~

What you are describing is actually a fairly common problem at homes with docks…

The electrician/contractor that ran the power never intended to have folks running A/C loads…Thus only ran wire sized for typical 15 amp receptacle loads…This smaller wire will have voltage drop over distance (from the house panel) and when the typical starting load of a 16K A/C is applied, that voltage will drop even more, which in turn will raise the amperage draw of the A/C trying to get started….lower voltage will cause higher amperage….

I don't think you have a problem with the A/C unit which is evidenced by the fact it will start & run on the generator & other shore sources…

Low voltage is a compressor killer…Not to mention that digital control…I wouldn't try to run it anymore on that dock power without an upgrade to the service supplying it…

Just for your info…From your description I believe you have "Rotary Air" unit & control…Dometic/Cruisair/Marine Air never made a 17,000 BTU unit…Rotary Air was owned by Westerbeke & now I think Northern Lights owns them…

Dometic does offer a SmartStart that could help with your problem: http://marine-ac.com/2010/11/0…..-assist-2/  but after installation you would still need to be sure your dock voltage does not drop below 100 volts and pay special attention to the temp of the wire feeding the dock…

If you do get a contractor out to quote wire/service upgrade…You need service for at least 30 amps…

 

Steve~  
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Kismet

posts 3

10:52 am July 7, 2011
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Hi Steve,

Thanks for your reply.  Ihad a feeling that his might be the issue.  Here is a description of the wiring to the boat.

From the main house's power panel 2 #8 wire, 120 volt, 30 amp a\c to the the boat house.  This run is about 300 ft.  In the boat house we put in 2 exclusive GFI circuit breakers that are 20 amp.  We couldn't find a 30 amp GFI Breaker for the name brand power box we are using.  From these 2 circuits we ran 2 #10 wire another 100 ft to the boat dock recepticles. 

Could we be losing the volatges over these long distances?  Or, I'm wondering if my power cords going into the boat are suspect.  I think highly unlikely.  It appears the "Smart Start" may be the answer.  Thanks.

 

Erik 
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Steve Pooler

posts 1127

11:15 am July 7, 2011
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The way to find out is with a meter & someone on the boat starting the A/C to see where the voltage drop is occurring…

 

Measure at the House, Boat house, then at the Dock…Each time while starting the A/C…This should tell you what wire run is not sufficient for the load presented…

No it shouldn't be your shore cord/s but remember they do add 50' of wire to the equation…450 ft is quite a long run…

I might also try a standard breaker…I've had troubles with GFI's & A/C's starting…Your local code may prevent you leaving the standard breaker installed, but it may help with answers on what wire run to upgrade…

 

SmartStart should help dramatically…They have for me on larger units, and now that you describe wire sizes I'm less concerned with overheating, but it never hurts to do the testing I described…

 

Steve~ 
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Kismet

posts 3

11:58 am July 7, 2011
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Thanks Steve, I will look into it.
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