Electric Power & Utilities/Home generator

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2 contractors have given me completely different proposals as to how large a home generator I need.  The issue is one of our air conditioning units.  According to the plate on the unit its LRA is 169, which is apparently high.  One contractor tells us that would require a 30kw generator (He is including some other circuits as well, but the driver of the recommended generator size is definitely the high LRA on the AC unit.)  The other contractor says that he doesn't size the generator based on the 169 locked rotor amps, and that all we need is a 20kw generator.  Is one or the other of them right?  Are there other issues I need to be considering?  Thanks,Harrison

Answer
Harrison,

Lets address your largest AC unit first. Both brands of generators I install Onan and Generac tout their 20 KW generators will start a 5 Ton AC unit and they do. The problem isn't running these things its getting them started.

There are "hard start" kits on the market which is a supplemental capacitor that is added to the existing start capacitor on the AC compressor. It will significantly reduce the starting current from 169 down to the low 140's or high 130's. Ask your HVAC man about them. They are cheap and easy to install. They also extend the life of the compressor which is an added benefit. The 20 KW Onan I use would have no trouble starting this 169 LRA.

So if this is the only reason why the one contractor wants to go to a 30 KW unit then it is probably not necessary but I don't know what other "loads" you may have such as lighting, sump pumps, refrigerators, freezers, the number of other AC compressors you have, electric heat, electric hot water heaters if any.  So he may be thinking of these things as well when sizing the generator to 30 KW.

It all depends upon what you have load wise and how much of what you have that you want or need to work at the same time. In other words how much are you willing to do with out when the power goes out ? Are you conservative ? Or do you want certain things like all your  AC Zones to work with out "load management" regardless of what you have going on in the house at the time ?

"Load management" is a great feature that helps reduce the size of the generator needed by prohibiting the AC to come on initially when the power goes out in the summer. If the generators computer determines there are too many other important loads on in the house at that time such as the washer dryer, lighting, refrig, freezer, furnace, sump pumps, fans, computers TVs and the like, it will not allow the AC to come on. When those other loads begin  cycling "off" and more capacity is available then the computer will allow the AC to come on. This is all so the generator does not become over loaded. In most house holds a 20 KW generator utilizing "load management is sufficient. Even a house with 5 AC zones.  

The Electrical contractor has to take stock of all your loads and find out what your needs are. Then he can come up with the appropriate size generator. Usually people are willing to "load manage" their AC in order to reduce the size of the generator or load manage other high users as well if necessary. In my area its all city gas not propane and many area's have low pressure service going into home's which often dictates the size of the generator. If your out in propane country you will be tied to the limitations of the size of the tank and the loads you need to run either managed or unmanaged. The only place you have more room for the size of the generator is high pressure gas service area's. So unless your in a high pressure area you probaly won't be able to do a 30 KW with out upgrading the gas service coming into your home assuming the gas main in the street would support it. And if your in propane country your going to need at least a 500 gallon tank for a 30KW.

All these things need to be considered before making a decision on size. If you have city gas and your on a low or high pressure system make sure these guys consult the utility as to wether or not the gas service is adequate. I've heard storys from inspectors that guys are installing generators with no knowledge of the ability of the gas service to support the generator and when theres trouble they are no where to be found. Ask your guys plenty of questions.



Bob  

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PV Electric Bob Mossman

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Master Electrician licensed (29 years) 6165B Contractor in Closter New Jersey with 38 years total experience in Residential - Commercial - Industrial work. Emergency Service - General trouble shooting and repair work - Paddle fans - Kitchen & Bath exhaust fans - AC lines - Smoke detectors - Telephone lines - Cable TV lines - Computer network cabling - Exterior lighting - Recessed lighting - Security lighting - (200 - 150 - 100 amp service up grades) - Electric Heat - New circuits - New appliance hook ups - Back up generators - and more ! FREE ESTIMATES Fully insured 201-358-1552

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