Generator size


Old 05-01-04, 09:53 AM
Howard Persun
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Generator size

I have an electrician redoing my electrical panel and am looking into setting some circuits to be able to use a generator in case the power goes out. Currently I am looking at a generator for 7550 running watts. I would like to have the following available:
Furnace: ~1725 watts
Sump pump: ~1700 watts
Water pump: ~7200 watts
Accordingly the water pump would take the whole capacity.
Would it work to have all these on that circuit on circuit breakers and only flip the circuit breaker on for the water pump when the other circuits are off and viceversa?
What is the exposure if someone were to forget and turn all on at the same time?
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Old 05-01-04, 10:24 AM
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I would predict that you might have some problems starting your water pump with the size generator you quoted. When a motor starts it consumes a lot more power while spinning up to speed. Your smaller generator may be overloaded at that time. The other thing to consider is something called the "power factor." A motor (like other inductive loads) would typically have a power factor in the neighborhood of 0.8 meaning that you can only get 80 percent of the rated power from your generator to do usefull work. That means that you would be a lot better off with a 10KW generator in order to get the maximum from your pump. I'm just making an educated guess here because I don't have any idea of what kind of motor you are using in the pump and don't know if you expect to use it's full capacity. I also don't know the true specs on the generator you are looking at. My sense is that you are cutting things just a little close. Your idea of just cutting off the breakers on the unused loads is OK. If you tried running all three at the same time you would certainly bogg the generator down a lot and you should trip the breakers on the generator itself. It wouldn't do the loads any good either as an unstarted motor consumes a lot more power that must be disapated in the motor windings and they would tend to overheat until you cut the power. Your generated power wouldn't be 60 cycles anymore either, but probably closer to 50 and that would also add to your overheating problems assuming that the breakers didn't open. Always give yourself lots of leeway when you are trying to start electric motors while running on generator power.
Old 05-01-04, 01:57 PM
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Water pumps need a considerable amount of power to get started. The generator you are considering is not large enough for your pump.
Old 05-01-04, 02:11 PM
Howard Persun
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Thanks guys. The starting watts on the generator are 13,500 watts. Even so, I thought I might be cutting it close. With just a couple of calls in my area this was the largest I found. Looks like I will either decide to go without water or look for something bigger. None of the things I am looking at putting on the generator would run for an extended period of time.
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