Sizing a Generator?

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Old 03-29-10, 08:02 AM
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Sizing a Generator?

Maybe I should start out with this question. When you install a generator, do you just decide which circuit breakers you want to power and then run wires outside to the transfer switch?

I've been reading a lot about how to size a residential generator. When you go through the list of items you want to power, that's not all you really need to consider, right? What I mean is, if I want to run my refrigerator I can't just add the amount of electricity JUST the fridge will use. I have to include everything that's on that circuit, right? If everything in my kitchen is running off the same breaker, I have to either make sure none of it runs during the outage or include it all in the sizing of the generator, right?

And how do I know what transfer switch to use? Do I just add up all of the circuit breaker amps I am running and that's the size of transfer switch I need? If you look on Lowe's website, their transfer switches range wildly in price. They have a couple 100 amp switches for $349 and $699. What's the difference? Even the $349 sounds pretty expensive. Do I even need a 100 amp switch?

Thanks,

Scott
 
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Old 03-29-10, 09:02 AM
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In my case, I'm just interested in powering the refrigerator and freezer during a long power outage. And I don't have a very high wattage generator, so that is about it.

I just run long extension cords to these when the power goes out. (I use 50 ft. 12 gauge wire heavy duty contractor extension cords sold in home improvement stores.)

And I might disconnect these and cook something in the microwave, then reconnect them.

Otherwise here are calculators to convert watts to amps...
(Use single phase.)
Power Calculators for quick conversions.
 
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Old 03-29-10, 09:20 AM
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Originally Posted by sday88 View Post
Maybe I should start out with this question. When you install a generator, do you just decide which circuit breakers you want to power and then run wires outside to the transfer switch?
I usually mount the transfer switch inside next to the main panel and only run the generator inlet wires outside.

When you go through the list of items you want to power, that's not all you really need to consider, right?
Add up the wattages of the items you want to run at the same time, then add about 10-20%. That's the minimum sized generator you should get.

If you want to power something with a large motor in it such as a well pump, then the wattage rating of the generator will need to be quite a bit larger than the motor to accommodate start-up current.

If everything in my kitchen is running off the same breaker, I have to either make sure none of it runs during the outage or include it all in the sizing of the generator, right?
Yes, it's called load management. You carefully select which appliances to use so as to not overload the generator. For example, unplug the fridge to use the microwave for a couple minutes.

And how do I know what transfer switch to use?
Depends on how many circuits you want to transfer to the generator. Many people do just fine with a four circuit panel (furnace, fridge, microwave, misc lights and TV). If you have modest needs and do not need to power a central heating system, you can probably get by with just a few heavy duty extension cords instead of a transfer panel.
 
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