Back Up Generator Sizing

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  #1  
Old 09-16-04, 07:22 AM
ramm
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Back Up Generator Sizing

I live on an 80 acre ranch, and have power outages frequently due to poor service, as well as hurricanes.
I have used a small back up generator, but it isn't sufficient to meet my needs. How do I go about sizing? Currently have two 200 AMP panels in home, supporting well, four heat pumps ([email protected] Tons), etc. I can't seem to find any data on surge requirements listed on equipment/devices, especially the Heat Pumps. Would my max requirment be met if I had 200 ampsx240 voltsx20r 96KW? Or would it be higher? My utility bill shows 177kwhrs per day average power use, but I think this is meaningless info for sizing.

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  #2  
Old 09-16-04, 02:16 PM
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The KWH value is not helpful, because it is the average over the day and not instantaneous information as needed for the generator.
The generator size has everything to do with what you want to run during an outage. Will you run all the heat pumps or none? Same question for your lighting, appliances, etc.
The best thing to do is purchase a clamp-on amp meter and measure for each hot conductor in the main panel, over several days (or at least several times), the amount of current you will draw when those selected appliances are on. Try to catch any startup currents on fans, pumps, refrigerators, etc.
Take the larger of the numbers and multiply it by 240V, then multiply it by a fudge factor like 1.1 or 1.2 and you will have your minimum size for the generator.
For example, I only put on the stuff I want and measure 30A on one hot conductor, and 25A on the other, I will need at least 8.64kW generator. There are other items to consider like power factor, future growth and such, but sometimes it is a bit past an average homeowners capabilities.
 
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Old 09-16-04, 05:08 PM
ramm
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Thanks, I will do that.
 
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Old 09-19-04, 08:10 AM
ramm
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I have measured the Amp loads, and it appears that I can meet minimum needs with a 40KW generator, linked to one of my 200 AMP panels with a transfer switch. However, the additional cost to get 80KW is relatively small.Can I use one 400 AMP transfer switch, splitting the output to the two 200AMP panels, or do I need two separate switches? I am going to hire an electrician to do the work, but Iwant to make sure I know what he is doing.
 
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Old 09-19-04, 09:01 AM
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When done properly, you can have one generator and two transfer switches.
 
  #6  
Old 09-19-04, 09:04 AM
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As a side note, you say you can meet minimum loads with a 40kW gen.
To help confirm your calculations, you measured somewhere around 160A on one hot conductor (or the total of two of the same hot conductors in the two sub panels), right?
40kW sounds like a lot.
 
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