# Sizing a generator



## Robbie Rob (May 14, 2017)

I would like to learn the proper way to size a generator ( whole house ). Would anybody be able to recommend a website that shows you how and the formulas or a generator calculator I can use online.


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## HackWork (Oct 2, 2009)

To size a generator you need to calculate the load for the house.

The truth is that the generator will not actually need to handle the full load, but the code now requires it to be sized that way.

Once you calculate the load of the house, you size the generator. If there are some appliances that the customers don't want, such as central AC, you can exclude those by using the ATS's load shedding, or adding your own.

So if a house's calc load is 100 amps, you would either need a 24Kw generator, or you could use a 20Kw generator if you shed off 15 amps.

Also remember that generators don't always produce their face value. a 20Kw generator might only be rated for 18Kw when fed with natural gas.


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## nrp3 (Jan 24, 2009)

Some of the manufacturers have sizing calculators, but the correct way is to size it the way you size the service, though if you use load management, you can shrink the size of the generator needed. Some things you may not run at all, some only if power is available or realistically able to run. Before you get too crazy on load shed, consider the labor and material costs, versus just purchasing a larger generator. Sometimes you may need to consider the smaller generator because of the limitations of the natural gas utility to support your generator or possibly the unwillingness of the customer to purchase a larger propane tank. Make sure your natural gas utility can support the load you intend to add as some older neighborhoods may not have the ability.


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## Jack Legg (Mar 12, 2014)

Mike Holt has a really good service load calculator for free

Super easy.. just plug in your numbers

With generators I always calculate both general and optional methods and use the smaller of the 2


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## HackWork (Oct 2, 2009)

I mainly do portable generator connections. What's funny about that is how a customer can run their entire house other than central AC exactly how they want to with a 6,500 watt generator. But the code would have made me install a 20,000 watt generator if it was automatic, and I would still have to shed some loads. Silly NEC.


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## nrp3 (Jan 24, 2009)

The portable generator thing with an interlock works great, but you need to be there to make it work, so manual load shedding is fine too. If you aren’t there to load shed the standby, it may not be able to handle the load if not sized right and or have load shedding.


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## HackWork (Oct 2, 2009)

nrp3 said:


> The portable generator thing with an interlock works great, but you need to be there to make it work, so manual load shedding is fine too. If you aren’t there to load shed the standby, it may not be able to handle the load if not sized right and or have load shedding.


That sounds an awful lot like a design issue and not a safety one :wink:


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