# Load Calculation for a Generator



## A Little Short (Nov 11, 2010)

I know how to do a load calculation for a service or feeder, but I have a question concerning sizing for a generator. I know about load diversity when sizing a service, but how do you do it for a generator?

I know both the heat and air won't run at the same time, however the circuits still need to be in the backup panel. Do you size for the largest load but still include the wiring/breakers for both units, or do you have to size as if both are to run at the same time?

I'm talking about for choosing the size of the genny.

Here is a couple of pics of the HVAC nameplate off their unit.

In case you can't make it out, the MCA of the cooling unit is 31.5 and the Max. Breaker is 50A
The heating unit's MCA is 52.5 and the Max breaker is 60A


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## jefft110 (Jul 7, 2010)

If that is a heat pump, then yes the compressor and heat strips sometimes do run at the same time.


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## A Little Short (Nov 11, 2010)

Then I suppose I would treat the HVAC like you do when doing a load cal. for a service.
100% of the compressor nameplate and 65% of the heating nameplate.

Just to give you an idea of what they want backed up,
This is all on the lower level of a two story house:
HVAC
Designated receptacles
Cook top only, no oven
All lights on lower level
Garage circuit including the garage door opener


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## Glennsparky (Nov 30, 2011)

The "emergency heat" coils in the air handler are real energy hogs. Oh well. 

Here's a genset watt calculator, just for kicks and giggles. 

http://www.generatorsales.com/wattage-calculator.asp


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