"Whole House" generator sizing.....

Status
Not open for further replies.
C

chevyx92

Senior Member
Location
VA BCH, VA
Residential customer has a 200A main service and wants to add a "Whole House" natural gas generator with ATS. My question is do I size the generator based on the 200A service size or do I do a load calc. on the entire house to come up with a more accurate KW rating needed? House is 3100 sq. ft., electric oven, electric water heater, well pump.
 
NEC 702.5 allows you to size it on the actual transferred load OR the actual connected load with automatic management devices employed.
 
I would size it based on the amount the home consumes. You can ask the HO to ask their serving utility to give you a consumption amount for a given season (Months). if thats a pain, then go DLC.
 
Last edited:
"automatic' is the keyword. If the homeowner is not at home and the load switches to the generator, you do not want the generator main to trip leaving the entire house without power
resulting in freezing, spoiled food etc. If they select loads that are not critical, you can handle that by separate panels or a load management system. Most of today's ATS include load management circuits.
 
We employ load-shedding on most every generator installation these days. Most of it involves electric heat pumps. The generator can usually handle the compressors just fine, but when you add in an extra 15 or 20 kW of load for the strip heaters that can cause problems.

So, we limit the strip heaters' ability to activate when using standby power.

Using load-shedding enabled transfer switches, as well as aux. relays can do the trick quite nicely.
 
resistance said:
I would size it based on the amount the home consumes. You can ask the HO to ask their serving utility to give you a consumption amount for a given season (Months). if thats a pain, then go DLC.
Click to expand...

That will only work with a 1 year max. demand data. 220.87 (1) exception.
 
kbsparky said:
Most of it involves electric heat pumps. The generator can usually handle the compressors just fine, but when you add in an extra 15 or 20 kW of load for the strip heaters.
Click to expand...
Do not think that is a real concern since the genny will be NG. Bet a dollar the heat is either gas or dual source with gas. I canot imagine anyone with a heat pump that would use electric strip heat with NG connection. It would be kind of ignorant to do so. Probable also safe to say they have gas hot water and cooking.
 
Last edited:
resistance said:
I would size it based on the amount the home consumes. You can ask the HO to ask their serving utility to give you a consumption amount for a given season (Months). if thats a pain, then go DLC.
Click to expand...

ceb58 said:
That will only work with a 1 year max. demand data. 220.87 (1) exception.
Click to expand...
It needs to be demand data and not just usage data. Simple billing that only shows usage doesn't tell you what peak demands are or even average demand for that matter. Some homes can have peak demands for 3-5 hours daily and then sit nearly idle the rest of the day, just depends on schedules of the occupants.

dereckbc said:
Do not think that is a real concern since the genny will be NG. Bet a dollar the heat is either gas or dual source with gas. I canot imagine anyone with a heat pump that would use electric strip heat with NG connection. It would be kind of ignorant to do so. Probable also safe to say they have gas hot water and cooking.
Click to expand...
I can show you some places that have both gas and electric heat in the same building, I'm not saying it is a good idea but it happens.
 
Status
Not open for further replies.
Top