Demand factor applied to 20 heaters?

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SmithBuilt

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Foothills of NC
An existing school building, has 6 gas/AC RTU's. The hvac contractor wants to install 20 VAV boxes total kw 122. The total AC load is 91kw.
During the winter the A/C compressors will not be running. The gas furnace would still be used. The VAV boxes would only come on if 1 or 2 rooms needed heat, if most of the rooms needed heat the gas furnace would meet the needs and not the VAV's. It is possible all the VAV's could run at once, but the AC compressors would not be running just the blowers.

Knowing the compressors would not be running in the winter is it permissible to use that available wattage for the VAV boxes?

Any suggestions on how to calculate per NEC? I would say additional load but its not really.
 
Using 220.87, you can use the previous years load demand provided from the utility +25% to determine the ampacity requirements of the main. 220.51 would require you use 100% for the added heat load. If you are sure the A/C units will not be running, you should be able to reduce this demand by the A/C unit demand of 91 kw and add the 122 kw for the heating units.
 
Thanks Bob
The problem is the school is comprised of 6-7 connected buildings on one meter. This is just one building fed from the main service.

I suppose 220.51 exception may be my only option. Taking into consideration the AC loads also.

I can understand why the cmp wants the heat load calc at 100%. But in this case the main heat will still be gas and the 20 VAV's will fill in as needed.
 
SmithBuilt said:
Thanks Bob
The problem is the school is comprised of 6-7 connected buildings on one meter. This is just one building fed from the main service.

I suppose 220.51 exception may be my only option. Taking into consideration the AC loads also.

I can understand why the cmp wants the heat load calc at 100%. But in this case the main heat will still be gas and the 20 VAV's will fill in as needed.
Click to expand...

I am not sure what you mean by " suppose 220.51 exception may be my only option. Taking into consideration the AC loads also. " but you would not include the A/C load in the main service calculations.

"I can understand why the cmp wants the heat load calc at 100%. But in this case the main heat will still be gas and the 20 VAV's will fill in as needed."

I don't find any way to reduce this load. The ole "what if" says that surely
the gas heat will go down.
 
bob said:
The ole "what if" says that surely
the gas heat will go down.
Click to expand...

That's a good point, I hadn't thought of. And inevitably the gas heat will go down and all the VAV's will be on at one time and possibly for a long time in the winter.
 
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