More about VAV's

[necnotevenclose]

I put together a spreadsheet to calculate VAV's. I input the load of the VAV and it spits a calculated breaker size. Based on the NEC I take the heat at 100% and the largest fan at 125%. The problem I'm having is that the VAV load is pushing me into a larger service. My question is: Is there ever a time where I can take a diversity factor of either the heat or the motor?

 

[iwire]

Based on the NEC I take the heat at 100% and the largest fan at 125%.

The NEC requires 125% of the total of both the motors and the heat.

The 2002 section is 424.3(B), I am not sure if it has moved for 2005.

 

[steve66]

I don't think so. I think you have to basically assume that on the coldest night of the year, all the VAV's will all run full throttle all night. At least in the Midwest, buildings with electric heat have large service sizes.


Depending on your exact system (i.e. no electric reheating of cooling loads), you may be able to take the cooling load out of the service size if it is smaller than the heating load. In other words, you only use the largest of the cooling or heating load if they don't run at the same time.

Steve