Climate-controlled public mini-storage units, HVAC electrical demand

[vecjohn]

Hi everyone,
I've been asked to work up the service and feeder sizing for a
public, climate-controlled mini-storage facility.
The 8, two-story buildings each have 4 HVAC units, for a total of 16 HVAC units. They will both heat and cool the storage areas to maintain a temperature between 50-80 deg. F. The HVAC units have A/C heat pumps and resistance heating elements.
The HVAC units are NOT interlocked; they are each controlled by an individual thermostat. The resistance heating elements are not stepped when energizing. The HVAC units are rated at 5 tons each.
My question regards the overall HVAC electrical demand for this installation. It seems logical that it should be less than 100% of connected HVAC load, but I'm not sure how to determine it.

 

[mc5w]

You mean 32 HVAC units. I would just simply add together the nameplate ratings and multiply by 125% just to be safe. If the heaters are interlocked with the compressors then you can drop the smaller of the two loads using this method. However, in some units the resistance can operate at the same time that a compressor is doing a defrost cycle.

U.S. National Electrical Code allows you to use 125% or the largest motor plus 100% of the other motors plus 125% or the resistance heaters. However, this ASS-umes that a larger installation will have power factor correction capacitors and that not all of the motors will be fully loaded all at the same time because of device oversizing. If this is in a demanding climate this calculation could be way off.

If your building is very well insulated and your units are oversized then I would not use a lower demand factor than what is stated above. However, the more that your heat wires and switchgear you will have lower equipment life and be paying for electricity and not get it. If your wires are larger than the minimum more electricity will get to the load.