Sizing of panels for Freezers

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mshields

mshields

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Boston, MA
I've got an optional standby panel with something like 16 -80 freezers on it (they happen to be 208V-sinlge phase and approximately 2kVA each). I generally, in keeping with Article 440.22, take these at 100% of their nameplate rating. I have a couple of questions.

1) Am I correct that there is no allowance in the code for applying a diversity factor. Realistically, the compressors cycle on and off and I know that when you put a meter on such panelboards, the actual demand load is consistently below 40% of the connected load.

2) If the code provides no relief here, as a practical matter, I'd be interested in the groups experience with regard to the actual demand load seen from such a panel.

Much appreciated,

Mike
 
130104-0011 EST

mshields:

I can not answer your code question. But I can give you a reason for the panel load you observe.

Go to my web page http://beta-a2.com/EE-photos.html and to the plot near the end, next to last plot at the present time, labeled P29. This is a plot of power vs time of day for a residential freezer, about 20 cu-ft.

The compressor operates using an on-off method (bang-bang servo). The duty cycle is about 40% under the ambient conditions at the test time. When you have many independent freezers, even though of an identical production run, there will be random differences in the total cycle period and the on time of the different units.

Using one freezer as a time reference, pick the turn on time as that reference point, and you can expect a reasonably uniform probability distribution of when all the other freezers will turn on and off. Thus, the average power load on the panel will be approximately the average duty cycle times the average power of all the compressors when running. In my plot the average on power is about 300 w, note there is also a residual power level of about 30 W. Duty cycle is about 40%. Thus, average power from the compressor on part of the cycle is 0.4*300 = 120 W. To this need to add 0.6*30 = 18 W or a total average for one freezer of 128 W. If this is the same for 30 freezers, then total average power is 30*128 = 3840 W.

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I would carry them all at 100%.


Utility fails, genset takes over, genset quits for a time, restart genset, now all compressors will be running, no diversity for a while.


It may sound far fetched but I do see gensets go down pretty frequently, low fuel, high temp etc.
 
iwire said:
I would carry them all at 100%.


Utility fails, genset takes over, genset quits for a time, restart genset, now all compressors will be running, no diversity for a while.


It may sound far fetched but I do see gensets go down pretty frequently, low fuel, high temp etc.
Click to expand...
Same is true with a utility power outage for any extended period of time and no backup power. When utility power is restored, all compressors will be running.
 
Smart $ said:
Same is true with a utility power outage for any extended period of time and no backup power. When utility power is restored, all compressors will be running.
Click to expand...

Of course, but the utility supply is typically much more stout than optional standby systems are. I run into this when starting up supermarkets after outages, on utility I can just turn it all on and walk away. But when using a portable generator I need to manually stage the loads. They do not like to rent a unit large enough to power it all at once.
 
iwire said:
Of course, but the utility supply is typically much more stout than optional standby systems are. I run into this when starting up supermarkets after outages, on utility I can just turn it all on and walk away. But when using a portable generator I need to manually stage the loads. They do not like to rent a unit large enough to power it all at once.
Click to expand...
Understood. But an optional standby panel with manual switching permits you to stage the load. With no backup or automatic switching, the entire load must be carried.
 
If the freezers are operated through a central energy management controller, the controller may switch on and switch off the freezers in accordance with a definite control algorithm so that a definite demand factor (<100%) may be assigned to the group of freezers. I think the code is yet to take into account of such type of controllers.
 
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