Not an Electrical Engineer

[dblyle]

Since I am not an electrical engineer, I have a question about Motor Control Centers and how to calculate the cooling load. I have a client with a MCC that he wants me to upgrade the AC. Obviously, the MCC is full of starters, breakers, etc. Do I size the AC based on the Hp rating of the starters, the Amp rating of the MCC or the total power coming into the room (5,000 Amps)? How would I go about calculating the load from the MCC in the room? I have looked at the ASHRAE manuals and can't seem to find anything on MCCs.

thanks for your help - DOUGLAS

 

[jimbo1184]

Your best bet would be to contact the local rep of the MCC manufacturer. He will get the most accurate answer. I typically have my EE contact their rep and pass the data along to make the calcs as accurate as possible. I tried searching in the ASHRAE books, but Icouldn't find anything as well.

 

[jraef]

Correct. There is no "cheat" formula based on expected load that would be meaningful. MCC manufacturers do have standardized tables of heat dissipation of basic units which they usually publish in the MCC manuals (Google Motor Control Center heat dissipation", but the total amount can vary greatly depending upon the mix of devices and what the devices are in the MCC. For instance, Varible frequency drives put out a LOT more heat than motor starters , which themselves put out a lot more heat than feeder circuit breakers. Someone would need to do a specific anaylsis of your MCC.

 

[CountOlaf]

I know this thread is a little old, but I wanted to contribute anyhow...

I agree that actual electrical equipment values from the manufacturer are (of course) the best, when available; but cooling load calculations are full of assumptions anyhow and it is nice to have some rules-of-thumb to quickly get an idea of how big of a system might be necessary. Plus some of the MCC's/electrical rooms I've looked at and have had to add some cooling to have some pretty ancient equipment of which I'm not sure the manufacturers are even around anymore.

At any rate, there's a rule of thumb in "HVAC Equations, Data, and Rules of Thumb" by Arthur A Bell which gives 500 watts per section (vertical 20"W x 20"D x 84"H). At this rate, a 7 vertical-sectioned MCC (decent sized MCC) only contributes 1-ton of heat gain. Not a substantial load. Obviously some other assumptions are necessary for transformers, switchgear, VFDs, etc. It's all in the book--a good reference for those doing HVAC design IMO.

It has some numbers for low voltage through medium voltage starters as well if you're inclined to tally up the actual "buckets" and/or have a single line diagram handy.

 

[jimbo1184]

CountOlaf is right, "HVAC Equations, Data, and Rules of Thumb" is a great reference book. I have this book at my desk and use it quite often.

 

[Guancho34]

Second Edition came out last month... time to update!