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Topic Title: MCC Main Circuit Breaker Selection
Topic Summary: How to properly select a mains breaker (ACB/MCCB) for a Motor Control Cabinet?
Created On: 13 March 2013 11:42 AM
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 13 March 2013 11:42 AM
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kudrenko

Posts: 4
Joined: 31 January 2011

Can anyone please explain me how to properly select a mains breaker (ACB/MCCB) for a Motor Control Cabinet? Assume I have a combination of DOL, ASD, VFD starters and all of them will be running at the same time.
Should I add FLC of each starter and select breaker of approximately the same size?
 13 March 2013 12:53 PM
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dlane

Posts: 684
Joined: 28 September 2007

A load flow study will tell you the FLC rating of the main incoming breaker.

The selection though will also be dependent upon the fault level the incoming device would have to clear, the type of protection required, the location of the installation, ambient temperatures, atmospheric conditions etc. control supplies available and number of expected operations. You may also need to consider the life expectancy of the installation and commonality of components, and of course there will be commercial considerations to take into account.

Incoming breakers can also be sized based upon the full load capability of the incoming supply, so you might use a 1600A ACB for a board that is supplied from a 1MVA transformer even if the load on the board is less than this. In that way, you would allow for future expansion or load changes on the board should the need arise.

Kind regards

Donald Lane
 14 March 2013 09:38 AM
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kudrenko

Posts: 4
Joined: 31 January 2011

Sorry for rather silly question, but is it possible to do it with limited amount of information? The only things I know are: motor sizes in kW, starting methods, ambient temp (<40) and that's pretty much it.

I've seen in USA they select breakers using following method: (FLC of biggest motor x 1.25) + sum of FLC of the rest motors = approx. size of main breaker. Do you know any similar techniques?
 14 March 2013 09:22 PM
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dlane

Posts: 684
Joined: 28 September 2007

There are several methodologies you can use to estimate the size of the incomer.

If you are using kW then you will also need to take into account power factor. i.e. 0.85 if unknown would be a reasonable value.

You could add any percentage on top of the total KVA rating you have, say 10 or 15% nominal. If you want a bit more extra capacity then go up to 25%.

You could also use the total of the maximum capacity of the MCC feeders in the board. i.e if you have 8 feeders at 63A then you would probably install a 630A feeder.

It all depends on how much future proof you want for your design.

You must however, know the fault level at the board, to know that the breaker you specify can handle the breaking current.

Kind regards

Donald Lane
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