Coordination of MCC Main/Feeder breakers?

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mull982

Senior Member
I was curious what method others use to provide coordination settings for an MCC Main breaker of Feeder breaker (If MCC is MLO).

Per the red book when modeling motors in an MCC motors 50hp or less are typically lumped into a single lump sum while motors over 50hp are represented seperately. Assuming that we are dealing with an MCC that has several motors under 50hp lumped together and single motors over 100hp represented seperately how do you typically go about setting the MCC main breaker?

Do you typically set the breaker to allow for some combination of the motors in the MCC starting? Largest motors? Largest 2 motors? Lump sum starting?

Do you try to coordinate the main breaker with the largest MCP in the MCC, accounting for the adjustable instantaneous setting on the MCP.

Just curious to hear what others do or have done when performing a coordination study, in order to derive settings for an MCC main or feeder breaker.

Thanks
 

jim dungar

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Retired Electrical Engineer - Power Systems
Do you try to coordinate the main breaker with the largest MCP in the MCC, accounting for the adjustable instantaneous setting on the MCP.

This is my first choice. Of course, knowledge of the actual conditions might impact my actual methodology.

For instance, a 400A MCC with a 100A across-the-line motor would be looked at differently than a 1200A MCC with the same loading.
 
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mull982

Senior Member
This is my first choice. Of course, knowledge of the actual conditions might impact my actual methodology.

For instance, a 400A MCC with a 100A across-the-line motor would be looked at differently than a 1200A MCC with the same loading.

Without any MCP info and just having motor information avaliable what would be your prefered method for coordination?
 

jim dungar

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Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Without any MCP info and just having motor information avaliable what would be your prefered method for coordination?
Select an MCP, coordinate the main to that MCP, then stamp and seal my recommendations.
 

mull982

Senior Member
Select an MCP, coordinate the main to that MCP, then stamp and seal my recommendations.

So it sounds like you are saying you always base the Main breaker coordinatnion off of the largest MCC MCP?

Do you ever take into consideration motor stating such as largest motor starting, largest 2 motors starting simultaneously, or a lump sum of motors starting?

Thanks for sharing your insight!
 

jim dungar

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Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
So it sounds like you are saying you always base the Main breaker coordinatnion off of the largest MCC MCP?

I never used the word always.
You asked me for my preference when I don't have any information except for motor size. In my first post I said:
Dungar said:
....knowledge of the actual conditions might impact my actual methodology.
 

MyBeardAndMe

Senior Member
Location
USA
Occupation
EE
Why wouldn't you coordinate with just the biggest MCP? If your biggest motors were 2-100 HP motors, you would just coordinate with one of them. The MCC breaker is protecting the bus. If your motor faults, then the MCP should take care of the fault and the MCC breaker should be fine. If you are worried about the inrush for starting a lot of large motors at the same time, model it as a lump sum and look at the curve in comparison to your curve for the MCC breaker.
 
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dicklaxt

Senior Member
Don't you have to take into account the connected operating load then add in inrush of the largest.
Fault and overcurrent settings will take out the main and maybe upstream feeded bkr as well if set to tight.


The MCP's them selves will protect the individual loads

JMO

dick
 

rcwilson

Senior Member
Location
Redmond, WA
1200A MCC. 35 KA fault level. 100 HP largest motor.

Motor inrush is about 600-800A so MCP will be set about 1000-2000A. Add 1200 A and set the main breaker instantaneous at 3000-5000 with a time delay to give time for MCP to clear. Check the acceleration curve for the 100 HP and make sure the main will not trip on short time during starting. If you lack data, assume 6 x FLA for 10 seconds and add a fudge factor for other loads.

If your worried about starting all motors, just assume they all start with 6x FLA = 6 x 1200 A = 7200A.

The point I am getting at is you can't know all the details and cannot exactly predict what the actual fault currents will be or how the breakers will react. Most of your calculations are +/-10% and probably more likely +/-25% just due to rough data. To worry about 3000 A versus 7200 A setting when fault levels are 35 kA, is not realistic.

But if you are trying to limit arc flash, you will need good data and crank the settings down.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
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EE (Field - as little design as possible)
So it sounds like you are saying you always base the Main breaker coordinatnion off of the largest MCC MCP? ...
Generally, Yes. And the closer the motor size is to the transformer capability, the more important it gets.

... Do you ever take into consideration motor stating such as largest motor starting, largest 2 motors starting simultaneously, or a lump sum of motors starting? ...
Absolutely. If the worst case conglomerate motor starting time-current overlaps with CB trip curve, I guarrentee the stars, planets and moon will line up - more regularly than one would think probable.

Get some log-log paper - the easiest is to print out a few pertinent CB trip curves on 11x17. Lay out the big motor starting curves. Lay out the conglomerate motors starting curve. Put the cable damage curves and xfm damage points on it. Doesn't have to be too accurate - save that for the ETAP presentation. This is just for a conceptual. Remember the definitation of "Coordinated" is "Daylight between the curves".

ice
 
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kingpb

Senior Member
You cannot properly coordinate without doing a short circuit analysis and then develop time current coordination curve(s). Without that you are just guessing.

The transformer maximum can be used to buy the equipment and get the right rating, but it cannot be used to determine actual fault current levels, unless you want to inadvertently set the devices higher than the available current and not trip them at all.:blink:
 
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