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Thread: Low Voltage MCC sizing

  1. #1
    Join Date
    Jan 2012
    Location
    Madison, Al
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    57

    Low Voltage MCC sizing

    I am updating an existing site that was built in the early 1960's. I need to replace the existing MCC that was used as a main entrance service and is also feeding a lot of motor loads. I am having problems with sizing the new MCC. Do I need MCC sizing software or is this something I can do with calculations?Some of the equipment is being replaced and I am adding new motors. I know the breaker sizes but can't figure out what the next step should be. Also there is a 500 KVA outdoor transformer that feeds this MCC which may or maynot be overloaded.

  2. #2
    Join Date
    Aug 2004
    Posts
    10,925
    I would run the calculations one would run for a feeder, and select the MCC busing and main accordingly.

    The calculations will tell you how big the transformer needs to be.

    The bus bracing is going to be affected by the available short circuit current from the transformer, so you will need to know that as well.

    if you are uncomfortable with doing it yourself, generally all the MCC folks will be happy to do it for you. You will need a list of all the loads to give them.

    I might be inclined to give serious consideration to having a MLO MCC with a fused disconnect right at the transformer to feed it, as opposed to an MCC with a main fused section. It may give you some options to dealing with potential arc flash issues.

    You may also want to consider breaking it into two MCCs with lower amp rated bus as a cost saving means. Some times that works, sometimes not.
    Last edited by petersonra; 01-11-12 at 11:51 AM.
    Bob

  3. #3
    Join Date
    Jun 2008
    Location
    Campbell CA
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    226

    Low Voltage MCC sizing

    Quote Originally Posted by lilsparky View Post
    I am updating an existing site that was built in the early 1960's. I need to replace the existing MCC that was used as a main entrance service and is also feeding a lot of motor loads. I am having problems with sizing the new MCC. Do I need MCC sizing software or is this something I can do with calculations?Some of the equipment is being replaced and I am adding new motors. I know the breaker sizes but can't figure out what the next step should be. Also there is a 500 KVA outdoor transformer that feeds this MCC which may or maynot be overloaded.
    I don't think you need software to size MCC, Calculator should be enough, software may make it easier. You didn't give much information, but here we go.
    start from all known motor loads existing, new, allow for future expansion, add 25% of the largest motor load. this will give you the MCC size.
    Now you may have other loads like cooling / heating, ligthing, receptacle loads, and etc.....
    If you have also large amount of other loads as I mentioned above, I would not use the MCC as service, I would go with a new MSB ( a meter / main with dist. sect.) and feed the MCC from that.

  4. #4
    Join Date
    Jan 2012
    Location
    Madison, Al
    Posts
    57
    Thanks everyone. I did find an MCC supplier to help. I am trying to track down the short circuit loading from the base utility which will be tough. I do have a combination of HVAC loads along with motor loads. However, since some of these motor loads aren't changing, they originally didn't have starters. I was wondering should I include starters? my MCC guy says no. Just wondering?

  5. #5
    Join Date
    Jan 2012
    Location
    Madison, Al
    Posts
    57
    Sorry, I also didn't include that I am space constraint so basically replacing in kind with everythingl.

  6. #6
    Join Date
    Aug 2004
    Posts
    10,925
    Quote Originally Posted by lilsparky View Post
    Thanks everyone. I did find an MCC supplier to help. I am trying to track down the short circuit loading from the base utility which will be tough. I do have a combination of HVAC loads along with motor loads. However, since some of these motor loads aren't changing, they originally didn't have starters. I was wondering should I include starters? my MCC guy says no. Just wondering?
    I see no reason to add starters for something that does not currently have one, unless the cutomer wants to pay for it.

    Feeder breakers is all you need.

    I will say that the feeder breakers MCC suppliers will sell you can be a little pricey compared to just a standard panelboard. if there are more than a handfull, you might want to consider a seperate panelboard to save money. They can also eat up a lot of room in an MCC.
    Bob

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