Results 1 to 6 of 6

Thread: Load factor and MCC and Transformer Size

  1. #1
    Join Date
    Nov 2014
    Location
    TX
    Posts
    171

    Load factor and MCC and Transformer Size

    Greetings,

    Based on the NEC when you size the feeder you have to add non continuous load and 125% of continues load. Based on this there is no place for load factor or demand factor.

    I was wondering when you size transformers and MCCs, where in the code states that we can imply the load factor.

    Thanks

  2. #2
    Join Date
    Nov 2014
    Location
    TX
    Posts
    171
    Quote Originally Posted by Electriman View Post
    Greetings,

    Based on the NEC when you size the feeder you have to add non continuous load and 125% of continues load. Based on this there is no place for load factor or demand factor.

    I was wondering when you size transformers and MCCs, where in the code states that we can imply the load factor.

    Thanks
    Any comments?

  3. #3
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    16,801
    Quote Originally Posted by Electriman View Post
    Greetings,

    Based on the NEC when you size the feeder you have to add non continuous load and 125% of continues load. Based on this there is no place for load factor or demand factor.

    I was wondering when you size transformers and MCCs, where in the code states that we can imply the load factor.

    Thanks
    I don't think it says that anywhere. Having said that, the code does not seem to require a transformer to have any particular size although the feeder ampacity is specified at a minimum level.

    The code also does not seem to specify the minimum rating of an MCC main OCPD or bus as long as the bus is protected. That suggests to me that the ampacity of the feeder to the MCC is controlled but the rating of the MCC bus is an engineering decision.
    Bob

  4. #4
    Join Date
    Jul 2006
    Location
    San Francisco Bay Area, CA, USA
    Posts
    7,806
    Quote Originally Posted by petersonra View Post
    I don't think it says that anywhere. Having said that, the code does not seem to require a transformer to have any particular size although the feeder ampacity is specified at a minimum level.

    The code also does not seem to specify the minimum rating of an MCC main OCPD or bus as long as the bus is protected. That suggests to me that the ampacity of the feeder to the MCC is controlled but the rating of the MCC bus is an engineering decision.
    Bob is correct, there is no official rule on considering load factor one way or the other. If you ask the MCC mfr they will tell you to size it per the total connected FLA, because that’s the only “safe bet”. But in truth, hardly anyone does that, it would be prohibitively expensive. It’s an engineered product so it’s assumed whomever is engineering it understands how the systems will be used and how much can be running simultaneously. The risk really is yours in that if you use a Main Breaker or a Feeder to the MCC that can’t handle that total connected load, the feeder or main trips and everything shuts down, so it becomes an economic loss. But the rules do dictate that whatever protective device feeds the MCC can be no greater than the main bus size.

    The same basic concepts apply to transformers too, with the exception of needing rules-based protection on both sides of it.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  5. #5
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    19,003
    I am curious as to what you think a "load factor" might be. The NEC has a number of places where it allows us to apply a "demand factor" to reduce the value of the calculated load. That is part of the process of determining the minimum size your transformer or your MCC will have to be.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  6. #6
    Join Date
    Nov 2014
    Location
    TX
    Posts
    171
    Quote Originally Posted by charlie b View Post
    I am curious as to what you think a "load factor" might be. The NEC has a number of places where it allows us to apply a "demand factor" to reduce the value of the calculated load. That is part of the process of determining the minimum size your transformer or your MCC will have to be.
    In this case load factor and demand factor is the same. I have seen in NEC that demand factor is used for sizing service disconnect. But I have not seen anything similar for sizing transformer and MCC bus rating.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •