Forums » Electrical Engineering

Connected load vs demand load for MCC

    • 6 posts
    December 30, 2019 12:35 PM PST

    I have a question regarding to connected load vs demand load for MCC. The MCC rating is lower then connected load but equal to demand load. Is this acceptable by standards?

    • 156 posts
    January 3, 2020 9:13 AM PST

    This is acceptable.

    • 75 posts
    January 5, 2020 8:47 AM PST

    Fadel -

    Give us some more context - and maybe a few definitions:

    "connected load"  Would this be the load as calculated by regulation, like an NEC load calc?

    "demand load"  Would this be the load as measured while the system is running (at full load)? 

    Is this a peak loading and the normal running current is down - say to 80% or less?  "demand load" is not continuous?

    "acceptable by standards"  Is this a legal/illegal question of the regulations, or is this a "will the system run with long term reliability"?

    Is this an industrial process, running Wide Open Throttle, 24/7? 

    Or, is this Grandpa's cottage down by the sea?


    Assuming I am understanding:

    If this is industrial process, about the only way that this can happen is if someone went in after the original installation was in operation and added load - and did not pay any attention to the load calcs.  That is the only way one could actually measure the "demand load".

    If this is the case:  The MCC will run hot, long term reliability is down.

    Legal or not, normal industrial design standards - not acceptable


    Optional scenario:

    This is an original design/installation and the builder/owner wants the cheapest possible installation.  Long term reliability/life cycle maintenance costs are not even considered.  The engineer has used previous experience to determine the "demand load". 

    Under the NEC, this would be illegal (although, it is possible to get around)

    For example:  Consider a 600A MCC with a 600A Molded Case Main CB.  (assuming I understood your definition of "demand load") The load is a continuous 600A.  By NEC rules, the Main CB is overloaded, 125%

    However, one could get a 100% rated 600A CB - uncommon, but possibly available.


    Under normal industrial design standards:

    100% loaded = runs hot = lowered long term reliability = increased maintenance = increased down time = loss of revenue = pissed off customer



    It could also be I don't understand the context.  and I'm all wet.