Forums » Electrical Engineering

125A main lug panel

    • 42 posts
    May 18, 2020 6:02 PM PDT

    Hi everyone,

    A main panel contains a breaker of 80A to feed a 125A main lug panel. If I am not wrong, NEC 215 allows this?

    • 75 posts
    May 19, 2020 12:22 PM PDT

    The short answer is:  I don't know.  Not enough information.  In particular:

    1. What is the load on the panel?  (1.25 x continuous + 1.0 x non-continuous)
    2. What is the ampacity of the feeder conductors? 

    In this case (MLO panel) the CB has to both protect the panel and the feeder conductors.

    Generally, one calculates the load on the panel (and adds a suitable, comfortable extra for future expansion).  Note the future expansion is not required by code.

    Then sizes the feeder conductors for the load.

    And finally sizes the feeder CB to protect the conductors.  Note some finagling allowed for next size up.

    So:

    1. The load would have to be less than 80A - right on down to zero if the designer wants. 
    2. The conductor ampacity has to be equal or greater to the load, but more than 70A.  If 70A (or less), the next-size-up-rule would not allow an 80A CB, one would have to use a 70A CB - or smaller, depending 

    For example:

    If the load is less than 80A, the feeder conductors are #4 CU, 75C insulation. all is good.

    Unless there is some other part of 215 you are looking at -- In which case, I still do not know.

    • 157 posts
    May 19, 2020 1:16 PM PDT
    Ronnie Rana said:

    Hi everyone,

    A main panel contains a breaker of 80A to feed a 125A main lug panel. If I am not wrong, NEC 215 allows this?

    Of course it is allowed as long as the calculated load is not greater than 80 amps.  

     

    I have installed many 200 amp main lug panels on a 100 amp feeder.  Not an issue


    This post was edited by Dennis Alwon at May 19, 2020 1:17 PM PDT
    • 29 posts
    May 19, 2020 2:56 PM PDT

    Totally agree with Dennis.