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Thread: FLA vs MCA vs MOP

  1. #1
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    FLA vs MCA vs MOP

    FLA = Full Load Amps (sum of the FLA of all the electrical data)

    MCA = Minimum circuit ampacity (It is chosen to guarantee that the wiring will not overheat under the expected operating conditions. The MCA should not be used to determine the normal operating current, but used for selecting wire sizes) 125% of the largest motor FLA + the remaining sum.

    MOP = Maximum overcurrent protection (The maximum overcurrent protection (MOP) is the maximum circuit breaker / fuse size required to properly protect the equipment under anticipated fault conditions.) 225% of the largest motor FLA + the remaining sum.

    Does this sound correct? I have seen some designs where the overcurrent device is sized the next size above the MCA and some between the MCA and MOP and some at the MOP what is the correct way? if the MCA is 175A and the MOP is 250A if would be easier to install a 200AS/200AF rather than a 40AS/250AF for space reasons in a panelboard? would the 200AS/200AF still be acceptable?

  2. #2
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    If the max ocp is 250 then you should be able to use anything between the min. cir. amp. and the MOP for the OCP. I always use the max size because later on there could be problems with the breaker being able to handle the older less efficient unit.

  3. #3
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    For sizing CB protecting motor, I will go with NEC rule (2.5 x FLA).

    If the result less than the MOCP shown on mechanical schedule ( it always does), I will go with mechanical schedule.

  4. #4
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    If you use the MOP for a breaker size, how do you size the wire between the breaker and the equipment?

    Do you still size it for the MCA, or does it have to be sized for the MOP??

  5. #5
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    Quote Originally Posted by steve66 View Post
    If you use the MOP for a breaker size, how do you size the wire between the breaker and the equipment?

    Do you still size it for the MCA, or does it have to be sized for the MOP??
    You would size it for the MCA. The unit has overload protection to protect the wire. The breaker is there for short circuit and ground fault in this case, not overload protection.

  6. #6
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    Quote Originally Posted by Dennis Alwon View Post
    You would size it for the MCA. The unit has overload protection to protect the wire. The breaker is there for short circuit and ground fault in this case, not overload protection.
    I knew that was the case for a single motor, but I was never sure about something like a packaged AC unit that might have a couple of compressors & a couple of fans.

  7. #7
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    Quote Originally Posted by steve66 View Post
    I knew that was the case for a single motor, but I was never sure about something like a packaged AC unit that might have a couple of compressors & a couple of fans.
    If the MCA and MOP are listed on the unit, that's all you need to follow. No calcs necessary.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  8. #8
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    what about FLA

    I believe you also need the FLA or RLA if you are doing load analysis of a system the FLA would be the full load or running load of the unit if you were to use the MCA for all the units you would have a false "system" load if you were doing a load analysis / study / planning of a campus system.

  9. #9

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