Motor MCA

[calabala]

Hi guys,
I was checking a single line diagram and it shows a motor of 17.9 MCA, 600V, 3-phase.

What does MCA stands for and how is it calculated?

 

[dpc]

HVAC equipment supplier gibberish? "Minimum Circuit Amps", most likely.

They use this for package equipment that may have multiple motors. It should be the sum of the motor full load amps plus 25% of the FLA of the largest motor.



"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[calabala]

Thx dpc! Appreciate your help.

 

[rbulsara]

To add to what dpc said, the load does not have to be all motors. There may be other items like heating elements. Some times all motors are not used at the same time. So manufactures are supposed to have either calculated per Code or tested their systems in various configurations they are intended to work and they post their "MCA".

So there may not be a formula for the "outsiders", but if you know the details you can calculate the ampacity required per Code.

Very reason MCA are given by the manufacturers is that you do not have to calculate and it does meet the Code. All you have to do is look up the correct data (column) for the combination/features you have.

 

[foretmark]

Is there any rule of thumb other than the above mentioned? Although it may err on the high side, I've added the motor fuse value to the highest branch circuit fuse and rounded up to nearest typical value when asked, which is not very often.

 

[ykee]

You should see MOCP as well as MCA. MOCP is the maximum overcurrent protection to size breakers/fuses, whereas the minimum circuit ampacity is typically used to size conductors.

 

[rbulsara]

There is no need for "rules of thumb". In the USA, NEC lays out very specific guidelines as to how to calculate a circuit ampacity for various type of loads.

For factory assembled equipment, manufactured published data is permitted to be used to size the conductors and overcurrent protection devices.

 

[EEJaime]

As DPC stated the MCA is 125% of the largest motor plus the sum of the remaining motors/loads on the unit. This is used for sizing the feeder. The MOCP is generally 225% of the largest motor plus the sum of the remaining motors/loads on the unit that is used to determine the upper limit of the overcurrent protective device which may be selected.

You will also see from some manufacturers, the "RLA" given, which is "Running Load Amperage", which is the actual expected running current under normal operating conditions. This would be what we include in our load summaries.

Regards,
EEJaime