FLA / MCA Calc

[bammor]

Afternoon I have a job that is requiring an array of fan motors to be installed in a four quadrant (square). Three of the four motors are to operate based on a sequence of events at one time; the fourth fan in the quadrant is a spare in case one fan motor fails coming on line through a series of sensors for detection that there is a failure of one of the three previous fans. Each motor has its own circuit and vfd or overload protection, to further explain each circuit ahead of the circuit protection comes from a single point disconnect with distribution lugs on the load side to each fan, customer supplies power to the line side of the disconnect for a single point hit. My question if you follow, do all four fans need to be calculated in the FLA/MCA or can I calculate three of the four knowing that only three will operate at any given time?

 

[GoldDigger]

I would ask whether the control logic detects that one fan is not moving air, even if it is drawing current or whether the spare fan only comes online after power has been positively interrupted to one of the initial three?
If the latter, and it the load calculation allows for the fact that all motors will be starting at the same time, then I would say that you could ignore the fourth fan.