wire twister
Member
I've inherited a roof top unit that the vfd is constantly going down on under-voltage. It is a 208v 3 phase unit with a MCA of 335A. Right now it is being fed by a 200' parallel run of 1/0's in the same conduit. I already know that the 1/0's are not large enough for the load and need to be replaced. My problem is the buildings voltage is low to begin with, 197 volts on the lowest leg, and the power company is being less than helpful since we are right at the 5% mark. Doing the voltage drop calculations using the 3 phase average at the MDP, 199.33v, I can only afford to loose 1.73 volts max to keep the VFD from tripping, VFD is set at 5% and they say it can't be changed. I have come up with running parallel 500's for a voltage drop of 1.5v.
Now on to my questions
Parallel 500's seem excessive to me. My calculations were based on the MCA of the unit. Is this correct?
Would the VFD trip on phase to phase under-voltage or average 3 phase?
Any help would be great I'd hate to do the install using flawed logic and be no further ahead than when I started.
Now on to my questions
Parallel 500's seem excessive to me. My calculations were based on the MCA of the unit. Is this correct?
Would the VFD trip on phase to phase under-voltage or average 3 phase?
Any help would be great I'd hate to do the install using flawed logic and be no further ahead than when I started.
Last edited: