Sizing a neutral feeder

[PQexplorer]

A 3-phase UPS supplying a data center (a combination of non-linear &linear equipment), if you know the current THD of any phase have not exceed 20%, and the neutral current measured in the neutral bus output is max 31A while each phase carries 100A,90A & 90A respectively, would you oversize the input (bypass) neutral feeder?

Of course, the neutral feeder will carry current only if the UPS is on bypass, because the input rectifier is only feed by the three phase wires "delta connected".

May I have to consider an unbalanced condition between current phases?.

Thanks for your anwsers and explanations.

 

[steve66]

Re: Sizing a neutral feeder

I don't think you would have to oversize the neutral.

Ignore imbalances between the phases. Even for a worst case imbalance, the neutral only needs to be the same size as the phase conductors. Assuming you have the loads somewhat balanced, you are already ahead of the game. That should be more than enough to handle 20% THD.

Steve