250HP 480V or 4160v

[ksavoie]

At what horsepower should I change from 480v to 4160v? I may have a 250hp or larger motor that I will need to feed. What is a general a rule of thumb for maximum horse power on a 480v circuit.

 

[dpc]

You might do a search on this forum - this has come up before. The problem is that the answer depends on the existing electrical system, the other loads, planned future loads, etc. There is not a hard and fast rule that can be applied to every situation.

If you have ONE large motor, it probably makes sense to stick with 480 V as long as you can supply it from your existing 480V transformer. That will usually be the limiting factor. You can buy a 1000 hp 480 V motor. But starting it might be a problem. If you have ten 300 hp motors, it will probably be less expensive to go with 4160V.

Likewise, you can buy 100 hp medium voltage motors, but you will pay a hefty premium until you get up around 300 hp or so.

And if this motor is to be fed from an Adj. Freq Drive, that's a different scenario entirely.

 

[jraef]

Good answer dpc. My observation is that 250HP seems to be the lower limit on practicality of changing to MV, but certainly smaller MV motors are made. Of the 4 industrial users I have seen that use that 250HP cutoff, all of have a lot of MV motors on site and two have their own co-gen plants, so as their own utility they have other benefits in doing so. Quite a few more users tend to cut off at 400HP and up, but as dpc says, if that is the only large motor it probably doesn't make sense to need an MV qualified electrician on staff to handle one load.

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