480V Motor HP rating

[jliu1]

Hello. I have a question on selection of voltage level for a group of motors. What is the maximum allowed HP ratings for 480V? In other words, when a motor have a larger size than that, it has to be rated at the next higher voltage level. I was told 200HP might be the number. But I could not find the basis in the standards, like IEEE. Please help. thanks.

 

[itsmoked]

I believe there is no standard on that. No one cares how many amps you'd have to shove thru a motor. However there will be a point at which the motor costs a ridiculous amount of money but that isn't a 'standards' based metric.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

There's no "maximum" size for 480 V motors. 500 hp motors are fairly common and even 1000 hp or 1500 hp is possible if you want to pay a premium. You have to be able to start it however.

In general, if you have a large number of motors over 200 hp, it is going to be more economical to use a higher voltage.

 

[7anoter4]

Hi jliu1
First of all I agree with all above.
I think is not a direct problem of voltage level but a problem of supply transformer size. Usually is limited to 2500-3000 KVA.
So you have to check if when starting the most powerful motor the voltage drop through the transformer [ loaded 80% or more ] is not too high and the voltage at the other still consumer terminals does not fall under the permissible[let us say 10% under the rated] .
A thumb rule is the transformer KVA > 3*maxMotor HP.
Regards

 

[alehman]

Up to about 1500hp, it's a matter of economics and the operation of your facility. I recently did a comparison on some 1000hp compressors and found 480V motors somewhat less expensive than 4160V. Next look at the costs of the starters and circuitry. Next look at your existing plant. Is it all 480 or all 4160? It's usually better to stay with the established standard unless there are overriding concerns. In my case I could locate a unit substation very close to the motors, so the circuits were short and 480V was the way to go.

Then you must consider maintenance. Is your staff trained on medium voltage? Would training be a significant cost or inconvenience? Also look at lead time for procurement of the equipment (and maybe future maintenance replacements).

 

[jraef]

1500HP is the practical limit for 480V systems. Beyond that and the cable becomes unwieldy and next to impossible to terminate.

But what you may have heard is an individual plant's assessment, which is not uncommon. I know of several facilities who have 4160V systems already running throughout the plant, who have internal edicts that all motors 200HP and up shall be 4160V. I even know one plant that draws the line at 150HP. They do it because they can. In most places, going to MV for such small motors is impractical and the cutoff is generally 500 - 600HP.