Recommended Maximum HP Rating for 480V 3

[timohearn]

I am interested in a recommended maximum horsepower ratings for 480 volt, 3 phase, 60 hertz. I realize that it is based upon a lot of factors, but my interest is in investigating whether there is a consensus within the technical community for the US market that could establish a recommendation. Thankyou in advance for your advice.

 

[EMan9999]

Hi,

I would suggest that the maximum working hp be lower than 200 in order to draw less than 300 Amps through the conductors. From my experience, 300 Amps would probably call for a 1.5" cable in most industries anyway (that's a large-sized cable in my opinion).

 

[dpc]

This really depends on a number of factors:

Number of large motors to be served
Availability of higher voltages
Voltage levels available
Constant speed or adjustable frequency

When I was doing power plant design, all motors 300 hp and larger were medium-voltage, generally 4160 or 6900V.

But if your system only one 300 hp or even 500 hp motor, it probably doesn't make sense to establish a medium-voltage system to handle it.

For adjustable frequency applications, you easily go up to 500 hp at 480V, if not higher due to the relative high cost of medium-voltage drives.

Below about 300hp you will pay a premium for medium voltage motors.

 

[busbar]

IEEE Std 141-1990 “Red Book” Table 3-11 Standard voltages and preferred horsepower limits for polyphase induction motors
460V 1 hp minimum - 1000 hp maximum
4000V 100 hp minimum - 7500 hp maximum

 

[rbulsara]

I second dpc in what he recommended.

One recommendation: If you decide to go with 480V , large motor (250 hP and above), at least specify starters with vaccum contactor (available in 2kV ratings). Because the major maintenace headache with regular air magnetic contactors is the pitting of contacts due to making and breaking of large currents. Vaccum contactor will avoid this headache.

 

[alehman]

Also see thread238-55851

 

[boomsystems]

I agree with Mr. bulsara. Based on our practical experience, system studies, voltage drop calculations during starting , network strength and cable sizing you can not have motor more than 250HP.

 

[alehman]

Cost and maintenance must also be a consideration. I agree with dpc, except for packaged equipment such as chillers, etc. we have found that medium votlage motors are a premium cost up to 1000hp or so. Also, experience of maintenance personnel and availability of replacement parts etc. should be a factor in the decision.

Voltage drop is a function of the length and size of the feeder cables. We regularly design systems for motors up to 1000hp at 480V, but only if a transformer unit substation can be located close to the motors. If you have higher voltage for other purposes or many large motors, higher voltage may be preferred.