Generator Size Vs. Service Amperage

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smithjo30

Member
Location
Wisconsin
Occupation
Electrical Engineer
Hello all,

I have a Waste Water Treatment Plant that has an 800A 3 Phase 480V service, and they currently only have a 180 kW Generator backing up a 300A MCC. The are other circuits that are fed from a 800A Main Circuit Breaker. The plant wants whole plant backup. Does my generator size need be as big as the system amperage or only as big as the existing load calculations, and adding roughly 40% for future growth?

The calculations I ran on the plant came up to needing a 348.42 kW size generator, times by 40% = 487.79.

487.79 kW = 586.72 AMPS

Would a 500 kW generator be sufficient in this scenario?
 
Did you account for starting the largest motors? Do you have control of the timing of connecting loads to the generator?

Even on utility power I worked on projects where we had to add sequential timing circuits to prevent multiple large (50 hp and up) motors from restarting after utility power was restored after a failure.

One of those was an industrial waste treatment plant that has multiple aerators with 2 wire control so they would all try to start when the power was restored.
 
don_resqcapt19 said:
Did you account for starting the largest motors? Do you have control of the timing of connecting loads to the generator?

Even on utility power I worked on projects where we had to add sequential timing circuits to prevent multiple large (50 hp and up) motors from restarting after utility power was restored after a failure.

One of those was an industrial waste treatment plant that has multiple aerators with 2 wire control so they would all try to start when the power was restored.
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I didn't. All motors on the Plant will be controlled by VFD's for what it's worth. The largest motor is a 15HP. What do you suggest I should change in my calculation?
 
smithjo30 said:
I didn't. All motors on the Plant will be controlled by VFD's for what it's worth. The largest motor is a 15HP. What do you suggest I should change in my calculation?
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Probably not as that is a pretty small motor. Most treatment plants that I have worked on all had 50hp+ blower motors.
 
Agreed. You have small motors, and the VFDs reduce starting current anyway.

You might want to get some utility demand information to double check your load calculations. NEC calculations usually come in quite high.
 
Also, remember that if you are installing a diesel generator, you want to run it around 60-85% of full load.

Running it at 1/3 load, or even 40-50%, can increase risk of wet stacking.
 
On a plant this size the utility should be able to easily tell your max demand over the last year. Size accordingly.

However, if there are a lot of VFDs, the generator head may need to be upsized.
 
Joethemechanic said:
Seems to be really small. I can see that is the biggest running clarifiers, bio contactors, bar screen rakes etc. But aren't there any big pumps or air compressors/blowers?
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This plant is for a small town that compared to some. Township of only about 2500 people. Hence the smaller motors and such. The generator fuel source has been decided upon as Natural Gas for those that wanted to know. It was asked of me to size the generator to whole plant backup with a 40% upsize as well.
 
Re-read this thread, I don't think that you as an electrician have sufficient information to calculate the generator size.

You can easily calculate the maximum possible power consumption of the plant, simply by adding up all of the rated loads. I assume that is what you did for your first post, and your 500kW answer is reasonable given the maximum possible power assumption. (Caveat about the front ends in post #9: these produce harmonics, and you may need a generator that has a larger alternator to deal with this, be sure to inform the generator supplier what the load will be.)

What you don't know is if your maximum power assumption is reasonable. It may be that the entire plant is designed with a ton of redundancy, and that only half the motors run at any given time. Or that everything runs at reduced power. In a very real sense, pumps don't consume power but instead convert electrical power into hydraulic power.

IMHO you need input from the engineer who actually runs the plant to I better understand the actual power consumption.
 
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