Service Size

[Sparky1981]

I am working on an industrial plant that has (6) 2000A services providing power to the facility (reasons for this are beyond my control). Anyway, one of the 2000A services has two 600HP motors connected to it. Nameplate says the FLA is 673A. I have sized the overcurrent protection for each motor as 1600A breakers, close to the max size NEC will allow. The motors will NOT start at the same time. The motors are of course connected to a soft start starter. My question is am I going to be tripping my 2000A main breaker when one of these motors is running and the other is starting? I wish the industrial engineer ordering the starters would get me some information on starting amps but I doubt I will get this in time before the gear is ordered.

I typically work in commercial so its not often I deal with motors this large. Figured I would pick your brains.

Any thoughts on this?

Thanks in advance.

 

[iwire]

I am not an expert on this but it seems to me it would be entirely dependent on how aggressively the drives are programed.

I have worked with some 500 HP motors for fans and the ramp up time was on the order of 5 minutes so the 'start up' current never exceeded the running current.

 

[jim dungar]

My question is am I going to be tripping my 2000A main breaker when one of these motors is running and the other is starting? I wish the industrial engineer ordering the starters would get me some information on starting amps but I doubt I will get this in time before the gear is ordered.

I typically work in commercial so its not often I deal with motors this large. Figured I would pick your brains.

Any thoughts on this?

Big bucks being spent, and no one thought of paying for a coordination study? One trip, during production, and I am sure they will be glad they saved that $1,200 dollars.

Assume that your main breaker has a trip point of 5 time, this means that it could handle starting currents of 10,000A. with one motor running at roughly 700A, you would have room for the next motor to pull 9,300A or roughly 13X FLA. If your soft start cannot limit your starting current to this level, it wasn't worth what you paid for it.

Remember you need to consider starting time as well as starting amps.

My gut feel, is that someone should be able to make your equipment work.

 

[Sparky1981]

Big bucks being spent, and no one thought of paying for a coordination study? One trip, during production, and I am sure they will be glad they saved that $1,200 dollars.

Assume that your main breaker has a trip point of 5 time, this means that it could handle starting currents of 10,000A. with one motor running at roughly 700A, you would have room for the next motor to pull 9,300A or roughly 13X FLA. If your soft start cannot limit your starting current to this level, it wasn't worth what you paid for it.

Remember you need to consider starting time as well as starting amps.

My gut feel, is that someone should be able to make your equipment work.

Yeah ideally I would have the start amp and start time in front of me, this is what I have been pushing for.

I will be doing a coordination study once I have the shops, I was hoping to get this tied up prior to shops.

I would hope that the soft start being provided with a 600HP motor can limit the current to 13X FLA.

 

[Sparky1981]

I am not an expert on this but it seems to me it would be entirely dependent on how aggressively the drives are programed.

I have worked with some 500 HP motors for fans and the ramp up time was on the order of 5 minutes so the 'start up' current never exceeded the running current.

The guy programming everything has built a few of these plants we are working on. My guess is with a 24/7 operation these motors do not need to be aggressively programmed and can start up over a long enough time period to keep the breaker form tripping. I will discuss with him the programming and make sure we tune it correctly to keep any nuisance tripping from happening.