Transformer sizing

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arenalights@gmail.com

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I am trying to size a 3 phase transformer to feed some motors, I am going to feed from a 120/208 panel and need approximately 175 amps for 5, 480 3 phase motors. Obviously I plan on back feeding the transformer to achieve this. Any help will be appreciated, thanks
 
In it's simplest form you have about 150 kva of load but a lot can depend on the individual loads. What size are the motors ? Will they all run simultaneously ?
Will they start at the same time ? Are they controlled by drives or ACL ?
 
RE sizing

RE sizing

arenalights@gmail.com said:
I am trying to size a 3 phase transformer to feed some motors, I am going to feed from a 120/208 panel and need approximately 175 amps for 5, 480 3 phase motors. Obviously I plan on back feeding the transformer to achieve this. Any help will be appreciated, thanks
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I have a 60, 2, 30's, and 2, 10hp mtrs. They could run together but most of the time the 60 and one 30 will run. thank you
 
Personally, I'd size the trannie for somewhere close (read nearest nominal size) the average of locked rotor current and full load current of the largest motor plus the full load current of the rest... unless one or more will not run simultaneously by at least electrical interlocking means.
 
If you contact one of the transformer manufacturers with the motor data they will assist you with sizing. My WAG would be a 225 kva.
 
augie47 said:
.... My WAG would be a 225 kva.
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I will agree with that as a minimum... I'm leaning toward 300kVA as optimal. The problem one usually runs into is secondary protection breaker sizing. It can't be any greater than 125% of the secondary rated output current, but yet it has to be high enough to allow starting of all the motors at the same time, if startup timing is not controlled, say upon power restoration after an outage... and at the least should be high enough to permit starting the largest motor with all other motors already running at full load.

ETA: Secondary protection can be as high as 250% in supervised locations... which can make a big difference in sizing the transformer.
 
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Asumming you likely need at least a 225 KVA that will mean a OCPD of 800 amp at 208 volt. Do you have the capacity and physical breaker space for a breaker with this big of a frame? Also FWIW, reversing a standard 480 delta X 208Y transformer is a poor idea for a number of reasons, not the least of which are the grounding on the 480 side. Sure you can do it but there are a lot of downsides to this. Better would be to get a 208 delta X 480Y transformer for just slightly more money.
 
texie said:
Asumming you likely need at least a 225 KVA that will mean a OCPD of 800 amp at 208 volt. Do you have the capacity and physical breaker space for a breaker with this big of a frame?...
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Don't forget the transformer size is assuming an unsupervised location, and actually is oversized because of the limitations of transformer OCPD vs. OCPD sizing for motor startup. Actual current draw at FLC is only going to be about 427A@208V (185A@480V). So the 208V source only has to have capacity for 471A with the extra 25% for the largest motor... but you are correct in that the OCPD rating and frame size will have to be substantially greater.
 
Is your posting name = email address for real? I sent info to it on a 300kva transformer that is not being reverse fed so made correctly for this.... As I said in email (in case it does not go thru) the inrush MAY be an issue on such a large xfmr if fed reversed - might be much higher than normal 10-20x and cause problems with trying to provide a decent size CB.

If you can add up all the starter max starting currents that might happen at the same time, you might find you can reduce the kva back down from the 300kva that is generally being described as best size to keep you out of trouble.
 
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