NEC Transformer Sizing

[timm333]

I have a question about transformer sizing for motor. Generators are generally sized 250 % of motor HP, for example if the motor is 100 HP then the generator required to start the motor at DOL would be 250 kVA.

Is there some general rule to size the transformer (delta-wye 13.8kV-480V) for starting the DOL motor? Thanks

 

[augie47]

This help ?
https://www.schneider-electric.us/en/faqs/FA102209/

 

[timm333]

Yes it is helpful, we size transformer which allows maximum 10% voltage drop on motor starting.

There are other applications which have high inrush starting current, like "transformer arc welding" and "lighting".

When sizing transformer for one 100 HP motor, one 250 W light, and one 10 kW welding receptacle: should we consider the inrush starting current of only motor for the sizing of the transformer, or should we also consider the inrush starting current of welding receptacle and light? Thanks

 

[petersonra]

Yes it is helpful, we size transformer which allows maximum 10% voltage drop on motor starting.

There are other applications which have high inrush starting current, like "transformer arc welding" and "lighting".

When sizing transformer for one 100 HP motor, one 250 W light, and one 10 kW welding receptacle: should we consider the inrush starting current of only motor for the sizing of the transformer, or should we also consider the inrush starting current of welding receptacle and light? Thanks

I would not be worried about the tiny amount of inrush current generated by the light. Compared to the 100 HP motor it just won't matter.

I don't think welders have all that much inrush associated with them so I would not worry about it either.

A 100 Hp motor is about a 100 KVA load once it reaches steady state conditions. The next standard size up is 112.5 kVA. After that it goes to 150, 225, and 300. I might use a 225 or 300 kVA xfmr for such an application, depending on cost constraints.

 

[timm333]

Thanks. When the source of power is utility, we install capacitor at the service entrance for power factor correction.

But when the source is onsite generator: should we still install capacitor for power factor improvement?

 

[GoldDigger]

Thanks. When the source of power is utility, we install capacitor at the service entrance for power factor correction.

But when the source is onsite generator: should we still install capacitor for power factor improvement?

You would not be doing it to avoid a penalty charge for high power factor, so what you need to know is how many kW and kVA your loads will run at max and what the kW and kVA ratings of the generator are. The high power factor will force the generator to deliver more current, but will not put any extra load on the prime mover (engine) of the genset.
Many generators are rated for a continuous kW figure at a PF of .8 or better. If your uncompensated load has a PF below .8 you may not be able to get full wattage output from the genset.
However, make sure that you do not overcompensate, as the voltage regulators of many generators do not handle well even a small capacitive reactance in the total load.

 

[JoeStillman]

You might want to ask how many starts-per-hour for that motor. Frequent starts don't give the transformer time to cool down between inrushes.

 

[don_resqcapt19]

This gets messy with the transformer overcurrent protection rules in 450.3 and the motor circuit rules in 430.52. The rules in 430 do not modify those in 450. You will have to size the transformer such that the maximum permitted secondary OCPD is large enough to permit the motor to start.