LV Contactor selection for Transformer Switching

[cherry2000]

Can somebody help me in selection of LV contactor (starter) for Transformer switching?
NEMA ICS 2 Table 2-4-17 gives the max kVA ratings for Transformers with 1 to 20 times of Inrush current.
Now for a 600V, single phase application, max. kVA rating is given as 135kVA.
However, for us , we have a 150kVA Transformer which is Oil filled.
I suspect that the inrush current will not be as high as 20times.
Now the question : If I can get the exact inrush current value, can I select a Contactor which may be lower in size compared to what NEMA ICS 2 prescribes?

 

[jraef]

You can do whatever you like, NEMA is not a regulatory agency. So from a prudent engineering standpoint I don't see why not if you get valid data showing the maximum inrush current.

But I would not assume the inrush will be less than 20x peak continuous current. This is a very real probability, depending on where (when) in the sine wave the contactor closes.

 

[cherry2000]

Thanks for the firm opinion. Here the Transformer is for a special application and has a reactance of 35%. Working with this value (works out to be 0.84 ohms), and some other values for saturation flux density, steady state flux density & residual flux density, one can arrive at the inrush which will never exceed the Maximum closing amps of 3163Amps for a Size 5 starter. Hence, even though, NEMA table shows 68kVA as the maximum 1phase, 600V Transformer rating, we can energize the 150kVA Transformer which has a low inrush of 3 times as per the manufacturer, using a Size 5 starter. In fact, the guiding criteria now becomes the enclosed amp rating for the Contactor (Transformer FLC =250Amps)which is 270Amps for a Size 5 starter.
Please give your comments on this analysis. I am attaching some sample calculations of Inrush current from a Transformer handbook.