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LV Contactor selection for Transformer Switching

LV Contactor selection for Transformer Switching

LV Contactor selection for Transformer Switching

(OP)
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?

RE: LV Contactor selection for Transformer Switching

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.

RE: LV Contactor selection for Transformer Switching

(OP)
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.

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