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I need a contactor for switching my transformer

We recommend our XT line of contactors for transformer switching applications. 

You determine your contactor size using the following formula:

Ie AC-3=X/6 * Ie transformer where X is the inrush current of the transformer and Ie transformer is the nominal current. 

For example

The transformer has a nominal current of 10A with an inrush current of 18 times the nominal current. So, the contactor much have an AC-3 current of 18/6 x 10A = 30A. using a XTCE032C ( 32A AC-3 ) contactor is recommended. 

For NEMA rated contactors we recommend the NEMA ICS 2-2000 standard, table 2-14-17 below for sizing the contactor

Use by permission from National Electrical Manufacturers Association: NEMA ICS 2-2000 Table 2-14-17