UL certification requirement in USA for instruments

We are coming up with a plant in USA which is a new territory for our operation. Does all instruments like transmitters, pressure switches, relays, valve electropneumatic positioners, etc., require UL certification for use in USA? Does control panels installed in safe area also require UL certification? Since instruments with UL certifications are costly, is there any exception to go without UL for some of the instruments? Does Pressure switch with potential free contact require UL certification if it is used in 24 VDC circuit?
 
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