Use an "on delay" relay to break the power to the contactor coil using the normally closed contact, break power to the relay thru a normally closed momentary 16mm, 22mm, or 30mm pushbutton in a nema rated enclosure for that enviroment, all can be 120 volt. Set timer for 20 seconds, push button, contacts will remain closed for 20 seconds, then open dropping out contactor. Beforwarned though a power failure will rest timer and activate load. Do not use if this is going to be a problem. They also make a pnuematic timer with built in pushbutton, but is large and bulky,hard to find, but will not have the unwanted activation problem. There are other types of timers that could be used, but this is the simplest.