Wire Nut Motor Leads?
Whenever possible, motor leads should be connected using approved mechanical fasteners because a wire nuts are more susceptible to vibration and might vibrate loose.
Wire nuts are only allowed on connections using wire up to 10 gauge wire.
Rated ampacity for 10 gauge wire is 30 amps which roughly equates to about a 10 horsepower motor. So, a general rule would be to not to use wire nuts on any motor 10 horsepower or greater.
I personally try to avoid using wire nuts on any motor. I prefer to use the insulated set-screw type electrical connectors whenever possible.
Refer to NEC National Electric Code number 110.14(B) for wire-nut connection information.