They are for most practical purposes. Standard TEFC motors rarely have an external temperature that approach the ignition temperature limits of most Class II materials. However sgunsel's comment is still very well taken because neither NEMA MG-1 nor MG-2 specifies a limit to the external surface temperature. So, unless the motor is listed for Hazardous Locations, there is no guarantee what the external surface temperature may be limited to. Note: NEMA motors for other than Hazardous Locations are not listed.
It may generally be assumed the external surface temperature will not exceed the maximum internal temperatures, since the ambient will cool the external surface. MG-1 limits the internal temperatures to a specific rise above a 40C ambient and requires derating if the ambient is higher. The typical industrial TEFC constructions use Class B insulation or Class F insulation with a Class B rise which results in 120C (80C over 40C) with a 1.0SF or 130C with a 1.15SF.
NFPA 499, Table 4.5.2 lists 120C for only one material as the lowest cloud or layer ignition temperature; the next lowest is 140C. So, in general, most TEFC motors will do fine in Class II, Division 2 – but you still need to check.
See Section 500.8(D)(2) for additional information.
Robert B. Alexander, P.E.
"I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)
Answers based on 2014 NEC unless otherwise noted.