I only get 9 "hits for vibration in the 2002 NEC, none of which seem to apply.
368.28 (Bussways over 600 volts)
400.7 (Cords)
422.16 (Appliances)
545.13 (Manufactured Buildings)
550.16 (Mobile Homes)
551.30 (RVs)
551.47 (RVs)
552.48 (Park Trailers)
695.12 (Fire Pumps)
I looked in 450 and some raceway articles and could find no requirement to flex in transformers, I have done it with many different methods, pipe, trough, flex, etc.
What I found strange is no requirement to use a flexible method to a motor.
