NEC section 250.102(E) (old 250-77) says that you may run the EGC on the outside of flex for sections not exceeding 6'. There are special flex fittings for this that have a ground wire lug on the exterior of the fitting. You're not permitted to wrap the EGC around the flex, as this would create an inductor or choke. It is most normal to just zip tie the EGC to the outside of the flex in a straight line. This is done sometimes when an industrial machine is wired in flex. The machines can sometimes vibrate the flex fitting loose, and the ground on the outside gives you a "backup" ground. Of course, running a seperate EGC inside the flex would do the same thing. Most electricians in this day and age prefer to not rely on any raceway system as the sole means of equipment bonding.
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