Where in the code book does it say you can't hard pipe to a motor.
I agree with what you say.I don't think it does, but the other side of the coin is, just because the code does not expressly prohibit something, doesn't automatically make it good practice.
The majority of industrial sites I work at would prohibit such a practice and I would think twice about what the perceived benefit would be.
The other hint is 430.223 "Raceway Connection to Motors". They make a point of allowing a flexible connection from a raceway to a motor. Yes they just allow it, not require it.
At any rate, even if the motor is rigidly mounted I'd consider at least just using MC. Makes it much easier to replace, and all motors vibrate a little.
I don't find anything faster or easier about installing a flexible whip on a pipe run as opposed to landing that same pipe directly at the device. Takes extra labor to make up the whip, and often extra time to get the conductors through it.I think we really use it because it is much quicker and easier than trying to hard pipe the motor.
If it's really that tough to pull te cables through 6 feet of sealtite and put 2 connectors on, then I'd consider that the value of MC cable in such a situation is really underrated, and makes the whole question a non-issue.
In my area they want the motor terminal box opening on the bottom side of the terminal box. Hard pipe would almost always require an additional conduit fitting or two, and an union (most all of the motor terminal boxes that I see have a threaded opening). Conduit labor is always greater than flex labor. The conduit fittings and union will cost more than the LFMC and two connectors.I don't find anything faster or easier about installing a flexible whip on a pipe run as opposed to landing that same pipe directly at the device. Takes extra labor to make up the whip, and often extra time to get the conductors through it.
13,000 horsepower, 4" rigid:I have always used flex, but either way, your looking at at locknut holding it together