I had someone tell me that using more than one box extension is a code violation. This came from what I regard as a reliable source. I've looked and don't see that it's prohibited in NEC. In fact, the UL White Book (QCIT) listing indicates that one or more box extensions may be used. Can anyone point out what might be a code concern on using more than one box extension?

It is a permitted practice.

Unforgiven:
Yes, as Ryan pointed out, it's a regular practice. The only code violation that I can envision would be the length of the conductors may be to short when installing multiple extension rings.

What I'm saying is, if the conductors in the original box were 4" extended past the front of the box, and you put two (2) 1-1/2" extensions on, then it could be a violation.

John