OK I learned something; the official term is "multiwire branch circuit", I didn't even know this was a thing before today. Makes sense, but go figure it's often done for cost cutting.
See section "E3701.5 Multiwire branch circuits" in this draft Florida code book:
http://ecodes.biz/ecodes_support/fr...idential/PDFs/Chapter 37 - Branch Circuit.pdf
See section "E3701.5 Multiwire branch circuits" in this draft Florida code book:
http://ecodes.biz/ecodes_support/fr...idential/PDFs/Chapter 37 - Branch Circuit.pdf