You'll get no sensible answers until you tell us what the wiring is for,
voltage(s) on it, and whether the wire type you are using is rated for
duct applications. (ie plenum cable.)
Local codes may say you can't even do it no matter what cable type you use.
If you run the wire in EMT and place the EMT inside the duct is the wire
considered to be within the duct? I have several places where wires,
some #4 AWG pass through return air ducts. They run in schedule 40 PVC
through the duct.
You definitely can't run PVC or any plastic wiring or conduit through an air
duct. I see your point regarding EMT or any metal conduit, but I believe
it's still illegal unless it's affecting something within the duct. It would
sure help if the NEC used clear wording though.
That is even a more clear situation 300.22(C) exception says where
stud space is used as an air return the wiring can only cross
perpendicular to the studs, not along the long axis.
Exception: This section shall not apply to the joist or stud spaces of
dwelling units where the wiring passes through such spaces
perpendicular to the long dimension of such spaces.
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