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[Jason Lavoie]

I know that mixing 70V and mic signals is bad news, but I've seen many installations where line level to 70V doesn't seem to have enough gain to cause a problem.
is there any good rule on this? or a max distance?

it seems silly to ask an electrician to run me two seaparate conduits to the same place if I don't have to (especially if one of them is only going to have one shielded pair in it)

Jason

[Duane Massey]

Jason, it's a risk, but not a very large one. I've had to run 70v, mic, line, and video in the same pipe on several intalls, and so far have had no issues, including one that was 350' n a very, very tight conduit. Had no choice, that what was already there in a concrete structure, and it was understood that there was no promise made that it would wrok, but (whew!) it did.

As a matter of practice, when we lay out our own conduit runs, we separate everything as much as possible.

[Brad Weber]

The general rule is to not do this and to avoid it if at all possible.  You may be able to get away with it but what is your backup plan if there is a problem?  If you are really that worried about the conduit, consider running the 70V line(s) as plenum cable, that is quite common.

I don't know your particular application, but also consider if the cabling may ever change.  I often oversize conduit primarily in order to accommodate future system changes and expansion, a fairly inexpensive investment that can easily pay for itself.