Something I picked up years ago at a previous employer is the "Rule of 6":
When running data lines in parallel with power for a distance of at least six feet, the data should be at least six inches away from the power.
Of course, this has to do with preventing the magnetic field in the power line from creating current (and signal) in the nearby data lines; we don't want that. I know this isn't the real specification, but as a basic rule of thumb it's served me well for many years now.
However, I can't find a source for this anywhere any more. At all. I have a bigger project coming up where I'd like to be more exact. Specifically, if I'm gonna be able to convince the higher ups to allow any space all — even sharing conduit — I may need something more concrete to point to than an old tradesman's rule of thumb.
What are the actual standards-based guidelines in this area, and where can I read the source material? Or can someone at least corroborate they've used the same rule?