Only recently have I begun doing A/C work. Heck, I even took an A/C course.
NEC clearly prohibits running the control wires together with the power leads. BUT...
The practice here seems to be:
If residential, run the control wires with the a/c line set. At the condenser, the control wires loop through the air and enter the condenser through a factory-provided bushing. Run this way, the wires are exposed to the much higher ambient of the line set.
If commercial, the control wires are run together with the power wires- even for 480 v. units.
As I see it, there are two issues here: insulation and induction.
Others have already addressed the insulation issue.
Most a/c systems seem to require the control wires to close a contactor, or otherwise deliver a few amps, so induction is not a problem. This may change as fancier electronic (plc-type) controls are used. PLC inputs simply read a voltage, so a voltage induced by the control wires proximity to power wires could cause control problems.
It doesn't help that the control (or t-stat) wires are often run by the HVAC guys.
The picture is also complicated in that some designs don't have control wires to the condenser at all.
Many times, the control wires and power wires come from differing directions, so they are run together only through the flex from the disconnect. This mitigates any risk.
Whatever the code may say, I've yet to see a condenser with two pieces of flex running to it.
I'm at a loss, and would like to learn if "code" contradicts "practice" elsewhere.
