Originally Posted by michael_d
The only real reason to run a dedicated circuit for the audio system all the way back to the panel - is to ensure the neutral is not shared with another circuit. Sometimes you will get some noise on a shared neutral.
I think your limiting your thinking on why you might need to run another line .. I stated two more above.

A fourth could be that your system constantly trips an arc fault breaker and you need to go back to the panels and to use a ground fault instead. Arc faults look for signatures of arcing. Some devices (like inverter circuits) can trip them because the signature looks similar. Note: a ground fault on the other hand detects a difference in current going out the line and coming back on the neutral. If not the same it must be going somewhere else ... like though you body.

Originally Posted by michael_d
Modern wiring (and also per last few NEC revs) requires ground fault protection on the majority of residential circuits at the breaker. And that requires home ran neutral wires. The previous practice of wiring a house would use 12/3 romax that would have two line wires, and one neutral. Now it's pretty much all 12/2.
Also think you swapped 12/2 and 12/3 (note: wire size (AWG) over the number of wires excluding ground ... I'll just call it X/2 or X/3). For leads coming from the panel (residential panel usually only has two phases) the extra wire is used for the 2nd phase to give you 220V... at least in the states.

Up to recently one could run 220V with only X/2 by using the neutral for the 2nd phase. But there was or there's going to be a code change to require X/3 for two phases. I'm not sure why but may have to do with arc fault breakers. Be interested in the answer if you know it ... Anyway I think this may be what you're referring to.

Most audio equipment runs on a single phase needing only X/2 wiring. This has not changed.

Ok that was long winded ... but I should put in a disclaimer ...
The above is my understanding and since I'm not an electrician, I do not keep up with all changes in the code ... So if I stated anything wrong I hope someone, in the know, will correct me