I no longer have a COF, so I'm not inclined to give wiring advice or get too deep into NEC interpretation. My earlier comments were about generic wiring of circuits and the rationale around a dedicated "line" for the EQ Rack, verse adding a receptacle to an existing circuit.

Having said that, in years past (not too long ago either), 12/3 was used throughout a structure for 110V/120V. It was quicker and saved effort with respect to running wire between the panel and the first Jbox, or just drilling holes in studs and joists for the romex. For example, I would run a length of 12/3 to the first convenient box in a bedroom where I planned to install 4 duplex receptacles. Per code at the time, 8 receptacles was the max I could feed from a 15A circuit. Well, the room also needed a couple of receptacles for lights and/or a ceiling fan. I would use one of the line conductors for the receptacles and the other line conductor for the lights, running 12/2 to the rest of the room box's. That left one N and one G for both the lighting and power circuits back to the panel.

This method of pulling/installing romex was common in the industry. It can cause issues though. Some light fixtures send weird amounts of current back to ground through the neutral, and sometimes, this is enough to create "noise" on the circuit used for the receptacles. I've seen enough stray current at times to trip a GFCI receptacle before. It can also be a PITA to troubleshoot.....

Nowadays, you just don't wire a house or other building using 12/3 for 110V/120V. 12/2 is used. This enables the wireman to do whatever he/she needs to do with the neutral wire back at the panel. Sometimes you only need to land it to the bus bar, or you might run it to an AFCI or GFCI breaker.

So back to the original question.....is a dedicated line needed? Yes, I think it is. And no, I would not consider adding a receptacle to an originally installed circuit a good idea, - unless that circuit is home ran (both L and N), and it has capacity for the added receptacle.

And I always use 12G wire (copper solid strand). I don't screw around with 14G. Not many electricians do. 12G is good for a 20 amp circuit, 14G isn't, well usually isn't.

For new construction, AFCI circuits are the norm if GFCI is not needed. A lot of electricians run combo circuits. The electricians I work with tell me it the next NEC will most likely call for all residential circuits to be AFCI, with exception of those that require GFCI.

For the receptacle, I have a preference for hospital-grade. They are stout as hell, near impossible to break when landing wires, and will outlast the home. More expensive, but not really that much more when you look at the total installed cost of the system.