I couldn’t find a discussion on this so let me ask.
I am putting in a dedicated 20 amp circuit for audio, as many have done before me. My question is, should I use a metal junction box or a plastic one? Other than being stronger, is there any benefit to a metal box? I vaguely remember from somewhere than a metal box helps shield EMI/RFI. Is that correct?
Further, if I use a metal box and an isolated ground duplex receptacle with it, I assume I do not have to ground the metal box. Anything else I should be aware of when using a metal (or plastic) box in this application?
Thanks!
Plastic boxes are fine unless you’re going to use stranded wire in metallic conduit. Then the conduit will be grounded by its connection to the service panel.
I have used romex and plastic boxes with great success.
Plastic box. Carlon blue tubing from the dedicated subpanel to the outlet box. Solid THHN in the Carlon blue. This is the (or at least a very good) way.
What is the difference between using THHN and Romex in the same gauge?
What does the Carlon blue tubing achieve?
Thanks!
Romex is not twisted. Some folks contend that even with AC lines, the parallel nature of a Romex run can introduce a capacitive aspect that may be detrimental to an “ideal” power run. Can anyone tell by listening? I’m not sure, but an alternative is twisted THHN (it has to be twisted a particular way, better done by machine (very few folks have the resources to do this)) - which completely removes that question mark, that’s the way I went. It helped that I do know someone who does audiophile power infrastructure, so he has the means to do the proper twisting.
As to Carlon blue, if you use THHN then by code it is required to be in a conduit. Carlon blue is a non-metallic conduit. Non-metallic conduit to a non-metallic outlet box(es) (all to code) eliminates the additional possibility (note I’m not saying probability) of ground loops, or ground imbalances across multiple dedicated circuits. There is only one ground from the dedicated subpanel (ideal if you have one) tied to the main ground in the external box outside your home. Direct, unbroken, point to point. No more grounding than is absolutely necessary. There are some folks who will prefer to have metal conduit from the subpanel to a metal outlet box. Could be done. More care is needed in doing this for an audiophile setup, IMO, and I’m not sure anything is really gained by it. All I do know is my infrastructure supports my system beautifully and (for whatever this is worth) it sounds dead quiet. Visiting audiophiles have commented as such, if that means anything. 
By all means, use metal boxes. It’s almost no difference in cost. I assume if you have a choice to use plastic, you’re installing it inside the wall. Then you can run flexible metal conduit or MC cable inside the wall. Flexible metal conduits you can pull more THHN wires in it if you decide you want additional circuits in the future. MC’s you cannot as the cable already has a fixed amount of wires inside the cable.