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I have a stud cavity through which I need to run some signal wiring that's sensitive to ingress noise. An industry-standard recommendation is to run the wiring in properly bonded metal conduit (raceway, per the NEC, right?).

There are plenty of 110V metal junction boxes in the same stud cavity. However, I can't mix signal and control wiring in the same junction box (or compartment of the junction box) with 110V wiring. Nor can I put it in the same raceway, right?

Supposing that I run new RMC and metal junction boxes for the signal wiring, how should I bond the boxes (or at least one box if the RMC provides electrical continuity to the other)? Do I use a bonding jumper from the signal wire box to one of the 110V boxes?

bonding jumper

The signal wires are going to be electrically isolated from the boxes (i.e. they pass through brush plates and aren't terminated at the boxes).

  • Can you run a ground wire to the same ground system that the electric service uses? In a house, the phone and cable TV wires are commonly grounded to the electric service either by a clamp on the meter box or directly to the main grounding conductor. – JPhi1618 Mar 27 at 15:14
  • Since you're doing this for noise immunity rather than safety, I recommend you ground the entire run from exactly one point, to avoid making ground loops or having any current flow on your ground. Agreed with @JPhi1618 that you want to ground it to the electrical system, ideally as close to the service panel as possible. – Nate Strickland Mar 27 at 16:04
  • @JPhi1618 The main service ground is a long way away--perhaps 100 ft or more. – watkipet Mar 27 at 22:42
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You probably don't need to go full RMC; cheaper EMT and plain stamped handy-boxes will do. Both types of conduit are acceptable as grounding paths for 200A++ of fault current, your milliamps of signal noise will be handled with ease. I never run ground wires inside EMT.

If you have a section of Conduit isolated from mains grounding, grounding it is simplicity itself. There should be a hole in the back of the box tapped #10-32. You simply put a 10-32 screw in there, shepherd's hook a 12AWG solid pigtail to it, and wirenut that to your grounding wire, any color you like as long as it's green, yellow/green or bare.

Then you exit the wire through a cable clamp or grommet, which fits in a standard knockout.

Then you run the wire in-wall to whichever ground point you deem appropriate. If you'd like guidance on that, look at the NEC rules for retrofitting ground.

  • When I'm running the ground wire to whichever ground point I deem appropriate, does it need to be in some sort of raceway or jacket (i.e. NM with a single conductor)? I ask because all the cable clamps I've seen are meant to be used with NM cable--not just a single conductor. – watkipet Mar 27 at 22:51
  • @watkipet -- grounding wires can be run by themselves as long as they are not subject to physical damage; if you or your inspector insist on a cable though, then an 8/1 Bare Armored Ground cable can be used for this. – ThreePhaseEel Mar 27 at 23:22
  • It's true, cable clamps are not well suited to single wires. Not a lot of good answers. You could wrap it with electrical tape enough to fatten it up. – Harper Mar 27 at 23:23

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