# Metal Conduit for EMI Shielding?



## sga2 (Nov 14, 2009)

I am about to start a DIY home theater in my unfinished basement and am concerned about picking up EMI in the low voltage cables (speakers, subwoofers, projector video, etc. )There are several electrical branch feeders serving the kitchen (directly above), as well as the house service main feeder, in the ceiling space of the HT room. Also - and I am not sure how much of concern this is - the microwave, refrigerator, and various small kitchen appliances are just a few feet away (kitchen is directly above the HT room). 

I am planning the wire routing carefully to maintain at least 24" (or more) between power and low-V cables and, when necessary, crossing at 90-degrees. However, it will be impossible to do this for every wire, and I don’t really know if EMI from the kitchen appliances is a concern or what is a “safe” distance to those appliances. I am going to route hard conduit from each low voltage box (speaker terminals, subwoofer terminals, CATV, HDMI, Cat6, etc.) to the A/V equipment rack in case I ever need to pull new wire for any reason. I have been considering using metal conduit (EMT or IMT) with metal boxes for all of the low-V wiring and bonding those to the grounding rebar (located in the basement) to provide EMI shielding, but I am not sure whether that is worth the effort and expense. Otherwise, I will use PVC conduit and standard low-V boxes. Wiring will be:
1. Speakers = Belden 5000UP (12/2 twisted, ~4 twist/ft)
2. Subwoofer, CATV and Component Video = Belden 1694a (RG6)
3. Cat6 = ??? (not purchased yet)
4. HDMI = ??? (not purchased yet)

Now to my specific questions:

1. How well should metal conduit/boxes bonded to grounding rebar work as EMI shield for these systems? Is this worth the effort or should I just use PVC?

2. Are any of these cables decidedly NOT worth running in metal conduit due the product’s inherent shielding? Since I have not purchased Cat6 or HDMI cables yet, feel free to give recommendations.

3. How much of a concern is EMI from kitchen appliances? How far away would be "safe" for the low-V cables?

Thanks to everyone for your input!


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## Anthony (Oct 5, 2006)

Well, metal EMT would be a very good shield. Up to 3/4" is pretty easy to work with (bends, fittings, etc) and isn't too expensive. The problem is that 3/4" is not very big and only a few RG6 feeds could make it through there and forget about hdmi (head is too big).

But first off, don't even worry about shielding speaker signals. They are higher voltage and much less prone to picking up interference. I wouldn't run them right next to the power lines for a long distance, but 6" to 1' separation and crossing only when necessary should be sufficient.

HDMI usually has a foil or snakeskin (metal) braid inside the sheath, so that provides some shielding. Same is true for some RG6 coax. It will be marked as shielded or quad-shield. These also reject a lot of noise, but again, don't run them right next to the power lines for a long time and if you need to cross them, go out of your way to cross the power line at a 90 degree angle. This will minimize/avoid any interference.

If you do get the EMT, make sure it is bonded to a ground somewhere. In most installs (power) it is tied to the panel or the first box in the run has the bare ground wire. In your case, you just need a little drain for any picked up voltage to disspipate through. a bare wire from the EMT to the nearest pipe or metal junction box should suffice. We're not talking high voltages here.

Also, I wouldn't worry about the noise in the kitchen appliances provided you take care of the separation distances, crossing angles, and that your kitchen and theater are on separate circuits.

I hope this helps.


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## sga2 (Nov 14, 2009)

Anthony - Thanks for the advice. You put my mind, and wallet, at ease.


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## Wayne A. Pflughaupt (Apr 13, 2006)

I would add that EMI is seldom a problem as long a good cable is used – meaning, having a robust shield. As far as basic audio and video signal cable is concerned, make sure you use good-quality installation-stock from a respected manufacture like Belden, Canare or Mogami. Most installation-grade cable has a foil shield, which is said to be 95% protective against EMI (maybe as high as 99%, I forget).

Regards,
Wayne


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## sga2 (Nov 14, 2009)

Thanks. Any thoughts on the cables I've already purchased (but not yet installed)?

1. Speakers = Belden 5000UP (12/2 twisted, ~4 twist/ft, but not shielded)

2. Subwoofer, CATV and Component Video = Belden 1694a (RG6)

Any recommendations on HDMI cable? I was eyeballing BJC Series-1 Belden Bonded-Pair for about 30ft run from AVR to projector.

Regards,
sga2


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## hjones4841 (Jan 21, 2009)

Monoprice has great deals on HDMI cables. Their cables work just as well as "the high price spread." I would advise against their "net" cables - they have a woven net looking jacket. Look nice but quite stiff to work with. I have a 15 foot "net" cable to my projector - went with that one to get the 24 AWG wire. Now they have a flat (really oval) HDMI cable that works great and is easier to bend.


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## Wayne A. Pflughaupt (Apr 13, 2006)

sga2 said:


> Thanks. Any thoughts on the cables I've already purchased (but not yet installed)?
> 
> 1. Speakers = Belden 5000UP (12/2 twisted, ~4 twist/ft, but not shielded)


No shielded is needed for speaker cable. Any installation-grade cable is fine (assuming it’s going in-wall).




> 2. Subwoofer, CATV and Component Video = Belden 1694a (RG6)


Bluejeans Cables uses Belden 1694 – that’s good enough for me.




> Any recommendations on HDMI cable? I was eyeballing BJC Series-1 Belden Bonded-Pair for about 30ft run from AVR to projector.


I don’t know much about HDMI cables, but if Anthony says they’re shielded with either a braided or foil shield, that’s the main thing you need to know. I guess the question that remains then is “how good is the shield?” (Especially if it’s a braid.) That can be difficult to determine without destroying the cable. Personally I’d feel confident with anything Bluejeans offers.

You might want to read the “Choosing a good cable” section of my Making Your Own Cables DIY article.

Regards,
Wayne


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## Anthony (Oct 5, 2006)

I've only hacked the one hdmi cable up and I remember undoing that tight braid shield. It may have had that waxy metal foil as well (it's been a while).

As an installer, I've had to run several right next to power cords (behind the rack) or skirt near power (in wall) and not had any problems, even with 35 foot runs.


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## Speedskater (Dec 23, 2007)

Sorry to be late, I had been away from a good internet connection for a while.
In this white paper "The Truth" they measure power line interference on page 16.
They have a neat test board setup.
See:

http://www.exactpower.com/elite/wpapers.aspx


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