Shielding conduits

On 7/27/2009 5:09 PM Aaron Fude spake thus:

Dunno about "intercom Cat 5", but for speaker wires, no, no nearby power wires can cause 60 Hz hum. If some audiophool told you this, you have my permission to smack him upside his head.

Regarding an intercom (I assume you're talking about something actually used as an intercom here, not Cat 5 wiring used for something else), either 1) nearby power lines won't cause much of a problem with hum, or

2) even if they do, it's an intercom, for chrissakes; hum comes with the territory.

You can smack the guy in the head who said 60hz will affect CAT 5 too. It is twisted pair and very tolerant of any kind of outside noise.

Virtually any 60hz hum you hear from anything is caused by ground loops.

In critical places like recording studios there might be issues to a perfect sound recording, keeping wires not touching and more than 6" distance is practiced from someone I know who builds studios. Ground loops cause issues easily heard but wiring issues I have never heard, my problems have been groundings.

Ask your friend who builds recording studios what kind of light dimmers he uses. The last studio I was in used variacs for dimming the lights. Guess why.

TDD

While not always necessary it is generally considered good practice to run signal and power cables in separate wall cavities. That is have a stud between them. This will usually take care of any problems that might develop.

Bill

The speakers will be un-affected.

As for the CAT5e you can always get shileded cat5e (you'll have to use shielded plugs/jacks too with this cable):

Older wired intercoms are always very susceptable to hum, cat5e used for data is not that susceptable anyway, same with cat5e used for phones.

In most cases just run the power away from the low voltage runs, and cross them at 90 degrees, you probably really dont need shielded cat5e but it cant hurt.

Unlearn what you have learned. You cant get away from 60 Hz noise. Cat

5 circuits use balanced differential lines that cancel induced common mode noise, power line noise is such. Use the cable the manufacturer reccomends for hooking it up. If the cable uses twisted pairs dont split the pairs. Follow the directions, I have never seen one hum that was hooked up right unless there was an electronic problem with the unit.

Jimmie

There is a constant barrage of electrical noise in this world! Cell phones, wireless devices, radio/video transmitters, electrical lines, etc.

The thing to do is use shielded speaker wire or shielded Cat 5 wire! Then just ground the shield at one end to prevent ground loops.

Protect the things which are sensitive to outside interference. That is much easier.

More...

On 7/28/2009 7:58 AM Bill spake thus:

Let me stop you right there.

This is pure, unadulterated *bullshit*.

There is *no way* a run of speaker wire can pick up *any* hum that would be audible in the speakers, no matter how long or how close to power conductors.

Leave this with the rest of the audiophool BS (monster cable, gold-plated connectors, etc.).

Looks like you've hit a hot button issue with some of the readers here :) The post actually belongs in comp.dcom.cabling group and would have easily generated 50-100 responses in the better days of Usenet.

Anyways, with all the advice to learn and "unlearn" that you got here I can easily imagine your head must be spinning right now. I would throw another point of view though: by NEC (assuming you are in the USA) you are actually required to keep 6" distance between power and signal and you cannot use the same outlet box for terminating them. "A stud between" rule is actually rather extreme and in many case impractical. So, if you kept the separation to keep your installation legal, you would have eliminated the worst-case scenario for EMI (interference). The separation is actually a fire protection requirement but works for EMI nevertheless.

It's rather amusing to see how many black/white opinions float around here yet no one asked what exactly would the power cable be feeding and what the low voltage cable will be used for, which makes all the difference. A higher load such as an AC power cable coupled to a non-balanced audio input cable will create the hum you would not like. EMI will not create enough current to drive speakers directly, that's true, but if you are amplifying the noise by coupling it onto the INPUT of your audio receiver, you will have issues. Also, an improperly installed phone cable given enough length will pick up some hum. By improperly I mean split pairs as in one wire from blue pair and another from orange, for example.

So, yes, it's probably not as terrible but you should not wrap your CAT5 cable around a power cable either. In any case you'd want to pass inspection should your township decide to check on that permit... So keep them separate.

Cheers.

DA

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Cite that

DA posted for all of us...

and this!

You now you could word that request a little more politely. The actual separation as required by the NEC is two inches.

"800.52 Installation of Communications Wires, Cables, and Equipment. Communications wires and cables from the protector to the equipment or, where no protector is required, communications wires and cables attached to the outside or inside of the building shall comply with

800.52(A) through (E).

(A) Separation from Other Conductors.

(2) Other Applications. Communications wires and cables shall be separated at least 50 mm (2 in.) from conductors of any electric light, power, Class 1, non=96power-limited fire alarm, or medium power network-powered broadband communications circuits. Exception No. 1: Where either (1) all of the conductors of the electric light, power, Class 1, non=96power-limited fire alarm, and medium power network-powered broadband communications circuits are in a raceway or in metal-sheathed, metal-clad, nonmetallic-sheathed, Type AC, or Type UF cables, or (2) all of the conductors of communications circuits are encased in raceway. Exception No. 2: Where the communications wires and cables are permanently separated from the conductors of electric light, power, Class 1, non=96power-limited fire alarm, and medium power network- powered broadband communications circuits by a continuous and firmly fixed nonconductor, such as porcelain tubes or flexible tubing, in addition to the insulation on the wire."

-- Tom Horne

"800.52 Installation of Communications Wires, Cables, and Equipment. Communications wires and cables from the protector to the equipment or, where no protector is required, communications wires and cables attached to the outside or inside of the building shall comply with

800.52(A) through (E). (A) Separation from Other Conductors. (1) In Raceways, Boxes, and Cables. (c) Electric Light, Power, Class 1, Non=96Power-Limited Fire Alarm, and Medium Power Network-Powered Broadband Communications Circuits in Raceways, Compartments, and Boxes. Communications conductors shall not be placed in any raceway, compartment, outlet box, junction box, or similar fitting with conductors of electric light, power, Class 1, non=96 power-limited fire alarm or medium power network-powered broadband communications circuits. Exception No. 1: Where all of the conductors of electric light, power, Class 1, non=96power-limited fire alarm, and medium power network- powered broadband communications circuits are separated from all of the conductors of communications circuits by a barrier. Exception No. 2: Power conductors in outlet boxes, junction boxes, or similar fittings or compartments where such conductors are introduced solely for power supply to communications equipment. The power circuit conductors shall be routed within the enclosure to maintain a minimum of 6 mm (0.25 in.) separation from the communications circuit conductors. Exception No. 3: As permitted by 620.36."

As you can see from the above quoted material there is a prohibition against terminating communications circuits in the same box as power and light conductors. Yes there are exceptions but the basic rule is there.

-- Tom Horne