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 Hexamexapex 06-30-2012 05:03 PM

Phone wires with line voltage

Recently a friend of mine told me that phone wires can be installed in the same bored hole as line voltage. His reasoning was that they are both "signal" wires so they can't interfere with eachother. I immediately wanted to call bull**** but decided to ignore the statement until I knew for sure what the correct reasoning is. I have always been told that the line voltage current can interfere with the low voltage signal ability resulting in static noise etc. So my first question: Will line voltage disrupt the low voltage and why? I also understand that the low voltage wire insulation can catch fire if it comes in contact with 120v line voltage. So suitable wire insulation comes into play here. So if anyone can add a bit more clarity to this situation I would greatly appreciate it. I would like to educate my buddy a little bit because he is shamefully talking out his ass and doesn't realize it...ha!

Anyway, thanks for any and all help!

Marcus

 AllanJ 06-30-2012 08:27 PM

When an AC branch circuit line in use with a reasonable current flow is juxtaposed for many feet with a telephone line or other low level audio signal line, phantom voltage induced in the latter may be significnt relative to the audio (or digital) signal causing audible hum or digital corruption respectively.

Some cancellation of the magnetic fields producing phantom voltage occurs because hot and neutral for the AC circuit are juxtaposed in the same Romex cable. Some cancellation of the induced phantom voltage occurs because the positive and negative, or hot and neutral, phone wires are juxtaposed and usually twisted together in their cable. But the cancellation is not perfect.

 Hexamexapex 06-30-2012 08:42 PM

Quote:
 Originally Posted by AllanJ (Post 955050) When an AC branch circuit line in use with a reasonable current flow is juxtaposed for many feet with a telephone line or other low level audio signal line, phantom voltage induced in the latter may be significnt relative to the audio (or digital) signal causing audible hum or digital corruption respectively. Some cancellation of the magnetic fields producing phantom voltage occurs because hot and neutral for the AC circuit are juxtaposed in the same Romex cable. Some cancellation of the induced phantom voltage occurs because the positive and negative, or hot and neutral, phone wires are juxtaposed and usually twisted together in their cable. But the cancellation is not perfect.
That was just the type of answer I was looking for. Thank you. I am currently a novice when it comes to electrical theory but am learning more each day. I thought induction and magnetic fields would come into play here, but of course how and to what extent I had no idea. This helps me better understand the term "twisted pair" for comm lines. So what is the difference between audio wave signals and digital signals? I know a little bit about pulse width modulation for digital sigs but not much more. So 120v line can be considered high voltage "signal"?

 AllanJ 06-30-2012 09:46 PM

A simple digital signal suggests a square wave analog signal, although the lengths of the pulses and spaces in between vary. There is just one pulse height and one base level for the spaces in between.

More complex digital signals may suggest stairsteps, with a base level and several pulse heights.

Yes, AC power can be thought of as a high level signal, it is represented by a sine wave.

 jbfan 06-30-2012 10:36 PM

Also, the phone line would not catch fire if it touch the romex.
If ti does, you had other issues anyway.

 Evstarr 07-01-2012 12:19 AM

To expand somewhat. An analog or POTS telephone line is unlikely to be affected by any induced current from a 120v household line. Computer data cables are pretty resistant at 10 and 100 base T speeds as well unless the cable length is excessive (300 feet or so)

A much bigger issue is networked computers fed off of different panels.
In an ideal world, all networked pcs should be fed off the same phase of an electrical service as the clock references the 60 cycle ac to help regulate communication. Another issue often run into is flaky behavior caused by differing voltages to ground among networked pcs.

 bubbler 07-01-2012 07:09 AM

Quote:
 Originally Posted by Evstarr (Post 955211) To expand somewhat. An analog or POTS telephone line is unlikely to be affected by any induced current from a 120v household line
Depends on the distance and the type of cable.

At my mother's back-to-back townhouse condo development, built in the 70s, all the phone wires are quad-cable (untwisted). The phone and electrical were all run in the same chase through the center of the building. She is about 20' from the demarc. The hum on her phone line was unbelievable and she was only the second unit, I can't imagine how bad it was at the 5th or 6th unit down the row of homes.

cat3 with some twists might have helped, but of course all the cables are stapled...

Fortunately for her technology has improved, and she has cable-provided phone service which is clear as a bell since the units were all wired for cable much later and it is run nowhere near the power cables.

 Hexamexapex 07-01-2012 09:39 AM

Quote:
 Originally Posted by jbfan (Post 955154) Also, the phone line would not catch fire if it touch the romex. If ti does, you had other issues anyway.
My thinking there was if the Romex were to become damaged and line wire were to become exposed and make contact with the phone wire sheath. Are we at risk for fire then?

 ddawg16 07-01-2012 09:39 AM

Most of you hit on the correct answer...I'll give you a bit more....

First off...there is not cancellation of any fields related to the neutral wire. In fact, you can place just the neutral wire next to a phone line and still get induced voltage. The induced voltage to another wire is a more of a function of current.

We have a thing called the "Left Hand Rule"....take your left hand...ball it up and point your thumb....if the current is flowing the direction of your thumb, a magnetic field will be produced the direction of your fingers.
So...if that wire carring current is next to a phone line....regardless of the voltage (neutral or hot), current will be induced on the phone line. The amount is just a simple function of how far the phone line parallels the AC line.

Key point here "Parallel". If the phone line crosses the AC line at an angle, the amount of induced voltage is a lot less. If it crosses at 90 deg...no induced.

If you open up a typical CAT5 cable, you will see all the wires twisted. If look even closer, you will notice that they are almost crossing at 90 deg to each other. This in effect cancels out any induced voltage and current.

If you can compare a Cat5, Cat5E and Cat6, you will see a difference in the twist. Each one has a progressivly tighter twist. A tighter twist means it takes more wire to reach a given distance....cost more....the Cat6 gets really close to a 90 deg angle.....

Cat5 or 6 makes really good phone cable and can be ran next AC wiring without worry of picking up AC hum.

One other issue with running phone line next to AC....if someone runs a nail through a wall and into one of the AC wires...it could also hit the phone cable and you have the danger of putting AC on the phone wire....

So....if y

 Hexamexapex 07-01-2012 09:46 AM

Quote:
 Originally Posted by bubbler (Post 955277) Depends on the distance and the type of cable. At my mother's back-to-back townhouse condo development, built in the 70s, all the phone wires are quad-cable (untwisted). The phone and electrical were all run in the same chase through the center of the building. She is about 20' from the demarc. The hum on her phone line was unbelievable and she was only the second unit, I can't imagine how bad it was at the 5th or 6th unit down the row of homes. cat3 with some twists might have helped, but of course all the cables are stapled... Fortunately for her technology has improved, and she has cable-provided phone service which is clear as a bell since the units were all wired for cable much later and it is run nowhere near the power cables.
So it would be safe to say that these days the wire makeup "twisted" helps to prevent any excess noise from nearby 120v lines. Although, to meet NEC code requirements and for safety related reasons it is best to keep high and low voltage separate.

 k_buz 07-01-2012 09:50 AM

Quote:
 to meet NEC code requirements
There are no restrictions on running them both thru the same bored hole in studs.

 Hexamexapex 07-01-2012 09:53 AM

Quote:
 Originally Posted by ddawg16 (Post 955360) Most of you hit on the correct answer...I'll give you a bit more.... First off...there is not cancellation of any fields related to the neutral wire. In fact, you can place just the neutral wire next to a phone line and still get induced voltage. The induced voltage to another wire is a more of a function of current. We have a thing called the "Left Hand Rule"....take your left hand...ball it up and point your thumb....if the current is flowing the direction of your thumb, a magnetic field will be produced the direction of your fingers. So...if that wire carring current is next to a phone line....regardless of the voltage (neutral or hot), current will be induced on the phone line. The amount is just a simple function of how far the phone line parallels the AC line. Key point here "Parallel". If the phone line crosses the AC line at an angle, the amount of induced voltage is a lot less. If it crosses at 90 deg...no induced. If you open up a typical CAT5 cable, you will see all the wires twisted. If look even closer, you will notice that they are almost crossing at 90 deg to each other. This in effect cancels out any induced voltage and current. If you can compare a Cat5, Cat5E and Cat6, you will see a difference in the twist. Each one has a progressivly tighter twist. A tighter twist means it takes more wire to reach a given distance....cost more....the Cat6 gets really close to a 90 deg angle..... Cat5 or 6 makes really good phone cable and can be ran next AC wiring without worry of picking up AC hum. One other issue with running phone line next to AC....if someone runs a nail through a wall and into one of the AC wires...it could also hit the phone cable and you have the danger of putting AC on the phone wire.... So....if y
So the key here would be to not run the low voltage parallel to ac line over long runs. It also appears that the twisted wires really prevent excess noise. This makes me wonder how induced current can be prevented by twisting wires like the cat5. Might need to study up on electromagnetic fields.

 Hexamexapex 07-01-2012 09:56 AM

Quote:
 Originally Posted by k_buz (Post 955371) There are no restrictions on running them both thru the same bored hole in studs. You can also run line voltage and low voltage in the same pipe as long as the conductors have the same insulation type/rating.
So even though the cat5 insulation and line insulation are rated differently you can still run them through the same bored hole? No fire risk in case of damage from nail or other accident?

 k_buz 07-01-2012 10:15 AM

Quote:
 Originally Posted by Hexamexapex (Post 955378) So even though the cat5 insulation and line insulation are rated differently you can still run them through the same bored hole? No fire risk in case of damage from nail or other accident?
Yes to the first question.

No more risk than a nail thru a single NM cable for the second question.

 Hexamexapex 07-01-2012 10:22 AM

Quote:
 Originally Posted by k_buz (Post 955403) Yes to the first question. No more risk than a nail thru a single NM cable for the second question.
I guess than the only real reason you would keep them separate would be to prevent interference. And even then, in this day and age, with the cat5 wire design, it might not be entirely necessary since interference would be next to nill in most applications?

I am going to beat this dead horse some more! Ha

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