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#1
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NEC question: low-voltage wiring crossing 120v wiring.
I am running network cables -- shielded, if that makes a difference --
above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? Perce |
#2
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NEC question: low-voltage wiring crossing 120v wiring.
On Thu, 29 Sep 2011 15:19:42 -0400, "Percival P. Cassidy"
wrote: I am running network cables -- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? Perce no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 |
#3
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NEC question: low-voltage wiring crossing 120v wiring.
On Sep 29, 5:30*pm, wrote:
On Thu, 29 Sep 2011 15:19:42 -0400, "Percival P. Cassidy" wrote: I am running network cables *-- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? *no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 What's the reasoning behind plenum rating a computer cable? Is there some fire-reasoning behind it or what? What's the difference between rated and non-rated cable? R |
#4
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NEC question: low-voltage wiring crossing 120v wiring.
On Thu, 29 Sep 2011 17:07:32 -0700 (PDT), RicodJour
wrote: On Sep 29, 5:30Â*pm, wrote: On Thu, 29 Sep 2011 15:19:42 -0400, "Percival P. Cassidy" wrote: I am running network cables Â*-- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? Â*no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 What's the reasoning behind plenum rating a computer cable? Is there some fire-reasoning behind it or what? What's the difference between rated and non-rated cable? R Fire rating. Plenum cable will not support combustion when flame is removed - will not flame on it's own - and different smoke. Non-plenum rated is not allowed in an air return. |
#5
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NEC question: low-voltage wiring crossing 120v wiring.
On 9/29/2011 2:19 PM, Percival P. Cassidy wrote:
I am running network cables -- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? Perce the only thing they taught us to avoid is running parallel to high current cables. But if you're using shielded cable, even that shouldn't matter. And i don't imagine any cable in a house to be high current except the range line and the line to the a/c unit. Lay away! -- Steve Barker remove the "not" from my address to email |
#6
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NEC question: low-voltage wiring crossing 120v wiring.
On Sep 29, 8:07*pm, RicodJour wrote:
On Sep 29, 5:30*pm, wrote: On Thu, 29 Sep 2011 15:19:42 -0400, "Percival P. Cassidy" wrote: I am running network cables *-- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? *no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 What's the reasoning behind plenum rating a computer cable? *Is there some fire-reasoning behind it or what? *What's the difference between rated and non-rated cable? R- Hide quoted text - - Show quoted text - If it's a plenum that means something is pumping air through it to the rest of the building. Can you think of a better way to speed combustion and spread smoke he asked rhetorically? |
#7
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NEC question: low-voltage wiring crossing 120v wiring.
"RicodJour" wrote in message ... On Sep 29, 5:30 pm, wrote: On Thu, 29 Sep 2011 15:19:42 -0400, "Percival P. Cassidy" wrote: I am running network cables -- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 [What's the reasoning behind plenum rating a computer cable? Is there some fire-reasoning behind it or what? What's the difference between rated and non-rated cable?] Some wire (non plenum rated), when it burns in a fire, will put off toxic smoke. You wouldn't want toxic smoke to be sent through all the air ducts! |
#8
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NEC question: low-voltage wiring crossing 120v wiring.
On Sep 29, 5:30*pm, wrote:
On Thu, 29 Sep 2011 15:19:42 -0400, "Percival P. Cassidy" wrote: I am running network cables *-- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? Perce *no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 If there is romex there already, then let's hope it isn't plenum space. Only in commercial buildings would I expect space above suspended ceilings to be used as air plenum (return air is drawn through it on its way back to the hvac system), and commercial building code anywhere in N.America requires power conductors to be in conduit or armored cable, and data cabling to be in either conduit (in which case, it might not need to be plenum rated) or a cable tray, the better to prevent it from collapsing into the space during a fire, blocking egress and creating a hazard for firefighters. At least that's how it was explained to me. Single-family residential code doesn't say anything about low-voltage cabling except that you can't put it in the same boxes as 120V. Having said that, I think laying it onto the suspended ceiling, as I understand the OP is proposing, is going to be a pain whenever someone wants to lift a panel. I'd find some clips to tack it to the joists. I wouldn't expect simple romex runs to be an interference problem for data communications. Fluorescent ballasts, light dimmers, fan speed controls and any kind of motor or switching power supply, maybe. Chip C Toronto |
#9
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NEC question: low-voltage wiring crossing 120v wiring.
On Sep 29, 10:43*pm, wrote:
On Thu, 29 Sep 2011 17:07:32 -0700 (PDT), RicodJour wrote: On Sep 29, 5:30*pm, wrote: On Thu, 29 Sep 2011 15:19:42 -0400, "Percival P. Cassidy" wrote: I am running network cables *-- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? *no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 What's the reasoning behind plenum rating a computer cable? *Is there some fire-reasoning behind it or what? *What's the difference between rated and non-rated cable? R Fire rating. Plenum cable will not support combustion when flame is removed - will not flame on it's own - and different smoke. Non-plenum rated is not allowed in an air return. Yes, but my question was whether the OP's above-ceiling space is actually a plenum. I had assumed the question was residential, a basement and the space was not used for airflow. Granted, that's reading a lot into it, but perhaps they were bad assumptions. Here are some interesting (but boring!) videos of the different cable ratings burning. http://www.l-com.com/multimedia/vide....aspx?ID=13100 Quite a difference between plain vanilla PVC and rated cable. R |
#10
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NEC question: low-voltage wiring crossing 120v wiring.
On 09/30/11 11:23 am, RicodJour wrote:
I am running network cables -- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 What's the reasoning behind plenum rating a computer cable? Is there some fire-reasoning behind it or what? What's the difference between rated and non-rated cable? Fire rating. Plenum cable will not support combustion when flame is removed - will not flame on it's own - and different smoke. Non-plenum rated is not allowed in an air return. Yes, but my question was whether the OP's above-ceiling space is actually a plenum. I had assumed the question was residential, a basement and the space was not used for airflow. Granted, that's reading a lot into it, but perhaps they were bad assumptions. Here are some interesting (but boring!) videos of the different cable ratings burning. http://www.l-com.com/multimedia/vide....aspx?ID=13100 Quite a difference between plain vanilla PVC and rated cable. You are correct: this is a residential setting, with a suspended ceiling installed in the basement. Perce |
#11
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NEC question: low-voltage wiring crossing 120v wiring.
responding to
http://www.homeownershub.com/mainten...ng-664640-.htm DA wrote: Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? For fire safety NEC wants you to stay 6+ inches away from the power cables and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) but might prevent some kinking and cinching. However, if you can return the cable, I would strongly advise to do so and get a smaller diameter UTP - you'll be able to pull more cables through the same size hole and that will come handy during the install. In a home made of essentially wooden sticks (often times "engineered wooden sticks") you don't want to make holes bigger than absolutely necessary. It may be a moot point in a wooden home, but I would always locate low voltage cables below high voltage power. If cables are the cause of the fire, it would always be the power ones, not low-voltage, and the fire would want to go up. The less burning material above it the better. But again, would probably make little difference if everything around is wood. Good luck! ------------------------------------- /\_/\ ((@v@)) NIGHT ()::) OWL VV-VV |
#12
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NEC question: low-voltage wiring crossing 120v wiring.
On 09/30/11 12:58 pm, DA wrote:
Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? My question was about the NEC, which is concerned with fire safety. For fire safety NEC wants you to stay 6+ inches away from the power cables and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) but might prevent some kinking and cinching. The shield is to eliminate interference to my amateur radio station. This was a problem with unshielded cables. However, if you can return the cable, I would strongly advise to do so and get a smaller diameter UTP - you'll be able to pull more cables through the same size hole and that will come handy during the install. In a home made of essentially wooden sticks (often times "engineered wooden sticks") you don't want to make holes bigger than absolutely necessary. No holes have to be drilled: the network cables pass over the tops of the partition walls. Perce |
#13
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NEC question: low-voltage wiring crossing 120v wiring.
On Fri, 30 Sep 2011 11:36:06 -0400, "Percival P. Cassidy"
wrote: On 09/30/11 11:23 am, RicodJour wrote: I am running network cables -- shielded, if that makes a difference -- above suspended ceiling where there is already 120V Romex wiring in places. This 120V wiring is stapled to the joists at intervals. Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? -- more precisely, should what is now Romex be redone with individual conductors in conduit? no issue even with cat5 or cat5e non sheilded - as long as your space above the ceiling is not an air plenum - and then you just need plenum rated cat5 What's the reasoning behind plenum rating a computer cable? Is there some fire-reasoning behind it or what? What's the difference between rated and non-rated cable? Fire rating. Plenum cable will not support combustion when flame is removed - will not flame on it's own - and different smoke. Non-plenum rated is not allowed in an air return. Yes, but my question was whether the OP's above-ceiling space is actually a plenum. I had assumed the question was residential, a basement and the space was not used for airflow. Granted, that's reading a lot into it, but perhaps they were bad assumptions. Here are some interesting (but boring!) videos of the different cable ratings burning. http://www.l-com.com/multimedia/vide....aspx?ID=13100 Quite a difference between plain vanilla PVC and rated cable. You are correct: this is a residential setting, with a suspended ceiling installed in the basement. Perce But we still don't know how the cold air returns work. If the cold air return just dumps into the ceiling, and the ceiling is open to the furnace room - and the furnace drawr return air from the furnace room - it's a plenum. Ditto if the cold air return had an opening into the basement, which is now semi-blocked by the dropped ceiling, so it is drawing return air from above the ceiling. Neither is a proper approch - but I've seen stranger!!! |
#14
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NEC question: low-voltage wiring crossing 120v wiring.
On Sep 30, 12:58*pm, DA wrote:
responding tohttp://www.homeownershub.com/maintenance/NEC-question-low-voltage-wir... DA wrote: Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? For fire safety NEC wants you to stay 6+ inches away from the power cables You have a cite for that? In addition to not believing it, it's hard to see what difference 6 inches is going to make. I regularly see low voltage cables of all kinds stapled near Romex. and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) Says who? A Farraday cage is a Farraday cage whether it's grounded or not. |
#15
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NEC question: low-voltage wiring crossing 120v wiring.
On Sat, 1 Oct 2011 07:42:27 -0700 (PDT), "
wrote: On Sep 30, 12:58Â*pm, DA wrote: responding tohttp://www.homeownershub.com/maintenance/NEC-question-low-voltage-wir... DA wrote: Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? For fire safety NEC wants you to stay 6+ inches away from the power cables You have a cite for that? In addition to not believing it, it's hard to see what difference 6 inches is going to make. I regularly see low voltage cables of all kinds stapled near Romex. and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) Says who? A Farraday cage is a Farraday cage whether it's grounded or not. My experience has been an ungrounded sheild just becomes an antennaa |
#16
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NEC question: low-voltage wiring crossing 120v wiring.
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#17
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NEC question: low-voltage wiring crossing 120v wiring.
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#18
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NEC question: low-voltage wiring crossing 120v wiring.
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#19
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NEC question: low-voltage wiring crossing 120v wiring.
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#21
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NEC question: low-voltage wiring crossing 120v wiring.
On Sat, 01 Oct 2011 22:05:39 -0400, wrote:
On Sat, 01 Oct 2011 20:32:11 -0500, " wrote: On Sat, 01 Oct 2011 21:14:12 -0400, wrote: On Sat, 01 Oct 2011 19:26:46 -0500, " wrote: On Sat, 01 Oct 2011 19:59:33 -0400, wrote: On Sat, 01 Oct 2011 17:21:18 -0500, " wrote: When I was in the cabling biz at IBM I set un an experiment with CAT-3 trying to break an Ethermet or a Token Ring LAN doing every urban legend bad thing anyone could think of (loops of cable over fluorescent ballasts, running next to 480v 1600a feeders, taped to the raceway, telephone in the same cable, exceeding the 300' rule, etc) Basically I couldn't break it. 10-baseT, 4/16MB T/R only? The original installation was 16mb TR but we also tried 10/100 Ethernet and it ran clean at 100 mb doing big file transfers and looking at the logs. For the "over length" test I hooked the kludge we had up to a new 300' spool so we had that plus the other hundred foot baseband cable we were playing with and whatever was in the rack, cables etc. I'm somewhat surprised 100BaseT worked over CAT-3. So was everyone but then there were a lot of people who said you would never see copper running at a gigabyte. Now it is old technology. bit? yes sorry That's sorta cheating, though. It's no one wire/pair. it is still 500 a pair. 1000BaseT uses four pairs, so it's 250Mb per. Three bits are encoded per pair, per symbol, so the symbol rate is 62.5MHz (MBaud). The whole thing got started when someone said we were having problems because the router (in a rack) was backed up to the equipment room wall where they had that 1600a service. The router was bad. Hate it when that happens. I hooked up the spool, just for playing with TDR to get a little more time down the wire but I had to try it. ;-) TDRs are great tools. Too bad they're so rare (I suppose the people who know how to use one are even more rare). We were doing TDR on a Tektronics 485. What were you using as the pulse generator? I used HP 140s (w/TDR plugins) in college and 7904s (7S10/7S11s) in IBM. There have been many times I wished I had one around. You just use that 18" cable nobody seems to know why the have and couple the trigger out on the side to the" in" with a BNC "T" and bounce that signal down the line. You sync on the fall of the trigger pulse and look for the echo. It really works pretty well but you need a fast scope since you are looking at nanoseconds. You ain't doing it with a 453 unless you have some real long wire. Ah, it seems they're using the trigger out as the pulse generator. Neat, but I'm surprised it's fast enough. That rise time is critical. Even the antique HP TDR pulsers were something like 25pS. I think a guy in Raleigh figured that out. It was part of the 7800 remote support training. I really only did it once to fix something but I did find the offending drywall screw in a coax on a 100' run in 3 tries. (found the screw heads that were close with a magnet and backed them out with a screwdriver) Third one was the charm. Once I took the screw out it worked so well they left the coax in the wall. Slick. ;-) |
#22
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NEC question: low-voltage wiring crossing 120v wiring.
On Oct 1, 11:56*am, wrote:
On Sat, 1 Oct 2011 07:42:27 -0700 (PDT), " wrote: On Sep 30, 12:58*pm, DA wrote: responding tohttp://www.homeownershub.com/maintenance/NEC-question-low-voltage-wir... DA wrote: Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? For fire safety NEC wants you to stay 6+ inches away from the power cables You have a cite for that? *In addition to not believing it, it's hard to see what difference 6 inches is going to make. I regularly see low voltage cables of all kinds stapled near Romex. and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) Says who? *A Farraday cage is a Farraday cage whether it's grounded or not. *My experience has been an ungrounded sheild just becomes an antennaa - Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - A metal shield is just that. It shields whatever is inside it from an electric or varying magnetic fields outside it. That is basic physics and requires no ground. So, the shield itself could be an antenna, but the conductor within is shielded. |
#23
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NEC question: low-voltage wiring crossing 120v wiring.
On Sun, 2 Oct 2011 05:47:01 -0700 (PDT), "
wrote: On Oct 1, 11:56*am, wrote: On Sat, 1 Oct 2011 07:42:27 -0700 (PDT), " wrote: On Sep 30, 12:58*pm, DA wrote: responding tohttp://www.homeownershub.com/maintenance/NEC-question-low-voltage-wir... DA wrote: Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? For fire safety NEC wants you to stay 6+ inches away from the power cables You have a cite for that? *In addition to not believing it, it's hard to see what difference 6 inches is going to make. I regularly see low voltage cables of all kinds stapled near Romex. and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) Says who? *A Farraday cage is a Farraday cage whether it's grounded or not. *My experience has been an ungrounded sheild just becomes an antennaa - Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - A metal shield is just that. It shields whatever is inside it from an electric or varying magnetic fields outside it. That is basic physics and requires no ground. So, the shield itself could be an antenna, but the conductor within is shielded. Not really true. Whatever (common mode) is inside can couple to the shield, making it the antenna. This stuff is symmetrical, so the same happens outside-in. The key is that the twists make the common mode cancel on the differential signal, within limits. The shield keeps in any common-mode radiation but it must be grounded to do so. The shield does *nothing* to magnetic fields. |
#24
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NEC question: low-voltage wiring crossing 120v wiring.
On Sun, 2 Oct 2011 05:47:01 -0700 (PDT), "
wrote: On Oct 1, 11:56Â*am, wrote: On Sat, 1 Oct 2011 07:42:27 -0700 (PDT), " wrote: On Sep 30, 12:58Â*pm, DA wrote: responding tohttp://www.homeownershub.com/maintenance/NEC-question-low-voltage-wir... DA wrote: Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? For fire safety NEC wants you to stay 6+ inches away from the power cables You have a cite for that? Â*In addition to not believing it, it's hard to see what difference 6 inches is going to make. I regularly see low voltage cables of all kinds stapled near Romex. and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) Says who? Â*A Farraday cage is a Farraday cage whether it's grounded or not. Â*My experience has been an ungrounded sheild just becomes an antennaa - Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - A metal shield is just that. It shields whatever is inside it from an electric or varying magnetic fields outside it. That is basic physics and requires no ground. So, the shield itself could be an antenna, but the conductor within is shielded. In the case where the digital data in the cable is interfering with Amature Radio signals, all an ungrounded sheild will do is collect and radiate that signal - in otherwords it will be a capacitively coupled (or possibly even inductively coupled) radiator. With one end grounded, it effectivel traps and stops the signal - keeping it from interfering with the radio. Grounding both ends can cause all kinds of other problems - but apparently there ARE situations where double-end grounding can be called for |
#25
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NEC question: low-voltage wiring crossing 120v wiring.
" wrote:
I'm somewhat surprised 100BaseT worked over CAT-3. When we moved into this house in 1996, I pulled CAT-3 through it, thinking 10mb is plenty. I am now running gigabit over it just fine. The longest run is maybe 25 feet, so that is a help. But it works. -- Doug |
#26
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NEC question: low-voltage wiring crossing 120v wiring.
On Oct 2, 12:14*pm, "
wrote: On Sun, 2 Oct 2011 05:47:01 -0700 (PDT), " wrote: On Oct 1, 11:56*am, wrote: On Sat, 1 Oct 2011 07:42:27 -0700 (PDT), " wrote: On Sep 30, 12:58*pm, DA wrote: responding tohttp://www.homeownershub.com/maintenance/NEC-question-low-voltage-wir... DA wrote: Is it OK for the network cables to lie on the ceiling support members, and thus beneath the Romex? Or should the network cables be above the Romex? Or should one or both be in conduit? Are you concerned with EMI (electromagnetic interference) or fire safety? For fire safety NEC wants you to stay 6+ inches away from the power cables You have a cite for that? *In addition to not believing it, it's hard to see what difference 6 inches is going to make. I regularly see low voltage cables of all kinds stapled near Romex. and that should also be far enough for EMI to be negligible. That shield on the network cables is absolutely unnecessary in a residential home, but since you already go it, do not ground it on either end. I presume you won't be able to ground it correctly at the outlet end anyhow, so just let it float. It'll be useless as an EMI shield (which you don't need anyway) Says who? *A Farraday cage is a Farraday cage whether it's grounded or not. *My experience has been an ungrounded sheild just becomes an antennaa - Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - A metal shield is just that. *It shields whatever is inside it from an electric or varying magnetic fields outside it. That is basic physics and requires no ground. *So, the shield itself could be an antenna, but the conductor within is shielded. Not really true. *Whatever (common mode) is inside can couple to the shield, making it the antenna. * It most certainly is true. Go Google "Faraday Cage" or look up Maxwell's Equations. This stuff is symmetrical, so the same happens outside-in. * Yes, it's symmetrical, which is why surrounding an antenna with a Faraday cage would render it ineffective. The key is that the twists make the common mode cancel on the differential signal, within limits. * That's true but unrelated to shielding. The shield keeps in any common-mode radiation but it must be grounded to do so. Following that logic, if run a microwave oven on an extension cord without a ground wire, it will spew microwaves all over the kitchen. Of course it doesn't, because it's a Faraday cage, which requires no ground. The shield does *nothing* to magnetic fields.- Hide quoted text - That is correct with regard to STATIC magnetic fields. It most certainly does shield varying magnetic fields. |
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