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Joined: Oct 2000
Posts: 2,749
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Question from the field:

Quote
We are installing Cat 5E cable to connect an emergency speaker system for Lehman Brothers. They are having us run this four pair conductor around these 40,000
sq. ft floors. They are having us use three pairs for power. This is supposed to adjust the voltage drop, one pair for voice.

I know its low voltage but having these pairs twisted in parallel, just don't sit easy with me.

Am I wrong?

Dan


Joe Tedesco, NEC Consultant
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Joined: Jul 2004
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G
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Cat 5 is wonderful stuff but is not the only wire you should carry on the truck. ;-)


Greg Fretwell
Joined: Dec 2003
Posts: 886
H
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Here we go again. CAT5 the universal cable. [Linked Image]

I can't wait until CAT6 and CAT7 is in use and CAT5 is no longer available. That stuff is going to be so expensive and hard to use that these hacks might even want to use the proper cable. By the way, the proper 4 conductor cable would probably be cheaper than CAT5e.

-Hal

Joined: Oct 2000
Posts: 2,722
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I 2nd the Motion!!! [Linked Image]

Forget the 24AWG UTP Cable idea (the CAT-5e), and get some rolls of 18/4 or 16/4 Cable instead!
Much easier to Terminate with the "Standard" type 4C Cable, as opposed to messing with the tight twists per pair, plus stripping the tiny Conductors of CAT-5e UTP.

You could use the "Basic" type Cable (no shielding, no "frills", untinned, for "All Uses"), or you can get the "Audio Quality", or "Architectural Speaker Cable".
I think Carol Cable sells a 1,000 ft roll of 18/4 "Architectural Speaker Cable" with CMR/CL3R rating, for something like $100.00 - close to the price of a box of CAT-5e.

Make sure to get Cable with the proper Jacket + Insulation needed - like "Riser" or "General Purpose" for Non-Rated areas (or in Conduits), or "Plenum Rated" for Plenum Rated Ceilings/Spaces.

If you do end up using the UTP, run "+" and "-" across each twisted pair, then join ends per Polarity at Terminations.
Example:
<OL TYPE=1>

[*] Pair #1: Blue on White = "+", White on Blue = "-";


[*] Pair #2: Orange on White = "+", White on Orange = "-";


[*] Pair #3: Green on White = "+", White on Green = "-";


[*] And Pair #4 would be the "Voice" pair.
</OL>

Good Luck!!!

BTW, what is the reason for using the UTP Cable on this task? Is there some specification, preference, or any other reason for its use?

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: May 2005
Posts: 177
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Maybe I missed it... but what kind of "power" is on those three pairs, and what are the load(s)?

Voltage drop can add up in a big hurry with multiple loads.

Joined: Dec 2003
Posts: 886
H
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Good question. Sounds like these are powered speakers, each has an integral amplifier hence the power and signal conductors. Not my first choice for a life safety system but it doesn't look like this is well thought out anyway.

I haven't found any UL listed emergency speakers like this (hmmm I wonder what they are using) only paging speakers so I can only go by those specs. Typically that would be 50ma @24vdc per speaker- and that sounds awfully low.

-Hal

[This message has been edited by hbiss (edited 05-20-2005).]

Joined: May 2005
Posts: 177
J
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Hal,

Yes, that 50 mA figure seems light. As an example, the Wheelock SA-H5-B is a powered 5-watt paging speaker that draws 100 mA idle and 450 mA full output at 24 volts.

To answer Joe's original question, there's nothing at all wrong with parallelling the pairs to obtain lower resistance. Those three awg-24 pairs are equivalent to an "awg-19.3" pair. However, I think the current would need to be limited or fused to the level of each conductor individually in case the others are damaged or broken -- a good reason to prefer larger, single conductors recommended by Scott35.

What I'm curious about is the planned wiring topology. If each speaker is being wired home-run to the power supply, then the three pairs might be OK. They exhibit a maximum round-trip resistance of .0185 ohms per foot, so for a cable run of, say, 200 feet, the voltage drop is 1.7 volts at 450 mA, which is a 7% drop.

But, if several speakers are daisy-chained, the drop could become excessive. If we consider 4 speakers at 50, 100, 150 and 200 feet, the voltage drops are 7%, 12%, 16%, and 17% respectively.

The SA-H5-B power input is specified at +/-10%, including the initial tolerance at the power supply terminals, so even a 7% drop is out-of-spec if the power supply is 5% low to start with.

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***BUMP***

Wondering what happened to the Original Poster (topic/question pasted here by Joe Tedesco), and if this thread has made sense (or added more Q's!).

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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