Bradford "BJ" James wrote on Thu, 28 October 2010 13:46 |
Hal, Charlie et all, I was being a bit generous with my distances. Distance from pole to pole is 100-120'. I was adding a bit extra since the existing unused insulators the electricians will be using are near the tops of the poles, and the cables will have to come down about 20' to their mounting location. So, not all the distances quoted are suspended. The poles are concrete and steel and the company running the cable is experienced. I'll make sure to ask when I quote them what cable to use, that they are able to safely install it. Also, looks like my max run will be 22 units. So that brings my total load to 660 watts assuming I use 30 watt taps on all units.
But, back to my question: is 12g UTP going to be sufficient for this straight run?
Pertinent info: Run "A"- Max potential distance 2500', 660 watt load, driven off one channel of a 2x 1200 watt amp(69.v) Run "B" will be less right now, but potentially the same as "A" in the near future. Thanks BJ
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While not very elegant this sounds like one I would try to do with an excel spread sheet.
Look up the ohms per foot for 12ga (.00187 ohm) or whatever size wire you want to use, then multiply that times the span between speakers (120') to get the wire R per leg (.224 ohm). Then calculate the effective impedance of a 30W/70V tap (hint = 163 ohms) . So the far last leg is delivering 163/163.224 of the voltage to the speaker as is present at the beginning of the run, for a - ,012 dB drop in voltage. Now this is additive, cumulative, etc.
Ok the next to the last leg has this 163.224 ohm load sitting in parallel with the 163 ohm drop so loss will be -.022 dB, loss at the last speaker is now -.033 dB
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But I am not a human spread sheet so fast forward to the back of the envelope.. 30 x 163 ohm loads is 5.4 ohms.. or approximately -0.35 dB drop in that first span.
I'm guessing (hoping) the average drop is half that so .17 db x 30, or around -5 dB at the end (that makes your last 30 watt speaker sound more like 15W or less.
I would really load up a spread sheet and do the individual span math because you will probably find the wire gauge matters more at the the start of the long run than at the end, and stuff like that..
Then if you tweak the the taps for more output at the far end, the spread sheet could show you have it impacts back up stream.
The sharp pencil part comes in with cost of thousands of feet of whatever gauge vs. pushing the system to higher voltage for lower losses.
JR