Low Voltage Question

Old 11-17-03, 07:32 PM
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Low Voltage Question

Here's one a little off the wall.

I'm installing some digital volume controls in a few rooms. The control hubs terminate in a structured media center, and require 12Volts, 75ma each. I will have 4-5 hubs. A cat5 control cable goes to the volume control in each room.

The control has a "mute on startup feature" wherein if the hub is powered on and off with the stereo amp, I can enable the hub to startup in a muted position. To do this, the manufacturer suggests using the amplifiers switched outlet (200Watt max on my amp--so I'm OK here).

Plug a transformer into the switched outlet and then run that 12volt line to the hubs in the structured media center. Then when the receiver is turned on, the hubs are powered up and boot up in a muted mode. So far so good.

Here's my concern. My 12volt run will probably be about 30-40 feet long. I plan on splicing the transformer leads to 14 gauge speaker wire to make the run. What kind of voltage drop can I expect over a run like that and is it OK to use an extra run of speaker wire like this. My thought is that I want to use the relatively large gauge wire to keep the voltage drop to a minimum so that the hubs are still getting as close to 12V as possible.

What do you think?

Thanks in advance.

Old 11-17-03, 10:58 PM
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Voltage drop will be 0.007V. Used an online calculator at http://mdmetric.com/tech/wire-gaugecalc.html to get the resistance per unit length and the used ohm's law to calculate the voltage drop. Basically you should be fine. You are correct in saying that the thicker the wire, the lower the resistance per unit length and the lower the voltage drop. 14AWG wire is used for much higher currents - that's why 75mA gives such a low voltage drop.

If you want to do the whole calculation manually

Ohms's law - V= I*R
Resistance of wire = (R*L)/A. Do not know offhand the relationship between A and AWG and also the R for copper. Need to look these up.

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