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DDASH 07-07-2008 09:31 PM

outdoor low voltage lighting
 
Qeustion:

I have recently replaced walkway lights. I had to upgrade my transformer to a 300w, although I am only drawing about 120w. My old transformer died and the old lights only used about 60w total.
I have no idea what size cable I am using- it was the existing and everthing seems ok, but I don't want a fire. My guess is it is 16 or 18 guage. It is a 50 ft run.
I know for 300w transformer they recommend a 12 guage wire but as I said, I am only using 120w. Does this matter, or do I need to run new cable?
I would rather not dig it up if I didn't need to.
Thanks!

jbfan 07-08-2008 06:30 AM

My guess is that you have #14 wire now. I have never seen #16, or 18 for lowvoltage outside wire.
If it is infact #14, then I would not change it, unless you add more lights.

micromind 07-08-2008 08:56 PM

If the load is 120 watts, and the voltage is 12, the current will be 10 amps.

The resistance of #14 wire is about 2.6 ohms per thousand feet. Your 50' run is actually 100' (50' out, and 50' back). You'll lose about 2 volts or so. (The reason it's not 2.6 is because the 12 volt lamps are operating at less than 12 volts, so the current will be less.) There's nothing wrong with this, the #14 wire can handle the load, and operating 12 volt lamps at 10 volts will make them a bit dimmer, but will make them last much longer.

If it were #16 wire, you'd be losing about 4 volts. The lamps would be noticeably dimmer, and if they're incandescent they'd be more of a reddish color than white. The #16 wire could still handle the current, but it'd be pushing it a bit.

If it were #18, you'd be losing about 6 volts. The lamps would be barely glowing, and would be reddish-yellow. You'd notice it for sure. The wire would become quite warm in just a few minutes.

If it all looks normal, I'd bet you have #14 wire. If there's a fuse in the transformer, make it 15 amp. That way, the fuse will blow way before the wire can overheat.

Rob


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