wiring for 12 volt halogen cable lights
I’ve installed some modern cable lighting in my
living room. Parallel bare wires carry the 12 volts.
The halogen fixtures can be put anywhere along the
track. Looks great. Rather than put the transformers
in the attic, I wired from the cable suspenders
up through the ceiling to some 12-gauge romex that
I strung down inside the wall to a plug-in (that is
a german style, so there’s no confusion about it
being 110, 220 or whatever). My problem is that I
can’t find nice “appliance” wire to go between the
transformer and the plug-in: when I tried typical appliance
wire, it got hot (going from 110 ac to 12 dc steps up
the amps, which needs less resistance I guess). Is
there some kind of cable I can use to solve this?
Transformer is 250 watt, and none of the other wiring
gets warm (though I did need to use heavier-duty
junction boxes than the usual 110 wiring).
Currently, I just use more romex, but it isn’t flexible
and looks ugly. Any ideas?
Replies
To make sure I have this straight, the transformer is not hidden away but is sitting out someplace where it's in plain view, yes? It has a regular power cord that plugs into a regular 120 volt outlet, and the cable you need will go from the twelve volt output of the transformer to the special plug you put in the wall, that feeds power to the lights.
Twelve volts at 250 watts works out to about 21 amps. According to Table 400-5(A) in the 1999 National Electric Code book for flexible cords, you need a cord with 12 gauge wires (I don't know for sure if this chart strictly applies in your case since you're only running twelve volts through it, but 12 gauge should be plenty in any case.) I assume it only needs to be a couple feet long.
I'd think a piece of type SO or SJO cable would do the trick. This is the thick, round, black cables like you'd see used for a heavy duty extension cord or a large power tool. It isn't necessarily beautiful stuff, but I'd think it would be fairly inconspicuous, and it looks like what it is - a power cord. You should be able to pick some up at your local big box store.
Yes, you got it right -- the transformer is in plain view
(easy to check on, etc) and the distance is as you say,
a couple of feet or so. I can try the cable you suggest and
see if that does the trick. BTW, your calculation agrees
with mine, which is why I went 12-gauge.
Edited 1/22/2005 2:28 pm ET by therm
A 250W transformer puts out a little over 20A at 12V. You need a minimum of #12 wire, just as you would for 20A at 120V.
You can find #12 black rubber covered cord if you shop around, or you could do something like take some #12 stranded "automotive" wires and run them through a piece of plastic tubing. The plastic tubing is usually available in a cloudy white, or you could go for the "industrial" look with brightly colored wires inside a clear tube.
If you'd prefer yellow, orange, or blue, buy a #12 heavy-duty extension cord of the appropriate color and cut it up.