View Full Version : Figuring Cable Loss
04-29-2011, 06:36 PM
I am adding and rearranging some lights on a job where the previous contractor (I do not know who) used 12-2 UF cable!
It is too extensive (and expensive) to replace it all.
Do I figure voltage drop on this type cable just as I would on 12-2 low voltage cable?
Thanks / Regards,
04-30-2011, 02:19 PM
I would just dig up a small section and see if you can find manufacturer information for te wire. They'll be able to give you all the figures.
04-30-2011, 05:58 PM
12-2 direct underground bury wire is fine and rated for low voltage systems as long as waterproof connections are made. It is not the best choice, Low voltage cable is, but I have seen enough of it that electricans must use it quite frequently. my experience is the voltage drop on it is a bit more than what you will find on 12-2 LV tables, but not much more so. the condition of the wire and corrosion will play a larger factor than the difference between solid and LV stranded.
FXL.com has good tables on LV lighting on their website over in the learning center.
04-30-2011, 08:49 PM
Cast makes a great device for testing voltage. It's the CTESTMR16 MR16 voltage tester and it allows you to check the voltage at the lamp and under load. The Voltage Drop tables will most likely be wrong and even if they are not much off, I would rather know exactly what is being delivered to the lamp.
Something similar happened to me not long after I went to work for Greg Yale. I was still new to this whole LV Lighting business and having trouble running a homerun through a couple of outdoor walls and under a sidewalk. My schedule was tight and the house electrician offered to do it for me - a really nice guy! I told him I'd leave the LV cable and be back on Monday.
I got back Monday morning and my very green crew spliced in the spider feeds as I set the fixtures. We fired up the transformer and the LV volt readings went nuts. I was expecting to land on the 13 or 14V tap and I had to go to 16 or 17 volt tap to make it work and that seemed to be unstable. It took me a few minutes to get up and look at was going on. I found the 12/2 UF homerun at the hub.
And then I found the LV cable where I had left it.
I pulled the LV cable with the UF to replace the homerun. Fortunately the trench was not tightly packed and it pulled easily.
I don't doubt you'll be OK with the UF still in place, just don't trust the tables and get good solid readings on your meter while under load. And seal both ends of the wire with Scotchcote to make certain you won't wick water into the UF cable. Any corrosion will really sink you eventually.
05-02-2011, 08:45 AM
Green / David / Tom,
Thanks for your replies!
Tom, I have the CTESTMR16 voltage tester and use it on every job.
I am a "stickler" for proper design / build. That's why I was asking about the UF cable in case there was something I needed to take into account on the front end design of the project.
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