UF-B for Low voltage???


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Old 05-25-09, 06:47 AM
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UF-B for Low voltage???

Hey Guys,

While doing some Landscaping work in my yard, I came across a 30 to 40 foot length of "Abandoned" 14/2 UFB , which was apparently used for some type of "Pond-Pump" at one time. Ive located both ends.

As it turns out, it lies in the perfect path for some LOW Voltage (12V) landscape lights that I was going to use.After checking the cable for "Externally induced Power", and shorts or opens within the cable itself, I "Trial" connected everything and they work just fine.

Any code issues with this application, or anything I'm Missing????
Thanx..
 
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Old 05-25-09, 07:40 AM
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I use some 12-3 UFB for low voltage. I haven't noticed any problems. Fine stranded wire is often recommended for DC, something about better skin effect I think, but this works fine for me.

Keep in mind the greater voltage drop at 12-18VDC which increases your amperage requirements. Find a table or calculate your voltage drop to avoid overloading the wire or the transformer. I guess you could also do it the easy way, which is to use a meter, but I would be cautious about running a transformer unloaded.

If available switch to the slightly higher voltage lead on the transformer. For example mine has nominal 12 or 16V I think. 12V is appropriate for the short distance but 16 is better for longer distances.
 
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Old 05-25-09, 09:55 AM
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As stated, low voltage wiring is usually stranded for ease of installation and such. UF-B is overkill, but considering you have it in place already, there's no problem with using it.

You'll have the same problems with voltage drop whether you run 100' of 12 ga stranded "low voltage" wire or 100' of 12ga UF-B, so you want to keep the runs as short as possible.
 
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Old 05-25-09, 09:55 AM
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There is no skin effect with DC. Skin effect is purely an AC phenomenon and even then only starts to become significant for wire this size well into tens of kHz.

Don't know about code issues. At 4 amps, voltage drop in 50 feet of 14-2 will be about 1.2 V, or 10% of 12 V, so that could be a problem if you need more than a few amps.

Try it and see. You might want to measure the voltage across the farthest load unless the total wattage is very low.
 
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Old 05-25-09, 11:00 AM
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Some landscaping luminaires which I used the 14-2 UF it is fine for short distance but when you get longer distance you will have to bump up the conductor size and 12 volts luminaires are very senstive with voltage drop.

If you have mulit tap on secondary side of your low voltage transfomer you can go up one step higher instead of 12v then go up 13 volts some case 14 volts but keep in your mind any luminare closer to the transfomer will be brighter than at the end so that something you have to watch out for it.

Otherwise I know you may say to me " nuts " but for first half or thrid of the lay out run with 10-2 UF that useally take care pretty good.

Merci,Marc
 
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Old 05-25-09, 01:37 PM
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Originally Posted by ByteWrangler View Post
There is no skin effect with DC. Skin effect is purely an AC phenomenon and even then only starts to become significant for wire this size well into tens of kHz.
Thanks. Do you have any idea why stranded wire seems to predominate with DC? Obviously it's easier to work with but is there any electrical reason?
 
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Old 05-25-09, 03:43 PM
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Originally Posted by ArgMeMatey View Post
Thanks. Do you have any idea why stranded wire seems to predominate with DC? Obviously it's easier to work with but is there any electrical reason?
I don't think there are any electrical reasons. Probably, as you note, it's because stranded is easier to handle.
 
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Old 05-25-09, 06:01 PM
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Easier to handle, and it works great with those little self-piercing connectors.
 
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Old 05-25-09, 09:00 PM
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As mentioned, there is no skin effect with DC. However, you are not using DC, you are probably using AC. I have not seen or installed any low voltage lighting systems that used DC. Your xfmr simply transforms 120V down to 24 or 12 volts. I have never seen them include a rectifier. 14 ga is pretty small. I never use anything smaller than 10 ga. Its stranded for ease of use reasons in these applications. Stranded or solid doesn't make any difference in any applications, electrically speaking, that any homeowner is going to come across. They will carry the same amount of amperage.
 
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Old 05-25-09, 09:11 PM
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Originally Posted by ArgMeMatey View Post
Keep in mind the greater voltage drop at 12-18VDC which increases your amperage requirements. Find a table or calculate your voltage drop to avoid overloading the wire or the transformer. I guess you could also do it the easy way, which is to use a meter, but I would be cautious about running a transformer unloaded.
Greater voltage drop doesn't increase amperage. Its the opposite. In this case, voltage is directly proportional to amperage. The more you drop on the wire, the less voltage that the load, in this case, lights, is going to see, consequently the light will get dimmer and use less electricity, not more. The wire simply acts as a load itself, which is very undesireable. Its wasted as heat. The resistance of the wire is fixed. E=IR.

There is nothing wrong with running one of these xfmrs unloaded. Won't hurt anything.
 
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Old 05-26-09, 07:19 AM
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Sorry for all the misleading info. I think I'm getting dumber as I get older.

DC? Did I say DC? I did! Oops.

Regarding the voltage/amperage, sorry, I got the theory wrong. I was thinking of the increased amount of output consumed as heat.

Regarding running unloaded, I thought that was in the Intermatic instructions, but it's safe to assume I was instead thinking about running an unloaded low-voltage transformer with a magnetic dimmer, which can cause a transformer to burn out. But I don't think landscape lighting is usually, or should be, run on a dimmer.
 
 

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