Replies

1. 
   fdampier5 | Nov 10, 2003 04:48am | #1

   Yes you can do it!    However you will find that if you go to 10 gauge the bulbs will burn brighter and last much longer 

     12 volts DC is 12 volts..  so the auto lights will work.   As to how weather proof they are and how long they will hold up that's another matter..

2. 
   EricGunnerson | Nov 10, 2003 05:02am | #2

   A little electronics math should help.

   12 gauge copper wire has a resistance of 0.00187 ohms per foot. A light 150 feet away would involve 300 feet of wire, or 0.561 ohms of resistance. Now, on to the bulbs. We need to know the wattages.

   There are lots of different wattages. Here's a little table showing the resistance of each bulb based on wattage (using Wattage = voltage * current, and then figuring resistance using V = IR).

   7W   1.7 ohms
   12W 1    ohms
   24W 0.5 ohms

   Now, we can calculate how much effect the cable has on the lamp brightness by adding in the total resistance. I'll give you the results as a percentage of total current (noting that brightness is non-linear, so 70% of current produces less than 70% of light output).

   7W  75%
   12W 65%
   24W 47%

   So, a 7W light at the end of 150 feet of cable will only see 75% as much current.

   Bumping up the size of the wire to 10 gauge doesn't really help that much - going to 10 gauge only gives you 82% on the 7W bulb, and less of an increase on larger wattages.

   The guideline is based on what the manufacturer thinks is an acceptable brightness. There's no hazard in using more wire than recommended, as long as the brightness is adequate to you. More wire gives a higher resistance and loads the transformer less, so there's no problem there.

   Hooking up three wires in parallel would reduce the voltage drop considerably, and give you more even brightness across all the fixtures. That would be the best plan.

   You should have no problem using automotive lights in the system, assuming you stick with acceptable wattages, and remember that the voltage drop is more important as the wattage goes up. Incandescent lights have enough thermal mass that they don't care about AC vs DC, given the same voltage.

   Hope that helps.

3. 
   FastEddie1 | Nov 10, 2003 05:39am | #3

   Some voltage drop will make the bulbs last longer, at the expense of dimmer bulbs.  Likewise, if you load the transformer to less thasn 50%  you run the risk of significantly shortening the bulb life.

   Do it right, or do it twice.

1. 
   dIrishInMe | Nov 10, 2003 08:09am | #5

   Thanks a bunch folks.  I think you all pretty much covered it.  The situation is better than I thought.

   I actually had complete basis electronics training 30 years ago, so I could understand all of what was said.   The formulas are way back in the fog somewhere, but how to use them and the concepts are still in in the realm of recall possibilities.

   It probably doesn't matter at this point, but to answer the question that Egunnerson asked, the wattage of the bulbs are eight 18w, one 20w, one 7w and then there are the four automotive lights which I'm estimating at 10w each, based on their brightness.   The automotive light fixtures are actually truck marker lights, so I'm sure they are weatherproof enough.

   Egunnerson, thanks a bunch for doing the math.  And I'm assuming you looked up the resistance of the wire too.  Thanks for giving me your time. 

   I guess my breaking it into 3 strings, effectively, each string will have roughly 1/3 the resistance compared to if it were all in one string, so the load on the transformer will be the same, but there should be less voltage drop along the length of each string since the transformer loading will be somewhat in parallel.  Of coarse, things are never that simplistic with the different lamp wattages, and each string will have a different number of lights.

   I'm assuming that since I'm not putting on a full 250w of lamp load, that will allow me to waste a bit of power on the extra wire too.

   sincerely,Matt

4. 
   CapnMac | Nov 10, 2003 07:09am | #4

   For the 7w bulbs and path or area lighting, the longer lengths are ok (as long as you can tolerate the previously mentioned intensity drop).  However, over 150', you are better served to move the transformer.  MIddle of the run works better all around.  But, that's for path & area lighting.  For spotlights (tree/feature lighting, for example), the wire lengths are significant.  (I've used equal length runs of wire to get tree uplights to match.)

   One thing I have learned from installing this gear is to lay it out first, unburied.  Ok, easy for me, I've got some rolls of "try" cable (chewed up in spots, too).  The cable is inexpensive compared to your time, and testing the end effect is definitely worth it.  You can also "see" what the finsihed product will look like--and change it if need be, all before burying it & finishing up.

   Automotive fixtures "ought" to work, but they are going to be more sensitive to line drop and amperage requirements.

   Occupational hazard of my occupation not being around (sorry Bubba)
1. 
   dIrishInMe | Nov 10, 2003 08:21am | #6

   Good tip on laying it out first.  I had read that in a book, but forgot it.  Actually, the only hazard to doing it that way is that it *may* take the entire project longer to get completed...  That was the way I did the front, and I think it took me 6 months to get around to burrying it since I had a working system on the first day - I did do some tweeking to the layout though.  Probably won't be an issue on this (backyard project) though since the dogs will probably chew it up if I don't get it burried in a hurry!  Your right though.  I need to verify I like the light placment, brichtness, etc, before "commiting", and you can't tell until you try it out at night.

   Matt