Ground lighting problems


Ok, I'm replacing the ground lighting in my yard and have a situation I'm having difficulty troubleshooting. Here goes:
I have approximately 30 fixtures, each with 20-watt bulbs (little quartz jobbies). I have a new 600 watt, 12volt AC transformer. I've run mostly new wire (some old stretches where it goes under concrete walks, etc. The fixtures are soldered in, not attached with the cheap clips. The total run is about 100 feet.
My previous system used a 300-watt transformer, but had only half as many lights and most of them were broken from years of being bumped by the lawnmower, tripped over by the kids, etc. At the end, only about half were working.
The power feed goes underground in conduit to my basement, where it's on a dedicated GFI circuit.
When I got everything in place and switched it on for the first time, the bulbs were bright near the source, and got dimmer, the farther away from the transformer you got. Towards the end of the line, the bulbs didn't light at all.
If I test the voltage at output side of the transformer, I get 11.4v. If I take out all of the bulbs, and test the voltage at the end of the run, I get 11.3 / 11.4 v. If I test any fixture (with no bulbs in anywhere), I get 11.4v.
For the next test, I put a voltmeter on fixture #5 (I'll number them sequentially starting with the one nearest the transformer as #1) and removed all of the bulbs. At the start, I showed 11.4v. If I put a bulb in fixture #1, my meter on fixture #5 drops to 11.0v. If I take the bulb out of fixture #1 and put it in fixture #2, my meter on fixture #5 drops to 10.6v. If I put the bulb in fixture #3, the meter drops to 9.8v. If I put the bulb in fixture #4, the meter drops to 9.5v. I've checked and rechecked all connections and voltage at the transformer. I've used different bulbs without changing the result.
For the next test, I leave the meter on fixture #5 and put bulbs sequentially in the first 4 fixtures. With a bulb in just fixture #1, I get 11.0v on the meter on fixture 5. If I put bulbs in both fixtures #1 and #2, the meter drops to 10.3v. Bulbs in 1,2 & 3 = meter drops to 8.8v. Bulbs in 1,2,3 & 4, meter drops to 7.0v. You can see the bulbs are dimmer as you go from 1 to 4.
I also measured the voltage at the input side of the transformer as I placed bulbs in the fixtures. I start at 118.0 volts, and drop to 117.9 as I insert the first bulb - then it recovers to 118.0. If I put bulbs in fixtures 1 and 2, then the input voltage drops to between 117.8 and 117.9, but recovers after a few seconds to 118.0.
I've taken apart one of the fixtures to check for shorts and find none. I've checked an un-powered (and temporarily unhooked) fixture with an ohmmeter with and without a bulb inserted. Everything looks normal. I've tried temporarily hooking everything to the old 300w transformer and using only the first 5 fixtures, but I get the same result!?!
I think I've eliminated everything BUT the feed line from the house. None of the tests has tripped the GFI breaker, but maybe it's bad anyway?
Is there something I'm missing? These are all wired in parallel, so there shouldn't be ANY drop in voltage when you insert the bulbs. I'd really appreciate your help.
Thanks!
Any idea what is going on here?
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Yep! Ohms law really works. You have to take in to account the resistance of your cable.

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Dave is "right on" as the system draws more current you get much higher voltage drop :(
voltage drop = R*I^2
so double the load & get 4x the drop, triple the load & get 9x the drop.....the wire resistance will each up your system voltage pretty quick as the load increase.
IMO low voltage lighting (esp run in a simple series layout) sucks on performance but it really is easy to install.
Parallel systems are somewhat better, resistance is the enemy.
Maybe you can get compatible lower voltage bulbs to use near the end of the run?
cheers Bob
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Another useful trick I have heard of, but never personally used is to make it a ring system, so you make a loop with both ends at the transformer (make sure you get the ends of the same wire on the same connection at the transformer, Let me know if this works I would be interested. BTW it also means if some doofus decides to dig a hole and cuts the wire, it will still work,
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wrote:

Another useful trick I have heard of, but never personally used is to make it a ring system, so you make a loop with both ends at the transformer (make sure you get the ends of the same wire on the same connection at the transformer, Let me know if this works I would be interested. BTW it also means if some doofus decides to dig a hole and cuts the wire, it will still work,
I've done a few with the loop. It does help a little. Also larger low voltage wire is a very good idea. Hopefully you have used at least #12 wire, but #10 would be better. By my calculations 600 watts at 12 volts equals 50 amps. You need some heavy gauge wire for that (#6) unless you ran separate lines to several groups of lights. I would also suggest that you not max out your transformer. Eliminate one or two lights so that the transformer is not operating a full load.
You could also relocate your transformer so that it is in the middle of the circuit and not at one end, but this will involve running line voltage to the new location.
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The other posters are right on. With low voltage lighting and only one transformer and lots of lights in a line , the bulbs near the end will be dimmer or maybe so dim you think they are are out. You probably need to string some more wire so that not more than about 4 or 5 bulbs are on each line. It will also depend on the size of the wire used to reach each bulb. If you run some wire about 1/2 inch in diameter not counting the insulation then you may get buy with only one long run. The smaller the wire, the more voltage drop as you add lights.
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Excellent - Thanks everyone. I'll post results when we decide what to do next.

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Tuner wrote:

Does your new 600W transformer have 2 (or more) secondary outputs? If so then each output is only good for 300W for a total of 600W. If there are 3 outputs then only 200W per line etc. My 500W Malibu LV transformer has 2 outputs so I can only use 250W each output.
If yours has multiple outputs then you will need to run a new wire from the 2nd output for your new lights. Each output has to be treated as if it were an independent transformer however you should try to balance the loads between outputs as much as possible. The packaging and instructions were vague on these details.
Note the GFCI is not a load sensing device, that is the function of the circuit breaker in the panel. In your case I would not expect the GFCI or breaker to trip.
You didn't say what gauge wire you were using. Each foot of wire adds resistance. Use the largest dia. wire you can find to cut down on line loss. I am pretty sure LV wire comes in 12ga size. If you have long runs it may help to use larger wire or double up the wire. You could even use some 10ga house wire type UF.
Kevin
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