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Old 05-08-2010, 08:51 AM   #1
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Low Voltage Transformer


I am trying to install a low voltage lighting system. I already have an older 200 watt transformer. It has 4 terminals (2 Load, 2 COM), and a High/Lo switch. I can't find any manual or instructions on how to connect this, because all the new transformers only have three terminals. How do I connect my wire to this? Do I need to use all 4 terminals? What is the High/Lo switch for? Am I supposed to run one wire from two of the terminals, and come back and connect the other end of the wire to the other terminals?

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Old 05-08-2010, 09:29 AM   #2
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Low Voltage Transformer


If you can find the part number of the transformer you can google it and probably find the specs for it online. If it is the one I am thinking it is the 2 load terminals are common to each other as are the 2 common.

I don't want to confuse you but some companies make an enclosure that houses 2 transformers (usually 2 300-watt). So to be sure it is a single transformer and not 2 transformers in 1 case, use a continuity tester to read between the 2 load terminals, there should not be any resistance, but if there is no reading at all it may be 2 separate transformers sharing a common enclosure. I have never seen a 200-watt transformer that was 2 separate 100-watt transformers in 1 but that doesn't mean they don't exist so check! The hi/lo switch is basically a dimming system for 2 levels of brightness.

As far as wiring this, if you are running a short distance, like under 50' you can just tie 1 side of the wire to either load terminal and the other to either common terminal.

For a short distance like that I recommend you use at least 12-2 LV cable but personally I use 10-2 for almost all runs. If you are using a smaller gauge like 14-2 that comes with some light sets you might want to loop back to the transformer and feed it in both directions. That means starting at the transformer and ending at the transformer.

*** This does not apply if your transformer is 2 transformers in 1 enclosure ***

If you have found your transformer is 1 200-watt transformer the following is how to loop your cable. You must observe polarity to do this; take your LV cable and note that the wire insulation on one side is smooth and the other side has a a raised strip or a groove or in some cases even a white stripe. The smooth side will go in one of the load terminals and the strip/groove side will go in one of the common terminals. You then run your cable out to where ever you are going to put you lights and then loop the same cable back to the transformer and connect the same way (smooth side to load and strip/groove side to common). This will feed the cable in both directions and eliminate a lot of voltage drop.


Last edited by sparks1up; 05-08-2010 at 09:33 AM.
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Old 05-08-2010, 10:48 AM   #3
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Low Voltage Transformer


Great thanks for the info. So for 50' or under I don't need to loop back correct? For over 50' I should loop back? I'm using 12 gauge wire.
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Old 05-08-2010, 11:04 AM   #4
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Low Voltage Transformer


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Originally Posted by Kevin321 View Post
Great thanks for the info. So for 50' or under I don't need to loop back correct? For over 50' I should loop back? I'm using 12 gauge wire.
You should be ok with #12 under 50', if I remember right I think it is good for up to 75'

Keep your wattages in mind! You only have 200-watts to use! 4 50-watt lamps and your finished, but you can use more lights if you lower their wattages to 35 or even 20-watts each.

Last edited by sparks1up; 05-08-2010 at 11:10 AM.
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Old 05-08-2010, 12:12 PM   #5
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Low Voltage Transformer


200w/12v = 17A

50' of two conductor copper #12 [100' total conductor length] with a 200w load at the end gives a 3v drop.
That seems excessive for a 12v system but you will get longer incand. lamp life.

If you post a diagram with lengths and lamp loads this can be figured out more accurately.

The thing is, incand. lamp life and brightness are very sensitive to applied voltage, with lamp life being more so. With a 120v system the wire resistance has negligible effect on these two factors, but at 12v the effect of wire resistance becomes pretty strong.
And the transformer has an internal impedance. If the voltage is 12.0v at 16.7A and the open circuit voltage is 14.0v, the internal impedance is slightly more than (14.0-12.0)/16.7A = 0.120 ohms, which is comparable to your wire resistance.

Last edited by Yoyizit; 05-08-2010 at 02:30 PM.
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Old 05-09-2010, 08:06 AM   #6
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Low Voltage Transformer


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Originally Posted by Kevin321 View Post
I am trying to install a low voltage lighting system. I already have an older 200 watt transformer. It has 4 terminals (2 Load, 2 COM), and a High/Lo switch. I can't find any manual or instructions on how to connect this, because all the new transformers only have three terminals. How do I connect my wire to this? Do I need to use all 4 terminals? What is the High/Lo switch for? Am I supposed to run one wire from two of the terminals, and come back and connect the other end of the wire to the other terminals?
I'm reasonably sure you should not loop the cable back and connect it to the other load terminal and that you will cause a big short circuit if you try that.

But you can loop back to the same load terminal. Being careful as mentioned above that the wire in the cable playing the neutral role going out is not connected to load (hot) coming back.

Looping back the cable makes the lights on the circuit a little more even in brightness as opposed to the first light brighter and the last light much dimmer.

You really should know the specs of the transformer before you use it. The best guess of what you have is 100 watts from each load terminal, do not assume that if you use 50 watts from one load terminal you can use 150 from the other.

You really need information on voltage versus current from a transformer. You can shorten the life 12 volt bulbs if your 12 volt transformer is delivering 15.5 volts because you only hooked up one light.
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Last edited by AllanJ; 05-09-2010 at 08:16 AM.
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Old 05-09-2010, 08:21 AM   #7
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Low Voltage Transformer


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I'm reasonably sure you should not loop the cable back and connect it to the other load terminal and that you will cause a big short circuit if you try that.

But you can loop back to the same load terminal.
That is why I want the OP to test continuity between the 2 load terminals! If there is no resistance the 2 terminals are basically the equivalent of a tandem lug. If there no reading at all then he may have 2 separate xfmrs in one case.

He may even be able to tell from simply looking at the terminal to see if it is 1 piece with 2 screws.
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Old 05-09-2010, 08:46 AM   #8
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Low Voltage Transformer


Kevin if you can please post the name brand and model number of the transformer so I can find the specs
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Old 05-09-2010, 09:14 AM   #9
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Low Voltage Transformer


This is solved with a spreadsheet and working backwards from the last lamp.

4 ea. 50 w, 12v lamps
4 ea. 12' sections of two conductor #12 copper

4.166666667 =each lamp takes
0.0384 = wire resistance for each 12' section

12 =#4 lamp voltage
12.16 =#3 lamp voltage
12.48 =#2 lamp voltage
12.96 =#1 lamp voltage
13.6 ='former voltage

Each wire section carries the current from all downstream lamps.
Lamp #1 will have 30% of the life of lamp #4, and 130% of the brightness [but brightness perceived by the eye is not the same as actual brightness].

If you do a star config. these lengths and sizes
50' of #10 = 31' of #12 = 20' of #14 = 13' of #16
will give you equal voltage.

Last edited by Yoyizit; 05-09-2010 at 02:34 PM.
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Old 05-09-2010, 10:39 AM   #10
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Low Voltage Transformer


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Originally Posted by sparks1up View Post
That is why I want the OP to test continuity between the 2 load terminals! If there is no resistance the 2 terminals are basically the equivalent of a tandem lug. If there no reading at all then he may have 2 separate xfmrs in one case.

He may even be able to tell from simply looking at the terminal to see if it is 1 piece with 2 screws.
The DC resistance of a low voltage high wattage transformer secondary is very low. It is also necessary to measure a higher resistance (a few to several ohms) between load and common to compare with and prove that a "zero" resistance between load and load (terminals) indeed represents a tandem lug.

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