Low Voltage transformer question


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Old 07-26-08, 07:22 AM
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Low Voltage transformer question

Hello, I put some low voltage lights down my lake front and bought a Malibu "power pack" transformer , unfortunately I didn't realize until I opened it that it is a two channel. I ran one line for my lights and it is all ready trenched and filled in . I'd like to bridge the two channels perhaps like is done with car audio amps. Can I do such a thing? Every transformer I find that is 900 watts is two channel.

Thanks
 
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Old 07-26-08, 10:54 AM
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What are the total number of watts that you are trying to run for your setup?
 
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Old 07-26-08, 12:35 PM
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850 watts total ...............
 
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Old 07-26-08, 01:39 PM
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Majorty of the low voltage luminarie and transfomer are very limited to the wattage due the low voltage amprage is pretty high.

Most of the LV circuit useally are limited at 300 watts per circuit { 25 amp @ 12 volts } .

unforeteally you can not " bridge " the circuits at all due the the transfomer design and the potary set up.

Merci,Marc
 
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Old 07-26-08, 01:52 PM
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The transformer is probably center tapped - the center and each end form the two taps. In this case you cannot. If it is two independent windings and they were phased properly you could. The only way to know this is to test it or check with the manufacturer.
 
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Old 07-27-08, 08:08 AM
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If you could 'bridge the outputs', then consider what the current would be:

850W at 12V means 71 amps.

You would need to use large gauge wire, and voltage drop will be severe.

You might be able to rescue the situation using higher voltage or lower wattage lamps (LEDs may be an option), however you are probably going to have to go all the way back to the design stage and figure out what is the correct hardware to install, and then redo the installation.

Things to consider for a low voltage lighting installation:
number of circuits
voltage drop on each circuit
access to conductors for repair
burial depth
etc.

-Jon
 
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Old 07-27-08, 09:13 AM
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Well that is a little disheartening . Thanks you fellas for your knowledgeable input. I used 12 gauge wire . I suppose it won't be too much trouble to get a little more wire and split the string in half and use both legs of the transformer independently.
 
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Old 07-27-08, 10:06 AM
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Could you post about your setup in greater detail? You really want to catch all of the design issues now, rather then go through all the wire laying work again only to find the next problem, and then have to do it a third time.

You say that it is a 900W transformer, and that you have 850W of load...that is too much for even _three_ 12ga circuits, presuming 12V lights.

1) What is the distance from the transformer to the lights?

2) How many separate circuits is the transformer rated for?

3) What is the voltage rating of the lights?

-Jon
 
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Old 07-27-08, 03:10 PM
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The lights are 12 volt, 50 watt. The lights start about 3 ft from my transformer and run about 150 ft down to my lake front. The transformer has two seperate circuits, each 450 watts capacity.
 
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Old 07-27-08, 03:54 PM
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Voltage drop is certainly going to be an issue over that length of wire at the low voltage. Your wire size will need to be increased.

Also watch how the ampacity goes up as the voltage goes down. For example 300 watts/120 volts is 2.5 amps, drop the voltage to 12 and watch what happens, 300/12 = 25 amps.

Some transformers also require that the distance to the first lamp be at least 10' so that it does not get overdriven by too much voltage.

Try to rework this so that the transormer is in the middle of the lights to minimize voltage drop.
 

Last edited by pcboss; 07-27-08 at 03:57 PM. Reason: added note about dist to xfmr.
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Old 07-27-08, 05:09 PM
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Okay, you need to learn about 'voltage drop' and 'ampacity'.

Ampacity is the maximum current that a conductor is permitted to carry. If you exceed the ampacity of a conductor, then it will overheat and the insulation will fail. Additionally, there are restrictions placed on the current permitted in small conductors (10ga, 12ga, 14ga). Generally, 12ga conductors are restricted to 20A, but I don't know if there are specific restrictions or permissions for low voltage lighting.

I generally work voltage drop calculations by hand, but you can find calculators on line.

Basically the wire itself resists the flow of electricity. In order to pass current through the wire, some of the voltage is used up overcoming the resistance of the wire. The amount of voltage drop depends upon current flow. Your situation (lights distributed along a wire) requires a simple but lengthy step by step calculation, which I'm not going to do; but I'll give an example.

Imagine that you have a 100W lamp at the end of a 150 foot run of 12ga. The lamp is nominally 12V, and thus requires 8.3amps. If the supply voltage is less than 12V, then the lamp will draw less than 8.33A, but for the purpose of calculation, you assume that the 8.3A value is constant. The resistance of the wire run is about 0.5ohm (you can find resistance tables online). The voltage drop is just the current value (8.3A) times the resistance (0.5ohm), giving about 4V. In other words, about 4V is going to be 'lost' in the wire feeding the lamp, and the lamp will only see 8V. The reality is, of course, that the lamp will draw a bit less current, and the voltage drop less...but you see the issue.

With 850W on a single circuit, a rough estimate of voltage drop shows more drop than the supply voltage, indicating that the drop is so great that the approximation cannot be close to valid.

As pcboss suggests, figuring some way to put the transformer in the middle of the run will reduce the problem of voltage drop, but this introduces the problem of outdoor 120V wiring; if you are ready to tackle outdoor 120V wiring, then you might consider using 120V lighting directly. The higher voltage means lower current, and thus significantly reduced voltage drop.

-Jon
 
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Old 07-27-08, 05:39 PM
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While it is not very efficient many outdoor systems use voltage drop to get the proper voltage to the lamps. They in fact tell you to not put a lamp in the first 10-20 feet of a run.

I have a ton of outdoor low voltage lamps and I purposely load it heavily to keep the voltage down on the lamps. In general 10% less voltage on a lamp gives you 10 times more life. Conversely 10% more will cause them to burn out much quicker.

Laws that apply to house wiring do not apply to outside low voltage wiring. Do what you want and see how it works for you. If the distant lights are to dim then you will need to use heavier wire, parallel wire, or a suggested center feed it.

Another thing you will find is that the connections for low voltage are crap. I can't tell you how many people throw these things away because they don't work and it is just a connection. I usually cut off the wire stab type connectors and hard wire it. I cut the wires and use standard wire nuts to connect them.

Another trick if you really want dim lights is to put a diode in series with one feed to the fixture. Low voltage lights are AC and putting a diode in series essentially cuts the voltage in half. Only 1/2 cycle gets to the bulb.
 
 

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