Led under cab. Idea ??

[Pirate]

Our under cabinet lights are switched from 1 switch.
Trying to figure out how to use existing wiring and switch to control 12 volt light tapes.
If I installed an extra box, next to the switch box, can I put a transformer in the box, and run the 120v power wire, into the switch box?
Then disconnect the original switched wires from the switch, and connect to the 12 volt output on the transformer.

[Roly]

Our under cabinet lights are switched from 1 switch.
Trying to figure out how to use existing wiring and switch to control 12 volt light tapes.
If I installed an extra box, next to the switch box, can I put a transformer in the box, and run the 120v power wire, into the switch box?
Then disconnect the original switched wires from the switch, and connect to the 12  volt output on the transformer.

 Can you use the existing switch to control the 120v to the transformer and wire the 12 v strips directly to the transformer ?  Roly

[Robert Adams]

What I do is put a switched outlet just above or just under the cabinets. Then use a plug in switching power supply. Don't use an old style 12v transformer as it cant adjust output to feed the leds properly.

        If you have existing 120v lights under the cab you can just remove it and then wire in a surface mount receptacle in the fixtures place.

         As for putting a transformer or power supply in a standard box there are code issues to doing so. Id have to look up the actual ones but its considered a no no (been too long and i just don't do it anyway). They don't like 120v and low voltage coming out of the same enclosure.

      As for your power supply. Always oversize it. Power supplies and LED bulbs regardless of brand are over rated. IE say a 60 watt supply is often 30 to 35 watts and with many LED bulbs it may be rated at 5w but when you test it they only pull 3w. Lots of this going on and often 3w bulbs are just rebadged as 5w and have a higher price but not a higher light output.

[EricU]

we had a light over the sink behind a fascia.  I put an outlet next to it behind the fascia so I could plug in the led power supply

[Pirate]

My object was to use existing 110 wiring to run 12 volts to light strips. Running new wiring would be a nightmare.

[Robert Adams]

My object was to use existing 110 wiring to run 12 volts to light strips. Running new wiring would be a nightmare.

           Gotcha. Just be aware that the larger wiring will have a greater voltage drop than smaller wire. Counter intuitive but when running low voltages over long distances on large wire it does goofy things.

          You could do what you propose and put a small switching power supply in a box of its own like you originally posted in the wall and have a blank cover plate over it. Like I said before there can be some issues in some places but its perfectly functional and as far as I'm concerned perfectly safe.

          The issue is going to be finding a high enough wattage power supply that will fit into a standard or deep box. There are some that will fit but they don't put out much. There was an in wall controller I saw somewhere that would do what you want but I don't remember where. A quick cruise through amazon looking at led strips etc and it may turn up.

[Roly]

           Gotcha. Just be aware that the larger wiring will have a greater voltage drop than smaller wire. Counter intuitive but when running low voltages over long distances on large wire it does goofy things.
     

????  Why  Never heard of this before.   Roly

[Robert Adams]

????  Why  Never heard of this before.   Roly

Not sure on the science behind it the field thats what happens. Most likely due to the cross sectional area of the wire but would need an electrical enginerd to explain it really. Similar issue when running speakers for a home theater. Typically on a really long run you would run thicker wire for the drop. Well if you do the speakers wont work as well or at all. Go back to small wire and all is good again. Even had an alarm guy have the same issue when he had a customer want to use existing wire. Maybe it needs to be oxygen free Monster cable...

[Phil Thien]

Not sure on the science behind it the field thats what happens. Most likely due to the cross sectional area of the wire but would need an electrical enginerd to explain it really. Similar issue when running speakers for a home theater. Typically on a really long run you would run thicker wire for the drop. Well if you do the speakers wont work as well or at all. Go back to small wire and all is good again. Even had an alarm guy have the same issue when he had a customer want to use existing wire. Maybe it needs to be oxygen free Monster cable...

I'm afraid all of that is incorrect.

[Phil Thien]

My object was to use existing 110 wiring to run 12 volts to light strips. Running new wiring would be a nightmare.

You cannot combine 12v and 120v circuits in the same box.

Avoid doing that, and feel free to run 12v over NM.

Similar discussion here:

http://forums.mikeholt.com/showthread.php?t=115729