Electronic transformers designed for Halogen lamps will kill LEDs.
Electronic transformers are very simple switched mode power supplies. They "chop" the input 230 V/125 V sine wave in a way that will yield an average output voltage of 12 V. However, the peak voltage will be much higher and will eventually kill LEDs (that's no problem with Halogen lamps because they react more slowly). In addition, the chopped waveform may cause radio interference if the cables to the lamps are too long. The waveform may also be the reason why your voltage readings are off.
Instead of an electronic transformer designed for Halogen lamps, you should either use:
- a conventional AC transformer (the heavy kind); or
- an electronic DC transformer designed for LEDs.
What watt power transformer should I be using?
You can use a power supply like the one pictured, rated for the amount of LED's you will be driving. Read the specs on the lighting. They should tell you how many watts per a given length of the strip. So if it says, for example, that needs about 1 watt per meter, then a 15-foot section would require a 5 watt power supply. Take your total length of 76 feet, and divide by the strip's requirements. It wouldn't surprise me if you ended up somewhere in the ballpark of 20 to 30 watts.
Can I run 14 gauge wire directly from a circuit 15 amp breaker to the transformer?
No. You can't put the power supply inside the wall, and you can't bring the wire out of the wall without going through a junction box. The best way to do this is to put a cord on that power supply, and plug it into an existing outlet. There is no need whatsoever to add a new circuit or connect these directly to a circuit breaker.
Can I plug in all 5 circuits of LED strips on top of each other to the output of the transformer?
Yes, you can wire multiple strings in parallel at the power supply. If the sections are short, you can also wire them end-to-end. Double-check the specs for maximum length of a single run.
Is this the recommended way of doing what I'm trying to accomplish?
The recommended way is to plug into an existing outlet. If your lighting requirements added up to 1000 watts, you'd want a new circuit. At 20, 30 or 50 watts, it is ridiculously small in household circuit requirements.
I would also like to add a on/off switch, do they make a wall switch for 12v?
Any switch can be used for 12v. I would put the switch before the power supply. Put a cord on the power supply like this cord with switch.
Can low volt wires come right out of the wall, or do they need a junction box before being connected to the LED strip lights?
The low voltage wires don't need a junction box, but they do need a junction plate, like those used for phone jacks.
Best Answer
Here's the normal way to do that, which your LED vendor was expecting you to do.
Bring your Romex cables into a 4 x 4 x 1-1/2" metal box ($1) attached to the actual ceiling. Sheath must extend at least 1/4" beyond the cable clamp. Wires must extend at least 6" beyond end of sheath. Box must be within 5' of the light's attach box.
Install a 10-32 ground screw in the ground screw site on the metal box. Both Romex grounds ground there. (in metal boxes always ground the box first).
Get a pre-made metal jacketed wire whip, or make one out of AC cable and knockout fittings. If it has a ground wire, you got MC cable by mistake. Must be <6' long so the shell can provide ground.
Up in the box, do the expected thing. Fit a blank junction box cover.
Down in the LED fixture, splice hot and neutral. Ground is delivered via the cable shell and properly fit connector, so it doesn't take any space in the box.