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Electrical Wiring

Low voltage wiring/transformer questions

16 years ago

I'm trying to figure out lighting for a wall unit, the transformer in particular. I'm looking at about 75W worth of bulbs on 15' of cable, using a 12V system.

The electronic transformer I'd like to use is rated at 150W but says not to use more than 10' of cable due to voltage dropoff. Since I'd be at 50% of the rated wattage, would I be ok with the 15' of cable?

Alternatively, the guy at the lighting store said I could use two runs of shorter cable, so I could use a 10' run and a 5' run, and splice them together at the transformer (basically putting the transformer closer to the middle of the wall unit than at the end). Is that correct, and if so could I do the splicing with simple wire nuts?

My other option is to use a magnetic transformer. What are the downsides of a magnetic transformer - I have heard that they might hum and run hotter than the electronic version?

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Comments (5)

DavidR
16 years ago
The type of transformer you use, mag or electronic, will make no difference in the voltage drop.

You don't say what size of wire you're using. If it's #12 wire, 15 ft at 6.25 amps (75 watts at 12 volts) will give you a 0.4v drop, which is reasonable. No. 14 wire will have a 0.6 volt drop, still within acceptable range. Any smaller wire will have too much voltage drop.
chiefneil
Original Author
16 years ago
Thanks davidr. Below is the wire I'll be using - the description says 10/2 stranded, so I should be ok. I'm actually a little dubious over their 10/2 claim as in person it looks like regular lamp/zip wire, but I'll go by their spec.

It was the guy at the lighting store that said magnetic transformers are not affected by longer cable runs. Or maybe he meant that I'd need a larger transformer and those are only available in the magnetic type. Anyway, thanks for clearing it up for me.

Here is a link that might be useful: Ambiance low voltage cable
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You've entered into the often misunderstood world of low voltage power wiring. Basically you have to treat the wiring just like 120V wiring with junction boxes UNLESS you use class 2 power limited transformers. This is what is required with typical low voltage puck light installation. Wiring must still be in wall rated to pass though the structure. Not all combinations of low voltage lighting installation parts by manufacturers are UL listed (and some will tell you that)so it is possible that you can't even connect what you have been given in a code approved way. For these parts to be compatible the transformer will have five 5 amp fuses or circuit breakers on it, each meant to supply up to 60 watts of lighting. So each fixture will have it's own run of cable back to its own connection point on the transformer. It's OK to lengthen runs using the manufacturer's voltage drop table and recommended wire size.
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Thank you, David and brickeyee! I know I can always count on your expertise. I have another question: Leviton magnetic dimmers vs. lutron magnetic dimmers. There are quite a few brands of magnetic low voltage transformers listing Lutron magnetic dimmers as their approved dimmer, but not leviton magnetic dimmers. I specifically asked Juno whether their magnetic transformer works with other brands of magnetic dimmer, for example, leviton. Their answer is "Unfortunately, at this time, we do not have any Leviton dimmers approved for use with the magnetic TL575 driver. " So basically no information there. I can see leviton is not listed myself! One of things I learn from this forum is that a magnetic dimmer should work with a magnetic transformer. Any reason why I shouldn't use a Leviton magnetic dimmer with a Juno magnetic transformer? Lutron is much more expensive than Leviton, what I am trading off by using a less expensive leviton? I am going to post this as a sperate posting just in case you don't see this.
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Depends on how you want to control them. If all you are doing is turn them on and off, it shouldn't be much of a problem. If you want to dim them, that's where issues start to crop up. Not to be too reductionist or too simplistic here, but most LEDs are solid state electronics, not really technically electrical, and require low voltage DC power, so not only is there a transformer (whether you see it or not), but usually some sort of AC/DC rectifier. Not all electronics behave well to being dimmed with typical rheostat wall switch dimmers. That's also why some LED lights have a slight delay turning on, the electronics. Even if they say they handle 120v and can be dimmed, it doesn't always work as advertised. Depending on why people on these forums recommend the Inspire LEDs (I'm not familiar with them) I would still stick with something either I know or people I respect know. If I want to dim LEDs today, I would buy a whole system that includes the lighting controller—like a Philips HUE system for traditional style light bulbs. LEDs have not been perfected to the point that anything you buy will behave properly. But if none of that matters, just get whatever you want that behaves like you want it (if you don't want the delay in turing on, for instance) Joe
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spook0829
16 years ago
My question is concerning a low voltage transformer that gives me the proper voltage output with no bulbs attached, as soon as I attach one lamp 7 watt bulb the voltage drops to zero and evidently no current. This is a Toro setup for outdoor ground lights. It just stopped working one day. Guess a new transformer is the answer. I connected the lamp right at the transformer eliminating any excessive voltage drop to the fixture which is about 25 feet away.
DavidR
16 years ago
The symptom would indicate very high resistance in the transformer. You have the right solution.
spook0829
16 years ago
Thanks Davidr, thought I would have to spend money.

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