Electricians: questions and help needed with outdoor lights

Factual Questions
1

I recently installed a bunch of low-voltage outdoor LED lighting in my front and back yards. I used two of these transformers (one in the front, one in the back) and fixtures like these. Two odd things happened that I’d like some expert advice on.

Issue 1. It rarely rains here in Vegas, but a week or two ago we had three or four days of nearly non-stop light rain. Not buckets, but pretty much constant for several days.

On one of those evenings I noticed that the lights in the backyard were out. I went out and checked the breaker that controls the outlet they’re plugged into, both mounted on the wall that separates our yard from the neighbor’s. The main breaker hadn’t tripped, but the GFCI breaker in the receptacle itself had. As I went to reset it, I got a shock. There was no exposed metal, just the plastic face of the receptacle.

The receptacle is a standard outdoor unit, with a nominally water-resistant cover, but when something (like my lights) is plugged into it, the cover can’t close, of course. So the face of the outlet was wet, and I suspect that the water conducted the current that shocked me. Once I reset it, the lights worked fine, and it stopped raining soon after and the problem hasn’t returned.

Do I need to replace the outlet? Did the shock I got indicate a faulty outlet, or was that to be expected when the face of the fixture was wet? I see I can get a box that is waterproof while in use, which seems like a good idea in this setting. I suppose replacing the outlet as well, only another $15 or so, couldn’t hurt. But is it necessary?

Issue 2. The other issue is not a safety concern, just a strange phenomenon I was hoping someone might explain. I strung 12 fixtures on a line from the 200-watt transformer. They were all LEDs, and none was more than 10 watts. The total was no more than 140 watts. So there was no question that I overloaded the transformer.

After I connected the 12th fixture, the six or seven fixtures farthest from the transformer all started flashing on and off together. I thought I had somehow damaged the transformer, perhaps by accidentally shorting the line at some point. But I switched the front and backyard transformers (identical) and the other one did the same thing.

Then I disconnected one fixture, and everything was fine. I reconnected it and disconnected a different one, to eliminate the possibility of a faulty fixture, and everything was still fine, as long as there were only 11 fixtures on the line. :confused:

I called Home Depot’s tech support line, and after stumping the first person was escalated up to “the electrician.” He said that you have to run 12 or more fixtures on two separate lines from the transformer. When I asked him for a more technical explanation, he said, “That’s just the way they are.” I don’t know if he thought I wouldn’t understand, or if he didn’t understand it himself. (I suspect the latter.)

But I’m sure there are electricians and engineers here who can explain it. What was happening?

I assume these transformers are designed for LED fixtures, since that’s most of what Home Depot sells these days. In my family room, I have an old low-voltage track light system that was installed before LEDs became available, and I found that when I replaced all the original halogen lamps with LEDs, they flashed, a lot like the ones outside did. Researching that, I found that some transformers need a minimum load, and the LEDs I was using didn’t meet that threshold. The solution was to keep one halogen lamp in the chain so that the total wattage was above a certain limit.

But the problem in the back yard wasn’t an underload like that, nor was it a wattage overload. It seemed to be a fixture overload. What’s the explanation?

Thanks.

2

I would definitely get the waterproof box in case #1.

For case #2, I wonder if the 200W transformer is overdriving the lamps because it’s oversized. That’s an off-the-cuff guess, so someone else can address it.

3

Possibly related to wiring run lengths? A lot of LED light strings have a limit on how long the total string can be (how many strings plugged end to end).

This might be even more relevant for an installation like this, which is running at a lower voltage than 120V AC.

4

In my experience LED bulbs tend to flash when they don’t have enough current, almost as if they were a capacitive load needing to be charged.

You’re calculating a 140W load on a 200W transformer; is there any chance of ground leakage? This would drop the voltage and thus the available current. Can you test the voltage at one of the flickering lamps?

5

Just out of the blue the thought occurred to me … impedance matching … something about the LEDs turning on and off … when they’re lit, voltage across drops until they turn off, then voltage builds up again until they turn on … like a capacitor or something …

ETA: … or perhaps as Balthisar suggests … current drops and lights go out until current picks up again … instead of voltage …

6

This is possible, although I removed a fixture closer to the transformer, leaving one farther out, and they all work okay. In fact that’s how I left it. But it is a relatively long run: about 100 feet. (To be clear, although I don’t think t-bonham@scc.net misunderstood, these aren’t string lights like Christmas tree lights. They are individual fixtures connected to a long cable.)

There could very well be ground leakage. Although I installed new wire for the last half of the run, I reused existing wiring that could be almost 20 years old. For most of the run, the cable is buried under soil and/or river rock and there are a number of splices with wire nuts. (Please don’t suggest that I pull up all the old wire and replace it: everything is working okay and it wouldn’t be worth the hassle.)

Maybe I’ll try testing the voltage at the end of the first line when I add the second line, (as recommended by the Home Depot electrician) just to see.

Anyone have any thoughts on my first question?

7

If you have an outlet cover similar to this http://www.homedepot.com/p/1-Gang-Horizontal-Vertical-Weatherproof-Universal-Device-Flip-Lid-Covers-MX1250S/202266485 you’re screwing up. Because, as you say, water can get into the outlet when it’s in use. That type of cover is only weather resistant when nothing is plugged in. IOW, it’s for outlets only used during fair weather.

You want a cover like this: http://www.homedepot.com/p/1-Gang-Extra-Duty-Non-Metallic-While-In-Use-Weatherproof-Horizontal-Vertical-Receptacle-Cover-with-Wasp-Guard-Clear-CKPM/203984705 where the outlet is fully covered while something is plugged into it. That’s an all-weather outlet cover. There’s an opening on the bottom surface that lets the cord out.

8

This is exactly why in-use covers are now required by code. People would plug things in and leave them plugged in and it would rain.
It’s not an issue with the transformer. Voltage drop is tremendous on a 12 volt system. 100 feet is an extremely long run for 12 volts. LEDs tend to flash on and off when they’re under-driven. User thicker wire (I use nothing but 12 gauge for 12 volt systems), run another wire out to the farthest lights, or put the farthest lights on another transformer closest to them.

9

Thanks. That’s exactly the same page I linked to in the OP. :smiley:

I didn’t know that. Thanks.

I just realized that although I have been calling it a 12-volt transformer, it has 12- and 15-volt taps, and I am actually using the 15-volt ones. The cable is 12 gauge the whole way, and the transformer says you can use the 15-volt tap for runs up to 150 feet. You can see that in the picture of the back of the box in the link in the OP. There’s a table with recommended gauges and lengths.

Thanks for all the answers, but I’m still waiting for an answer to whether that outlet is faulty or whether getting a shock just because it was wet was to be expected. Don’t bother saying, “I don’t know, but you might as well replace it,” because that’s probably what I will do anyway, just to be safe. But I’m curious to know if a non-faulty outlet would do that.

10

IANAE, but my guess is there’s nothing wrong with the outlet. The GFCI is designed to trip when the amperage on the hot leg varies from the amperage on the neutral leg, indicating that current is going somewhere else. In your case, the water was conducting current to ground, causing the GFCI to trip. However, since it was just a GFCI outlet, current was still delivered to the back of the outlet, where the water could still reach it and give you a shock.

A GFCI circuit breaker feeding this outlet would cut off all power to the junction box and prevent this in the future.

11

That the box says you can do it doesn’t necessarily mean it’s not the problem. 140 watts is a awful lot for a single line on a 12 volt system. I’d either connect both ends in a loop (be sure of polarity so you don’t create a short), or if that’s not possible run a second wire out and connect every other light to each wire.

12

It sure sounds like voltage drop in the wiring. If you have 140 Watts on a 12 volt system, that’s 12 amps. That is a lot of amps! If your run is 100 feet, that’s 200 feet of voltage drop (assuming all the load is at the end, which it probably isn’t). If you used 12 gauge wire for 200 feet, that is about 0.3 Ohms, which doesn’t seem like much, but multiply that by 12 amps and you get a voltage drop of 3.6 Volts. LEDs have a nonlinear output characteristic and require more voltage when they are hot. My guess is that you dropped the voltage to the point that they go on, but immediately heat up and turn off, which repeats in a thermal relaxation oscillation.

13

Follow up: a day or two after my last post, I looked out and saw that only the two fixtures closest to the transformer were on; everything else was out. :mad: I pulled the plug until I could work on it.

So today I went to Home Depot and in addition to buying a new GFCI outlet and waterproof-while-in-use cover, I also got two 100-foot rolls of 12-gauge outdoor wire, just in case. Then I went out back to figure out what was wrong.

Remember how I said…

It turns out that that is what we now know to call an “alternative fact.” When I got my fresh new 12-gauge cable home, I noticed that what I bought previously, and, more importantly, all the original stuff in the ground that I was reusing, was a little smaller. Sure enough, what I thought was 12-gauge was actually 14. :smack:

Remember how I said…

…well, that ended up being exactly what I did. (But thanks to you all for not being smartasses and suggesting it anyway.)

I could have spent a lot of time troubleshooting and trying to find the fault(s), but since I already knew that the old wire had lots of splices and other potential problems, I decided a fresh new cable run would be the quickest and easiest fix. So after about three hours of work, I had laid out the new wire, connected all the fixtures, pulled all the old stuff out of the ground, buried the new line, and had everything up and running properly (knock wood).

With any luck, everything will keep working from now on.

Thanks again for all the suggestions and info.

14

Congratulations on fixing both of your problems! If I use 14 gauge wire in my calculation (post #12), the voltage drop for a 200 foot loop is about 6 volts, a major issue for a device designed to work on 12 volts.

15

I meant to thank you specifically for that post, which was very helpful. Although don’t forget I’m using the 15-volt tap for the long run, not the 12-volt. Also, the load is spread fairly evenly along the length of the line. However, with the old wiring there were a lot more zig-zags and splices, so the old run was somewhat longer.

But your post explains why the lights at the far end of the run were flashing and the closer ones weren’t. Thanks for fighting my ignorance, JWT!

16

My pleasure.

This is a nice illustration of why Thomas Edison lost the electricity wars with George Westinghouse. Edison was promoting direct current. The problem was that, if you used safe voltages in the home, you either had to place the generator close to the home, or use large and expensive conductors. Westinghouse owned the patents on the transformer and promoted the system we now use, which uses enormous voltages for the long distances, than transforms down to lower voltages at the point of use. This requires alternating current.

17

Now you’ve confused me again. The output of this unit is AC. Or are you talking about the problem with low voltages regardless of whether it’s AC or DC?

By the way, what is the name of the characteristic that makes electricity either AC or DC? In other words, if the correct response on Jeopardy were, “What is AC or DC, Alex?” the clue would have been, “This is an electrical circuit’s _________.” Fill in the blank. I was wondering this just the other day.

18

DC is direct current, positive and negative. AC is alternating current, changing polarity from + to - sixty times a second in the USA, 50 in Europe.

Edison claimed that AC was unsafe. AC was used to electrocute people in prison. Edison made news releases: “Carnivorousplant was Westinghoused to death in the Arkansas State Prison last night.”

19

Not sure what you’re responding to, carnivorousplant. If it’s to the second paragraph of my last post, I understand the difference between AC and DC. I’m looking for the word that describes the characteristic of electricity that is either AC or DC.

20

Frequency. DC’s is zero. Anything else is AC.