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Old 03-02-2013, 12:32 AM
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JimLewis JimLewis is offline
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Join Date: Dec 1999
Location: Beaverton, OR
Posts: 6,818
Originally Posted by starry night View Post
I couldn't find the entire Article 411 code online, but I am pretty sure the wire size uou listed must be incorrect. Most of our landscape lighting fixtures come with a 16 gauge lead. Also, the AOLP guidelines mention landscape lighting wire as small as 18 gauge.
Right. I understand that most fixtures don't come stock with 12g wire. Obviously, I've seen a lot of those fixtures too. And that's the first question I asked as well. This was one of Kichler's lead designers/engineers who is now their national trainer, by the way. And that's the first question I asked him after the class. If we have to use 12g wire, then why do your fixtures come with the smaller wire?

It's a complicated explanation of why this rule is in effect, from what I understand. And I'm just taking this from what I heard from one person. I may not be able to explain it correctly. But I'll try my best to explain it how I understood it; Transformers have internal breakers in them, or at least some do. Not the breakers we see when we take the cover off. This breaker is way inside, in the part you cannot see. This breaker turns off the transformer if it gets too hot, or too much amperage. So let's say that number is 15 amps. At 150 feet, 12g wire can safely handle 15 amps all the way back to the transformer. So if it gets over 15 amps, it will trip this breaker. And let's say there's a pathway light out at around 150 feet. And someone backs into it with a car on the way out of the driveway. So now the pathway light is broken, leaning over, and the wires are shorting/sparking. If you had 12g wire all the way up to it, it would start to draw more than 15 amps, and when this draw hit the transformer it would trip the breaker and turn off the transformer. That way, the transformer notices the danger and turns itself off, hence avoiding danger and maybe a fire. But let's say you didn't use 12g wire. You used 16 gauge wire. So now that same fixture is broken/shorting/sparking. But since the 16g wire you installed cannot handle a 15 amp load, it cannot draw the 15 amps that the short is trying to cause. It only draws 10 or 11 amps. So the transformer doesn't trip. It doesn't realize there's a problem, because your wire isn't big enough to basically "report" the problem back to the transformer. So the transformer just keeps sending the electricity out there, not knowing there's a problem down line. It just keeps running and running, causing the fixture to short/spark, etc. even more. Now we have a problem.

That's how it was explained to me. And that's why he said NEC requires 12g wire. Now maybe it's more complicated than that. Maybe you only have to use 12g wire on transformers that are set to trip at 15 amps. I don't know. Or maybe the law says that you can use whatever wire you want, as long as it's strong enough to carry the draw that the transformer needs to trip, in case of a problem. All I know is he just said we need to be installing 12g wire or larger. Nothing smaller.

As to why the fixtures use smaller wire? Well, I gather that it's because those leads are really short. He said that's why they keep their leads short at Kichler. Because they can carry a 15 amp draw (or whatever the number is) for a short distance. But they couldn't carry it back 150 ft. So I guess it's okay to use a smaller wire as long as it's just a little bit.

I don't know any of this for sure. This is just what I heard from a guy who is an engineer at a lighting company. And I knew he understood all this a lot more than I did.
Jim Lewis
Lewis Landscape Services - Oregon - Portland Oregon Landscaping Company

landscape design Portland Oregon
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