There are better quality transformers rated for indoor or outdoor.

Can you be more specific? brands, models etc?

Some of those transformers have a photo/light sensitive switch to turn them on at Sun down. Otherwise I have no idea why they would insist on outdoor mounting. Probably just never thought someone would mount it indoors.

The indoor/outdoor issue is a UL or ETL listing for safety and not for sensors, although some transformer have built in photocells which will need to exposed to the exterior light level somehow. There can be indoor mounted transformers with exterior sensors.

Q-Tran and Vista have indoor/outdoor rated models. I believe FX Luminaire does also and I'm probably leaving out a dozen other brands. The common thread will be that they are higher quality than Malibu. The transformer must be listed appropriately. (You can't use a general purpose transformer for landscape lighting.)

So, what you are saying is that homeowners have asked for transformers that can be mounted outdoors? That may be but I am not sure there is that much customer feedback in the transformer marketplace from homeowners. I ended up buying a transformer that is rated for being mounted indoors or outdoors and mounted it indoors mainly because I had feeds going 3 sides of my house and the conduits were aready installed into the house. There were very specific requirements for installation of the transformer indoors such as clearances, mounting on non-combustible surface, etc.

What puzzles me is what in the construction of a transformer would cause it to be hazardous to be mounted indoors if it was only rated for outdoor use. To me, it seems only the clearances required around it for heat dissipation and being mounted on a non-combustible surface and not near a combustible surface would be the main requirements for mounting indoors.

Can anyone shed light on this? What exactly is different about an outdoor and indoor transformer that limit where it is installed (other than a rating that says "Don't do it" from some agency)? (no pun intended)

I'm not an expert on these matters, but my guess would be that the transformer was tested for UL rating in an outdoor environment. Thus, its listing is only for that environment.

Some of the issue is legal arcana. The NEC says that you must follow the manufacturer's installation instructions. If they say "mount it outdoors," then mounting it indoors is a code violation.

"What puzzles me is what in the construction of a transformer would cause it to be hazardous to be mounted indoors if it was only rated for outdoor use. "

If it catches on fire or fails outdoors the risk is far less than indoors.

Transformers have a lot of wire insulation, and often paper between the layers of windings to supplement the thin insulation on the wire.

They tend to produce a LOT of hazardous things when they burn up.
If it is outside it is annoying but not nearly as dangerous.

You do want to wake up in the morning, right?

Thanks, but they do have indoor rated transformers, such as the one I purchased and installed so does anyone know what they do differently that don't make them burn up?

There were instructions about not mounting my indoor unit on a combustible surface and having an airspace etc., but there must be more to the unit that is different from an outside unit.

While I don't know the answer to this off hand, it seems a combustible surface can just as easily be on the outside of a home as well as inside so I don't think that is the issue here. A couple other thoughts, however, do come to mind.

One is that the insulation and/or plastic case may produce harmful fumes if burning and preclude an indoor use rating. Similar to the difference in ratings for riser and plenum rated wiring.

The second is that the commonly used and supplied LV direct burial wiring that comes with the transformer, especially in kits, is not in-wall rated (for the pass through) and that indoor use is excluded because of this.

"does anyone know what they do differently that don't make them burn up?"

More margin.
Larger conductors, higher temperature rated insulation, better steel lamination and design, and lower temperature rise in use.

Temperature rise is a huge driver for transformer reliability.
If the core saturates magnetically the internal self heating goes very high very quickly.
Making sure the conductors are large enough to not contribute to heating helps a lot, as well as providing enough metal in the core to stay well away from saturation.

High quality transformers are HEAVY for their size.

I looked up the specs on UL 1838 which covers landscape lighting and it still did not indicate what the difference is between indoor/outdoor - indoor - outdoor power units other than the obvious wet/dry issues.

It appears the real difference (putting the wet/dry issue aside) is in the transformer's 'application' and whether it's intended use is for indoor or outdoor LV lighting, along with secondary and primary wiring requirements.

Then, after doing a little more research, I found this excellent technical article (in pdf format) that explains the UL and NEC requirements concerning LV landscape lighting.

(Not sure why it's on UL's Denmark DEMKO website but it's a safe link)

Here is a link that might be useful: UL/NEC Low voltage lighting requirements

The NEC does not address things like transformer manufacture, just requires equipment to be listed.

"I looked up the specs on UL 1838 which covers landscape lighting and it still did not indicate what the difference is between indoor/outdoor"

Why would it? UL 1838 only covers Low Voltage Landscape Lighting Systems, and these are normally outside.

You need to look up the standards for dry transformers.

brickeye

I really don't understand your reply.

NEC does not cover transformers - UL, as a voluntary approval organization for manufacturers does, and in UL 1838 they do cover indoor/outdoor - indoor - outdoor power supplies, without elaboration. The original posting was in regards to the UL label that indicated "for outdoor use only."

While I used my UL account to log in, I do see the information is available to the general public - such as yourself. You may wish to view it.

NEC on the other hand does have requirements for in-wall low voltage wiring and this appears where the discrepancy exists.

The transformer here, along with it's typical wiring and ratings, is meant to only be used for outdoor landscape wiring and not in-wall low voltage, in house, wiring.

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