Should I change how this switch is wired for a low voltage light?

Question

In my kitchen I have these three switches. From left to right they are:

1. Cabinet accent lights
2. Under-cabinet task lights
3. Garbage disposal

The middle switch no longer functions properly (it looks like someone banged a pot into it causing something inside to break) so I decided to replace all three with new switches.

When took out the switches, I discovered that the left switch for the cabinet lights is connected to a 12 volt transformer for a low voltage bulb. The transformer, visible in the image above, was tucked behind the three switches.

The weird part is that the left switch was not switching the 120 volt power to the transformer. Instead the transformer was always powered and the switch is switching the 12 volt output of the transformer.

Is this safe to have two line voltage switches and one low voltage switch next to each other in the box? Is it even legal (in Montgomery County, MD) to have the transformer tucked in the box like that? Even if it is safe and legal, would I benefit from changing it to switch the input because that will eliminate phantom draw?

Answer 1

Is it legal to mix line and low voltage?

Not generally... but there's an exception in your favor. When the entire low-voltage system and everything it connects (electrically) to is contained within "Class I wiring methods" (i.e. normal AC house wiring), then that is allowed. The rationale is that if mains voltage were to cross over into the low-voltage wiring, it's made of the right stuff to handle it.

Bringing an ethernet cable through a power box to a port is Right Out... because the PC, router, and cable/DSL input would all need to be inside a Class I wiring method. That would make the PC rather hard to use!

The only time Ethernet gets used inside AC mains conduit is to interconnect SCADA equipment, which is designed to live inside Class I wiring enclosures.

Can you have a transformer tucked in a box like that?

Only if

• its design follows the rules in the UL White Book for such devices, AND
• it is UL-Listed (or equivalent NRTL) confirming the above, AND
• the instructions (which UL approved) say you can do that.

If you look at this device, you see it is RU-Recognized... which means UL has approved it as a component inside a product which might be UL-Listed. However it is NOT the same as a UL Listing. Component instructions don't usually discuss usage, but if they did, they wouldn't authorize this use.

So no. You'd need a different transformer, that is approved for this use.