Installed low-voltage under cabinet lighting and now the breaker trips...

I'll attempt to be as clear a possible...

I installed under cabinet low-voltage lights by routing the wiring to a transformer that converts from 12.5V to 120V. I ran a black and white wire from the transformer to an existing outlet that I replaced with a light switch to control the lights.

I used the existing twist on wire connectors from the existing nomex - black (hot), white (neutral) - in the existing pigtail. Then I connected the new switch with the previous ground from the outlet, the black (hot) from the outlet, and the white (neutral) from the outlet and attached them to the pigtail. The transformer has no ground wire attached to it, nor a place for it.

The lights are on when the switch is in the "off" position, and the breaker trips when I flip the switch to "on." I'm not sure if it matters, but the light switch is now the second switch off of a GFCI, and it continues on to two previously existing outlets which both still work.

I've done some research and found some things about a "switch loop" - is that what I have on my hands? If so, how do I fix it? If not, what to do?

Sorry for the long post...

 

Welcome to the forums! I believe your transformer will covert 120 v to 12v. Just to be clear. In addition, you don't switch a neutral, ever. What you have done is built in a dead short when the switch is thrown. You need to break the hot wire and install the switch between the two blacks. Leave the white wire off the receptacle. What you want is a switch loop. What you have is something else. In relation to the receptacle, where is your switch? If next to it, just bring the hot wire from the receptacle to one screw of the switch, and the hot wire from your transformer to the other screw. Bundle all the neutrals together and tie them to the neutral lug of the receptacle.

 

If you really just ran 2 individual conductors to the transformer you did it incorrectly. Individual conductors are run in conduit.

Also the countertop receptacle circuits cannot serve lighting loads. You should remove what you have installed. It does not sound safe, nor is it code compliant.