I'm installing 120v to 12v transformers for low voltage lighting. Inside the transformer are output connectors for the low voltage wire, and those output connectors are rated for 14 awg wire. I'm using 10 awg wire. What is the proper, and code compliant way, to connect the 10 awg wire to the 14 awg output connectors?
The correct answer is to use approved wire twist on "wire nuts", or split bolt. The package description will usually say how many and what sizes are allowed.
For example, the common red twist on "wire nut" will hold up to five #12s, officially. There is a diagram and chart on the product which gives examples for mixing different wire sizes.
Goodness, that seems like awfully heavy -- and stiff -- wire for low-voltage lighting. Did you get a lot of 10ga free or something? You could attach short lengths of 14ga to the output screws and use appropriate wire nuts to join to the 10 ga inside the box. Not sure that is at all kosher, but it's workable.