Is it ok to pigtail different gauge transformer wires with a 15 amp circuit?
Just fix this with a pigtail + wirenut
Your breakers are GE breakers that do not support a &quot;double tap&quot; connection.  Fortunately, this is easy to fix; since this a 15A breaker, a length of 14AWG THHN and a wirenut suitable for the wires involved can be used to combine the new pigtail with the existing branch-circuit and transformer hot wires, while the other end of that pigtail simply lands on the existing breaker hot terminal.  Make sure to do this with the breaker in question turned OFF, of course, and you'll want to use an inch-pound torque screwdriver to torque the breaker and neutral screws to spec, as well as cranking that wirenut down tight!