Came out to service call in regards to a customer having issues with his AC system. Customer stated that he bought the house recently and he was replacing the thermostat. He hooked up what he believed was the common wire to the transformer. Customer stated the unit hasn’t worked since and has a burned smell from thermostat. I checked system over and found a bad transformer. After initial inspection of wiring, I found the customer to have wired the common wire of the thermostat to the 208v side of the transformer. Showed customer the transformer and discussed repair needed and what I could do to help prevent this from happening again. Customer had me replace the transformer. After replacing the bad part. I wired in the common wire correctly and added a 3 amp fuse to the line. That way if a short happens, the customer doesn’t lose another transformer. Cycled system on after repairs were made and system came on with no issues. Went over operation of system with customer and advised him on