What could be causing my Trane XL80 transformer to keep shorting out?

I just bought a Trane XL80 dual stage furnace, I ran 18-5 thermostat wire and used all five wires, I have been through two transformers due to them shorting out. Could using the blue and green wires for the thermostat cause the transformer to keep shorting out. I am not sure if this is the cause because it does not short out until I try to turn the heat on. If anybody knows if this is the problem or something else could be causing it to short out I would really appreciate some help. I live in Northern Indiana and the cold is setting in. I have 4 kids and really need to get this figured out. Thank you for any advise on my problem. By the way, I am using this furnace as only a single stage furnace for the time being. Please help, I am getting frustrated and honestly desperate to get heat before the cold really sets in.