My A/C unit stopped working during the night and I could not turn on the heat or air. I recently had to replace my 24V transformer so I thought to replace it first. When it came in I installed it and everything worked for about 30 seconds before the transformer got hot and everything stopped again. What is causing this?

My A/C unit stopped working during the night and I could not turn on the heat or air. I recently had to replace my 24V transformer so I thought to replace it first. When it came in I installed it and everything worked for about 30 seconds before the transformer got hot and everything stopped again. What is causing this?

Something in the low voltage part of the system probably shorted,, without being there I would guess the first place to look be the contactor in your ac condenser unit,, this happens a lot,do a close eyeball check as best you can on the lowvoltage thermostat wires that run from the furnace to the ac for frayed ,chewed or shorted wiring
You could also get yourself a inline fuse holder, with a 3 volt fuse, to put in place of the hot side 24 volt wire on the new transformer before installing and if the short still persists it will only bloew a fuse ./not the transformer

wmproop meant to say 3 amps, not 3 volts, fuses are cheaper than transformers.
Low Voltage wiring goes from the Air Handler to the thermostat, and also between Inside unit and outside unit.
I take it you are not using heat, which leaves Outside Contactor coil or inside blower relay coil.
I would disconnect a wire from outside contactor coil, turn back on, if it still blows a fuse, I would suspect the blower relay/contactor.
Check any visible wiring for damage.

3 amp===3 volt,, they are spelled different,, yes ,sorry about that