If you are already powering the wifi stat with its own external transformer, you don't need a C (common) from any other source.
To control the a/c system, you just need to connect the R, G and Y terminals to the stat. That gives the stat the ability to start the compressor while making sure the fan will also be started. It also allows the stat to run the fan manually without starting the compressor.
To control the a/c system, you just need to connect the R, G and Y terminals to the stat. That gives the stat the ability to start the compressor while making sure the fan will also be started. It also allows the stat to run the fan manually without starting the compressor.