Need C wire but using 4 wires

[NYCApartmentGuy]

I have read the forums and am still really confused. In my apartment we have 5 thermostats. I purchased 4 basic Nest Thermostats and a learning one (for the bedroom). The Learning thermostat installed perfectly. For the Next Thermostats I have tried in 2 places and It says I need a C wire. 

I have access to the window HVAC unit but I am unclear if I should purchase an external power adapter - or the Goggle adapter which I can install in the until. 

My question is if, I purchase the external power source, it has two wires, but I only have 1 C. I can splice the power and get it to the unit, using 2 of the spare wires, but does the 2nd wire go into the OB? 

I have 3 more units to do after this and can you help? 

[Markjosephp]

Hello NYCApartmentGuy,

Thank you for taking the time in letting us know about your inquiry. I'd be happy to take a look into this for you.

Do you mind me asking what type of HVAC system do you have? Also, could you please share with us a picture of your previous thermostat wiring configuration?

Looking forward or your response.

Thanks,

Mark

[Markjosephp]

Hi there,

Checking in to make sure that you've seen my response. Please let us know if you have any other questions or concerns.

Best regards,

Mark

[Markjosephp]

Hi NYCApartmentGuy,

It has been a few days since the last reply, and I am going to lock the thread in 24 hours. If you have any questions, please feel free to create a new post.

Best regards,

Mark