Wiring Ecobee enhanced on NCM300 no C wire

[Mestorer]

Hi,

I’m trying to replace the existing thermostats with Ecobees on a zoned system. When I looked at the wiring at the thermostat, there were four wires (W, R, G, Y), no C wire though. Looking at the zone controller the C terminal is empty on the Zones 1 and 2 Tstats. There appears to be an extra wire that wasn’t used which I could use to connect the C wire on the zone controller to the c terminal on my Ecobee but I need to double check if it really is there. I also have the PEK power adapter but that seems silly if there’s an extra wire. Finally, the NCM300 is powered by a stand alone transformer. There’s no C wire connection between the zone controller and the HVAC unit. Which is the right answer? Is the standalone transformer enough to power to ecobees via the c terminal or using the PEK adapter?

Thanks, I included a couple photos in case it helps.

[Steven W.]

You can use the "G" green wire as your "C" Common wire. Your fan will still run when it is needed.

[Mestorer]

Thanks Stephen. If I want manual control of the fan (to say circulate air 15min/ hr), which I believe I will lose if I use the G wire as C, can I use the blue wire and connect C on the thermostat to C on the zone controller? My understanding is that the dedicated 40VA transformer has enough power to power the two dampers in my system. I estimate about 35VA peak load at the most.

[Steven W.]

That is correct if you use the G wire as the common wire then you will lose the ability to run your fan manually. Yes 40VA transformer is more then what you will need.

[Mestorer]

Thanks a lot Stephen. Installed the thermostats today and tested all the functionality. They work great. Turns out the blue wire was available so all I did is connect it to the C terminal at both ends. Thanks for the advice.