hvacthermostat-c-wire
I got a new ecobee thermostat that requires a c wire. my current thermostat has 4 wires plugged in, and there are 2 extra wires not being used coming out of the wall one blue labled y2, and a brown one lableled w2 aux. they both have current flowing. Can I use either of these and connect to the c terminal?
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Yes, tapping along with the red wire under the (C)OM is where it goes. If you'll notice, your (Y) is already double taped.
The schematic image you've attached is difficult to see, so I can't say for sure. Based on the wiring, I would say that the B terminal would work as C.
Since I'm not there and can't test things, I have to make some assumptions. Based on the wiring, it looks like the one cable goes to the thermostat and another goes to the outdoor air conditioning unit. Since the red wire from the A/C cable is grouped in on the Y terminal, and the white wire from the A/C cable is on the B terminal. One can reason that the B terminal should be connected to one side of the transformer coil, while R is connected to the other.
You can test this theory by setting a multimeter (or voltmeter) to AC volts, and testing between the R and B terminals. You should read ~24 volts.
You can either connect both the white wire that's already on the B terminal, and the new C wire to the screw terminal. Or you can connect the white wire and your new wire together with a spare bit of wire, using a twist-on wire connector. Then connect the other end of the spare bit of wire under the B terminal screw.
Best Answer
It looks to me that you can connect either wire to the same terminal as the yellow wire. I would use the brown simply because it is as close to black than any other color. To be honest T'stat control wiring really doesn't have a color code but everyone sticks to the color that the letter indicates for clarity's sake. So disconnect the brown from your control panel and connect it to the C terminal and then connect it to the C terminal in the T'stat.
Good luck