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Discussion Starter #1
Hi, I am replacing my old thermostat with a Nest smart thermostat. The Nest requires a 24V input. I removed the old thermostat and luckily there are two unused wires that are taped up in the junction box, which trace back to a 24VAC transformer in the attic. So it appears I have the wiring already in place for my new thermostat.

Being curious, I tested the voltage to ground coming from each transformer wire by touching one probe of my multimeter to a wire, and the other probe of my multimeter to ground. The multimeter reads a voltage of about 12V coming from each wire individually. However, when I measure the voltage across both wires (by touching one probe of my multimeter to one wire, and the other probe of my multimeter to the second wire) I get a reading of 24V.

What is going on here? I have done some reading and think it may have something to do with the transformer output not being grounded (instead I think it is floating). Does that sound correct?

Thank you,

- Scott
 

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Normal readings. The only reading that matters is the reading across the two transformer wires. The transformer secondary is isolated from ground.
 

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For future information:

If you are using a digital multimeter, you may not be getting a meaningful reading. If you have an analog meltimeter use it and see what the readings are.

If you were already using an analog meter, then tell us more about that transformer. Does it only have two leads? What ground point are you choosing for the meter probe?

If you were using a digital meter and have no analog meter to try, then hook some load between one of those transformer leads and the ground point you used before. That load can be some handy 120 volt electrical item you have, such as an electric clock, radio or similar item, a Christmas light string will work. (a resistor between 1k and 1meg would also do). With that load hooked in there, measure the voltage across those leads now. If the voltage disappears under those circumstances, you can ignore that reading you got before. It's caused by what is often called a ghost voltage or phantom voltage that is indicated when a very high impedance digital test meter is being used on a very high impedance circuit.

I just use a little $10 analog multimeter for testing things like that... less baffling and I don't have to drag the toaster around for loading the circuit. :biggrin2:
 
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Discussion Starter #5
Hi surferdude2: Thank you for your reply. I was using a digital multimeter. As for the transformer, there are two wires coming in (110V mains) and then the two wires coming out which carry the secondary voltage. The ground I was using was the metal conduit coming into the junction box where the wiring for the thermostat is housed. I am not at home right now to test your suggestion, but I can do that when I get home.
 

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It's mostly for your insight and may help you sometime on another project. Every little bit of electrical knowledge is worth having.

Joed has given you good advice, you'll be OK if you don't do any further testing but you will have missed becoming smarter than the average homeowner. :plain:

Best wishes, SD2
 
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