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A voltage drop of 13 volts on the red thermostat "24v" wire. It reads 28v at air handler and 15v at the other, unconnected end. I was thinking a resisted short so I re-wired to an unused wire, same result.

I believe it is 18 gauge wire over a 25 foot run. Can any one explain the voltage drop the same constant voltage drop over the two different wires?

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  • what is the transformer size (Watt)
    – Traveler
    Dec 8, 2022 at 6:18
  • Your Voltage to drop should not be more than 10% at 1 Amp load, so you have a wire/insulation problem somewhere along the run, or bad transformer
    – Traveler
    Dec 8, 2022 at 6:27

2 Answers 2

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Nothing causes a voltage drop in a wire that is not passing current.

There is something not-wire in there somewhere.

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  • That's a good point. OP said "unconnected", which means no current draw by the tstat. So there has to be something loading down those wires or a voltage divider of some sort someplace along the route.
    – SteveSh
    Dec 8, 2022 at 12:48
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If I understand you're right you replace the wires and still have large voltage drop.

That would indicate something is wrong with the transformer.

Find or get 24 V lamp and connect to the transformer to see if it can handle the load

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