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?

  • 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


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

There is something not-wire in there somewhere.

  • 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

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.