The 'typical' cable is 18/5. The cable I have is 20/5. The installer commented that the wire is a bit on the thin side. My comment was unless the thermostat draws huge current, the losses will be small and since my furnace is literally just below the thermostat, I don't have a long run of cable to worry about. Anyway, there was no problem. Instead of making up excuses, the guy should just install it. If the wire is too thin and it doesn't work, you will see it in the results.
Here is a link showing wire gauge, amp limits, resistance, etc. Bottom line is there should be no problems with 18-22 gauge wiring.
https://www.powerstream.com/Wire_Size.htm
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Tangerine, EQ, Simplii,