What wire gauge should I use when wiring the T6 Pro thermostat?
Use 18- to 22-gauge thermostat wire for all wiring connections. Shielded cable is not required. Always disconnect power before beginning installation to avoid electrical shock or equipment damage. (Pages 4–5)
From the official Honeywell Honeywell Thermostat Manuals Pro Series Honeywellhome And Others manual, page 4
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