Add-A-Wire Accessory, Common Wire Kit for All 24VAC Thermostats (4 to 5 Wires), White$$Tools & Hardware Other

Makes one wire works like two To add a common wire to a 4-wire system, or to add cooling to a heat-only system For use with all 24VAC systems Includes yellow, green, blue wire splice (directionally sensitive) Cannot be used as 'R' power or 'C' common wire