I've noticed that in many/most modern furnaces the thermostat cable enters and connects to the control module in the same compartment with various line-voltage wiring. In fact, much of the line-voltage wiring terminates on the same control module. There is no partition to keep the low-voltage and line-voltage wiring separate. This would seem to violate the frequently quoted rule about separation of low- and line- voltage wiring unless thermostat cable insulation is supposed to be rated for line voltage. The thermostat cable I've seen is CL2 with a 105C temperature rating but says nothing special about voltage. Is it 300V or 600V rated?
Dan Lanciani ddl@danlan.*com