Hello trm3650 and Welcome!
Post Brand/Model of A/C Unit and Furnace and type of thermostat
15A is MAIN DC for controls
Typically that FUSE would feed DC directly to furnace and it then gets triggered by DC back from thermostat to start furnace FAN
Some designs have the DC going to a control module in A/C Unit
SO Bran/Models will be helpful
** I grew up in Casper
Went to East Jr High, Dean Morgan Jr High, Natrona County High and then Kelly Walsh where I graduated (Dad was oilfield and we moved to/from/back to Casper a LOT!)