I'm not entirely sure if this belongs here, so if it belongs somewhere else let me know.
Home thermostats (in the United States at least, I'm not sure about other locations) for newer homes have a few different wires (depending on capabilities for the home) that are used for turning on the heater, turning on the AC, and turning on the fan. There is typically two more wires, one that is the hot (connect this to the AC wire will turn on the AC, connect this to the heater to turn on the heater, etc), and one that is the "common" wire and is used for powering the thermostat itself.
These all run at 24VAC that comes from a transformer stepping down the mains voltage somewhere in the system.
Are there any building codes or product standards that would tell me what the minimum current carrying capability of this circuit is? How much current can the transformer handle? How much current is the wire supposed to handle?