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?


3 Answers 3


The only wire currently sold by US big-box stores for thermostat use appears to be AWG18 x whatever number of conductors.

Most of the transformers are 20-40VA (some are 10VA though) and are so marked, along with Class 2/Class 3. Many are impedance-protected as well as fused so they may just limit the current and get hot if you short the output (and if a primary fuse does not blow). They probably have terrible regulation as a result.

Anyway, a 10VA transformer would be be able to supply about 400mA RMS at 24VAC or if you full-wave rectify it and filter it about 250mA at 30VDC or so. Scale up for the VA of the transformer.

If you do test the short-circuit current of the transformer you might want to find out where the primary is fused and make sure you have an equivalent spare on hand, because the primary fuse (often something like a glass or ceramic fuse on a furnace control printed circuit board behind a service panel on the furnace) may well go. It won't blow a mains circuit breaker, more likely a somewhat hidden fuse.

Note that it's normal to have the supply for the central air control in the furnace and if you want to remove power from the 24V and the 240VAC main power you have to turn both off.

Also, keep in mind that the 'C' wire is not always available at the thermostat location, especially in older systems. For that reason smart thermostats often have a battery inside and may cycle the heat or cool to keep the battery charged, even if unneeded-- this is so they can have the largest possible market. Homeowners may be fine with rewiring an existing thermostat, but fishing a new wire through various finished interior walls is a bridge too far.


If you are asking how much current you can depend on getting from an existing transformer to use to power a smart thermostat, I believe the answer may be not much. The transformer installed inside an HVAC unit may be sized to operate only relays in the unit assuming a "dumb" thermostat that needs no power. The thermostat wire is sized for mechanical durability, not for current carrying capacity. Codes may require the circuit to be Class 2, 30 volts and 100 VA maximum, usually 24 volts and no more than 4 amps. It could be much less than 4 amps and most of the capacity may be required for the relays in the HVAC unit.


Normally voltage distribution losses use cables for Ac designed with not exceed 5% of the cable voltage at max current.

But 24Vac used for furnace relays and doorbells are quite low current far less than the ampacity of the wire gauge specified for a 10'C rise or higher.

Installation standards are regional so there is no universal answer.

Do you have a specific region or wire gauge and extended application in mind?

  • \$\begingroup\$ I'm planning on building one of those "smart thermostats" (aka diy nest), but I would like to know my power budget. All the 24VAC used for relays are pretty low current, but what about the common line (the one used for powering the thermostat)? That one COULD actually be pretty powerfull.. \$\endgroup\$ Oct 4, 2016 at 18:01
  • \$\begingroup\$ Just rate your wire ampacity to the short circuit current of the transformer, not that high <1A for the relay , I think \$\endgroup\$ Oct 4, 2016 at 18:11
  • \$\begingroup\$ How do I nondestructively figure out the short circuit current of the transformer? \$\endgroup\$ Oct 4, 2016 at 18:13
  • \$\begingroup\$ read the part number rating & measure DCR \$\endgroup\$ Oct 4, 2016 at 19:07
  • \$\begingroup\$ I don't know where the transformer is. I expect that I will have to find it, but I was hoping that I could cite code instead. \$\endgroup\$ Oct 4, 2016 at 19:08

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