What is the rated power of American central air-conditioning control circuit?

I want to know what is the rated power that American central air-conditioning 24V control circuit can supply, which is also the power supply for Nest. Nest gets its power from the 24V control transformer, and converts it to DC. But what is the current value a device can draw from this 24V? Thanks!

More: I care more about the 24VAC HVAC system's capability of its power supply. E.g. if I design a similliar device like Nest, but my device consumes more power, whether could I still connect it to this 24VAC line?

Now it is clear of one thing, 24V will provide power for both the control relays/heating/cooling parts and Nest. So how much power could Nest get? This is my key question.

Many systems have a single transformer located in the furnace / fan coil unit. That transformer should be stamped with a "VA" rating. Typical sizes range around 40VA to 75VA.

Roughly speaking, for electronic loads one should assume a power factor of about 85% (electronic control board, nest thermostat, etc).

The total continuous current a 24 VAC control transformer can supply in a residential system is given by:

I = (VA Rating) x (Power Factor) / (Secondary Voltage)

Or in the case of a 40VA transformer:

I = (40 VA) x (0.85) / (24 V)

I = 1.4 amps

This amount includes the controls installed in the system, and there may not be much to spare for additional loads. When in doubt, increase the VA rating of the transformer to allow for the additional load.

Many electronic loads are not continuous and in those applications a peak current of 30% - 50% higher may be acceptable as long as the sustained load does not exceed the rating.

More:

Q: "how much power could Nest get [from the HVAC transformer]?"

A: It varies wildly and in my thinking this is a secondary design consideration. The 18 gauge thermostat wire has a resistance of about 0.6ohms/100ft, so roughly 3 amps is the maximum current that can be drawn before the supply voltage drops more than 10%. Deduct several hundred mA for contactor loads, and the thermostat could use perhaps 2.5 amps, not accounting for the system transformer size. If the thermostat is a really great product, people will upgrade their system transformers to install it. To improve market adoption, later revisions could be lower power units that would not require a system transformer upgrade. In the case of the Nest, the manufacturer resorted to using what looks like a LiPo battery. LiPo batteries are expen\$ive, but the marketing guys must have felt it was worth it.

• Sorry! I ask another question related to Nest battery in Nest Community, and got answer shows that 24V AC is not enough for Nest electronics. You may check it here: community.nest.com/message/53953?et=watches.email.thread#53953 Which one is right? Commented Nov 14, 2015 at 8:12
• Power factor is not the same as efficiency, but is the ratio between actual power supplied to a load and VA (actual power + reactive power). Since transformers are rated in VA, one must account for this. (regarding the post you linked to)
– user39367
Commented Nov 14, 2015 at 14:35
• Re Nest power requirements, the manufacturer is disturbingly vague, but for their 3rd generation unit they state "less than 1KWH per month." This means the unit requires less than 1.5 watts power on average or very roughly 5% of what a 40VA transformer can supply. This should not be a problem.
– user39367
Commented Nov 14, 2015 at 14:37
• @ Tom Xue - You appear more concerned about the Nest thermostat than power available from the typical residential HVAC transformer. According to the manufacturer, the Nest has a rechargeable lithium ion battery which is used for high power activities such as WiFi transmission, and the battery is kept charged by the HVAC system transformer. This makes the unit compatible with the majority of systems, but it will not operate without the rechargeable on-board battery (if that is what you meant to ask).
– user39367
Commented Nov 15, 2015 at 3:06
• @ chris Thank you for your feedback! Please check my main post, I update it. Commented Nov 15, 2015 at 7:29