Can I directly add a Common wire to this point on a 24VAC transformer, or will load be too high? (70mA)

Question

Referencing This excellent answer
I have a two wire furnace controller, 24VAC, no common. I measured the current draw of the relay (when the thermostat wires are shorted), it draws 0.2A
Wish to add an Ecobee Lite 3, which draws about 1.9 watts (just 80mA).
Am reasonably comfortable with general electrical/electronics. Have opened the control box, and located the "second" terminal of the 24VAC transformer, where one would tap a common wire.
Question: Is there any issue if I just directly solder a new common wire straight to the transformer and use that (+"R") to power the smart thermostat?  That would mean a draw of 280mA on a system rated for 200mA.  Is that likely to be an issue? Potential consequences? is the worst case scenario just the ecobee lite turning off / relay not having enough power to hold closed, or could it be something more problematic, like making the transformer fail?
I would tap the common wire at the location shown by the blue arrow in this image:

Ed Beal · Answer

If you cannot find a rating on the transformer I would not add the extra load.
With small signal and control transformers it is best to have a “buffer” so you do not over heat it. Small transformers when overloaded will break down much faster than large power transformers as they get hot at first the voltage rolls off and in some cases if they get hot enough and break down the varnish on the wires they can do all kinds of bad things.
When I size control transformers when using relays I usually target a 20+ increase in size so the “pulse” from pulling in the relay doesn’t cause an excessive voltage dip (under sizing with relays can cause chatter and buzzing) chatter is hard on the relay and even worse for the electronics in the circuits think inductive kickback. With the loads you mention I would want a 350ma transformer.
A transformer that is oversized will usually last “forever” but fully load or overload the transformer and the life curves roll off very quickly.

Can I directly add a Common wire to this point on a 24VAC transformer, or will load be too high? (70mA)

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