I believe the formula for this is-
40VA means that at an effective voltage of 24V it can handle an effective current of 40VA / 24V = 1.67A

Is the transformer just a plug-in transformer type? As steve_fl said, the 110 current draw will certainly be less than an amp.

That's more like 40watts - not 40amps.

Not unreasonable at all.

Here is a link that might be useful: 120/24v Universal Mount Transformer (40VA)

I'm trying to figure out how to reply to each response but I'm just not seeing it so.

Thanks Steve for the formula.

jreagan this is a bare bones AC transformer that needs an enclosure and maybe a ground somewhere. That brings me to the power cord that has 3 wires. I was hoping for a simpler solution like a RadioShack device with built in plastic case.

dennisgli this link shows something like what I have. Model Number PF42440 Home Depot

http://www.homedepot.com/p/Packard-40VA-120-20-240-Volt-24-Volt-Secondary-2-ft-Mount-Transformer-PF42440/203565058?N=5yc1vZc4nlZbae

How are you planning on mounting/enclosing that Home Depot transformer? Why do you want a llastic case?

Not sure why they sold you a bare transformer when you can easily find a plug-in transformer that produces that voltage and wattage. Here's one for $9.50

Here is a link that might be useful: Wall Transformer

dennisgli I'm not trying to find a plastic case for this transformer, I'm trying to find an alternative power source that has it's own case. Like so many of the others I have. All are plastic.

I am sure now I was mistaken by ordering the exact one Honeywell tech support recommended. At the time I was not aware this would be the one I received.

Thanks for the link jreagan. This one looks perfect. Just what I was hoping.

Red.

This transformer looks like it could mount to a standard electrical box through a knock-out on the side. The black and white wires would connect (wire nuts) inside the box. The red/green would be 24v.

That's what I thought. I've decided to go with the one in the link denisgli provided. I

t does seem od that this rather large transformer is only a little over 1 amp but I'll just toss it back in the box and stick in on a shelf somewhere with the rest of my collection of maybe useful trinkets.

Thanks weedmeister.

Correction:
I decided to go with the device in the link provided by jreagan.

Typically transformers for things like doorbells or HVAC systems are hard wired so you don't have to worry about somebody unplugging them - or the plug falling out - etc. The one that I linked mounts on the knockout on a standard electrical box or circuit breaker panel.

If you want one that can be unplugged then the other one should be fine. But do note the reviews that say that it has an internal fuse - so if you short the output you will have to buy a new one.

This post was edited by dennisgli on Sun, Mar 2, 14 at 12:58

BTW - when I replaced my thermostat with one that needed power I just replaced my ancient control relay box with one that has a built-in transformer and fuse for powering the thermostat. It was under $50 and made for a clean installation.

Here is a link that might be useful: Universal Switching Relay w/ Internal Transformer