It doesn't NEED 400 mA, but the only maximum current rating I could find on Arduino spec sheets was the ~550 mA rating of the USB power input for the Arduino. I selected a fuse at 500 mA and backed off 20% to come up with some rough capacity number.
I'm not sure how much current an Arduino draws. I could easily back down to whatever the max Arduino rated current is. I don't plan on powering anything else with it, just the Arduino itself. Is there a good place where I can find the expected/maximum current draw of the Arduino?
As for not having any values, I'm not there yet. I'm just trying to get close to what the circuit needs to look like before I do my circuit analysis.
I was planning on coming off a main. I know this circuit provides no isolation. I'm planning on having this inside an enclosure and not exposed, and I work in an environment with three phase 6900 VAC loads and have had electrical safety training, so I'm not too concerned about it from a personnel safety standpoint, only an equipment protection standpoint (if I am underestimating the risk please let me know). The fuse and varistor should provide some level of protection to the arduino, and I might need to put a little more into the circuit to limit potential failures, particularly the consequences of an instantaneous short of C1.
What I want to do with the arduino is set up a device which measures power consumption for a load using inline shunt resistors. I'm going to have an enclosure, with one end plugging into a wall outlet, and the opposite end plugging into the load to be measured. On the front of the enclosure will be a display (likely a 7 segment) with a button to swap between W and kWh, a reset button for clearing cumulative kWh, and a power switch. I was hoping to power the Arduino using the same 120VAC that I'm measuring as that is what is present in my system. The Arduino will measure the current from the load, the current that its own power supply is drawing, and the voltage across the two rails and compute instantaneous power draw and cumulative energy use. I'd also like to be able to transmit this to another system for trending.
There's no real reason to avoid a transformer. I was having a hard time finding/selecting one, and I do not want to pull apart a "Wall wort" and repurpose it. I believe a transformer also takes up more space and generates more heat than a capacitive AC-DC rectifier, but I could be wrong.
I appreciate your comments.