With the concerns you state, and especially with having the relay installed in a wall, I guess the answer is:
Don't.
You might add a fuse, but then, you'd have to find a rating in the relay's data sheet telling you if it is approved for usage with a fuse, and if so, what the fuse's rating would have to be. All in all, you would have to do the same qualification for the entire system (relay, fuse, type of outlet socket, possible loads) that the original vendor did for the combinaiton of the relay and fountain.
The relay will not limit the current (amps) to a level that is safe to itself. When overloaded, it will likely act like a short circuit for moderate overload conditions and will fail open (or explode, or burn) for severe or ongoing overload conditions.
The thing to remember is this: Once you exceed any of the ratings, the manufacturer won't guarantee anything they tell you in the data sheet any more and you are on your own.
Also, with having the relay installed in a wall where it can't really get rid of any heat it produces, I recommend using the derating curve for 60...70 °C, which tells you to rather not exceed 2...3 A.
A hacker's answer, however, is:
Yes, your setup might work, ...
... but you would have to make sure all is well and within the specified limits and characteristics.
A USB device requiring charging will usually signal its presence either by attempting to negotiate digitally or by placing certain combinations of resistance on the data lines (depending on its sophistication).
At end of charging it will decrease charge current to essentially zero but is liable to maintain a voltage monitoring presence on the line. It may also maintain a resistive ID on the data lines.But that is less certain.
It is extremely unlikely that a phone will draw NO current when connected. Load will be very small when fully charged but is liable to be detectable.
[1]. Wikipedia on common universal power supply.
[2]. Here is USB.ORG's developers approved class specification documents download page. The documents are longer than the title. Much.
For all there is to know about the battery charging interface you could download.
- Battery Charging v1.2 Spec and Adopters Agreement (.zip format, size 589 KB)
The old version is also there as
- Battery Charging v1.1 Spec and Adopters Agreement (.zip format, size 292 KB)
Also see these - with the 2nd possibly most relevant
- USB 3.0 Connectors and Cable Assemblies Document Rev. 1.02
Cable and Connector Class 2.0
Series 'A' Plug form factor Guideline 1.0
USB Connector for Mezzanine Applications Guidelines Rev. 1.0
Micro-USB Cables and Connectors v1.01 Spec and Adopters Agreement
OR (recommended), just try it.
Get a microusb cable which you have access to the conductors of. Plug into a socket you can probe or cut the end off a cable or ... .
Plug in a phone and measure the voltages bewtween data + and - and power + and -.
Repeat with a few phones.
A pattern will emerge.
Tell us about it.
Note that as the spec has evolved so too will the phones, so what phones do now will be subtly (usually) different than a year or few ago. Drwaing current somewhere is liable to be a common thread.
I haven't tried this with recent phones but intend to. May not be soon enough for you though.
Best Answer
I agree that the stray magnetic field from your power cables seems like it would be enough to at least discriminate between "at least 5 amps on" or "0 amps off". Perhaps you could build up a system based on one of the magnetic field sensors listed at Open Circuits and press it against the power cord at some convenient point.
As Joby Taffey pointed out, most equipment plugs into an electric socket, and so inserting a meter between the equipment and the electric socket is common:
These things are generally calibrated to linearly measure current, so they are total overkill for merely discriminating between "on" and "off". And they won't work as-is for your application, since there is no socket between your well pump and the distribution panel. But perhaps using one of these off-the-shelf devices, hacking it open and pressing the magnetic sensor against the weak magnetic field that leaks out of your romex cable, will be able to discriminate between "on" and "off" and take much less time and effort than building up something from scratch.