I meant Arduino Power Selector. But since I'm not a USB board user, I make a mistake. Sorry for that.
That's what I thought you meant, which is why I thought it was worth clarifying. 
Fortunately we're all human around here as far as I know. 
But Ananjar cannot use the USB 3.3 volts as only source for Arduino (and relay), in this case.
The power supplied from USB is actually 5V which the FTDI chip uses to also supply a regulated 3.3V. But as you point out, the provided current--a theoretical 100mA--500mA and ~50mA respectively--is the main factor he needs to take into consideration.
Hopefully I'm not just confusing the issue for the original poster.
--Phil.