My USB cord looks like this one.
The male ends are plugged into
- a USB port on my computer
- a sugar cube, which is plugged into an outlet
The female end is plugged into a printer cable, which connects to my Mega.
I am not sure what the current draw of my arm is, but according to the product page, the 4 servos need a "working current" of 100mA each, or 400mA in total. Given that an outlet can provide 15 amps (according to Google), 400mA doesn't seem like too much.
Would it be better to forget about powering through a USB altogether and just use the screw terminals?