Wiki is your friend:
"The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices.
A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0."
I think it is more likely that the scanner is drawing more than 500mA. I had not known of that Y cable idea..... I'd try that.
