Supply current from USB

What is the typical current draw on a USB power source? Specifically, if powering the Morph with the Innovator’s cable from an Arduino Mega, how much current does it draw from the Mega? I’m seeing the Mega’s regulated 5V output sag to 4.5V when running the Morph. Assume that all 24 LEDs might be on at times. Obviously, if the cable is connected, Bluetooth should be turned off.