Current Rating of Stepper Shield Ground Wire
The shield is exactly that, a shield, it is not technically a conductor. It is basically a flexible metal case around your cable.

Under normal circumstances, there should be no current passing through the cable shield.

However, in some situations where chassis grounding is being used, or the ground at either end of the cable is at different levels, it IS possible to have significant currents passing though the shield. This should be avoided if at all possible.