The maximum cable length is 5 meters, or about 16 feet. Since each hub is a repeater, the signal can be propagated through additional cables for up to six total hubs, that is five connecting cables of up to 15 feet each with the final upstream and downstream hubs at each end. The intermediate distribution hubs can optionally provide additional power to their downstream hubs or simply distribute what is provided by the upstream hub. Most often the use of additional power at the hub is via a power supply transformer plugged into the wall. It can be plugged in to provide additional device power at any time.
Most systems boards made in the last two years include two USB root hubs.
A connecting cable from the system board to a screw down strip at an available card slot provides the external USB connections. ATX systems boards already have integrated external hubs and no internal cables are necessary. The USB port extension that was added to my computer had five wires, one more than the basic four needed for use. The fifth wire is a heavy black wire and it was suggested that the wire be clipped before the hub is used. That was done with no problems.