Rack Space / Rack Unit / RU

A rack space is defined as 1.75 inches from top to bottom. Standard rack equipment conforms to this by either being 1.75 inches high, or some multiple of that.

Equipment height is often stated in "rack units," where 1U (one unit) is 1.75 inches, 2U is 3.5 inches, and so on. When racks are built for equipment, the "rack rails" have holes in them that correspond to this spacing. Therefore equipment of any size (as long as it's a multiple of 1.75 inches) can be mounted.

The standard for rack equipment is established as a 19 inch width between the center of holes on opposite rails, with 1.75 inches designated as the "standard" height of a space.

Speaker Cable Gauge Guide

The resistance to electrical flow (impedance) inherent to a speaker cable results in some amount of power loss over the length of the cable. The power lost depends on the length of the cable and the impedance of the speaker connected to it. To minimize power loss from your speaker cables, use the following table to select the appropriate gauge of speaker cable for your system.
The recommended gauges are based on a power loss of less than 0.5 dB due to speaker cable impedance.

Up to 25 feet:
4 Ohms - 16 gauge
8 Ohms - 18 gauge

Up to 40 feet:
4 Ohms - 14 gauge
8 Ohms - 18 gauge

Up to 60 feet:
4 Ohms - 12 gauge
8 Ohms - 16 gauge

Up to 100 feet:
4 Ohms - 10 gauge
8 Ohms - 14 gauge

Up to 150 feet:
4 Ohms - 8 gauge
8 Ohms - 12 gauge

Up to 250 feet:
4 Ohms - 6 gauge
8 Ohms - 10 gauge