Designed to reduce the space required for repeater and termination equipment in a telephone company central office, the 19-inch rack format with rack-units of 1.75 inches was established as a standard by AT&T in 1922 — typically with 3 kW of power density. The rack format has remained a relative constant — albeit the technology mounted within it has changed considerably. Standard power densities of stayed around 3 kW to 5 kW per rack, too, until about 10 years ago.
Today, advancements in chipset technologies for both CPUs and GPUs continue to drive compute densities upwards, with the average density of more than 11 kW per rack and deployments that reach as high as 50 kW. Modern IT server architectures such as 5U, hyper-converged infrastructure, rack scale designs, and multiple other forms of high-performance computing (HPC) are rapidly becoming mainstream.