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The ratio of length to width is more important than actual data center dimensions. Actual dimensions depend on the number of cabinets you need to accommodate, which can vary widely.
A width to length ratio of about 3:4 seems to work best, but there are several other factors to consider for a data center building design. Remember that if you add or delete a foot from the perimeter of a rectangle, it can amount to a large increase or decrease in square footage. Therefore, the space required around the perimeter for large air conditioners and power distribution cabinets, plus the code-required aisles in front of them, can quickly reduce cabinet row lengths and cabinet counts significantly. A 3:4 ratio should get closer to a 2:3 ratio for actual cabinets when you remove side space.
With in-row air conditioners, rear door heat exchangers or Direct liquid cooling, the usable floor area for cabinets is usually about the same, but the row lengths are unimportant since the cooling air doesn't have to travel any distance. Anything from a width to length ratio of about 2:3 on up to 1:1 (square) could probably work for data center building design. Power distribution becomes the main consideration rather than cooling.
Columns are the great enemy of data center building design. They take up cabinet space, and can also force aisle widths that can waste space. Columns in a room can often force it to be close to a square.
Lastly, for very large data centers, it is generally more efficient to break them up into about 25,000 sq. ft. modules or sections. That allows you to design and use different technologies and/or tier levels in different sections.
In short, there is no right answer when planning data center building design. Building parameters usually dictate that choice. But if you build from scratch and have the opportunity to decide your own data center dimensions, avoid columns and build a clear span space -- that's far more important than the actual ratio. If the architect says you can achieve a certain clear span width in the room with the planned structure, take it. Make the room longer to achieve your needed square footage, and don't get hung up on dimensional ratios.
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