Rack Essentials

Racks organize IT equipment into standardized assemblies that make efficient use of space and other resources. At the most

basic level, a rack consists of two or four vertical mounting rails and the supporting framework required to keep the rails in

place. The rails and framework are typically made of steel or aluminum to support hundreds or even thousands of pounds of

equipment. The rails have square or round holes that allow you to mount rack equipment to them with screws. (In the case of

the square mounting holes, the screws connect to removable cage nuts placed in the required locations.) Some equipment

includes horizontal rails or shelves that you mount in the rack to provide extra support.

Rack Standards

The width of the rails, the horizontal and vertical spacing of the mounting holes, the size of the equipment cabinets and other

measurements are standardized. This ensures that standard rack equipment will always be compatible with standard racks.

Most IT equipment is nominally 19 inches wide (including mounting hardware) and follows a standard set by the Electronics

Industry Alliance (EIA) and now maintained by the Electronic Components Industry Association (ECIA). The current 19-inch

rack standard is called EIA-310-E, which is essentially equivalent to IEC-60297-3-100 or DIN 41494 in other regions. (There’s

also a standard for 23-inch wide telecom equipment. The vast majority of IT applications use 19-inch racks and equipment.)

Rack Units

Although 19-inch racks are always the same nominal width, the height and depth vary. The depth of the rack rails is usually

adjustable to some degree. The height of the rack is divided into standardized segments called rack units. Each rack unit

is 1.75 inches high, and the height of a rack or an equipment cabinet is expressed as the number of rack units followed by

the letter “U”. For example, a 42U rack contains 42 rack units. That does not mean the rack is exactly 42 x 1.75 inches high

because racks usually include at least a little extra space at the top and bottom that isn’t usable rack space. It does mean that

the rack will accommodate any combination of standard rack equipment up to 42U—whether it’s 42 x 1U switches, 14 x 3U

servers or 21 x 1U switches with 7 x 3U servers. Remember that the rack also has to be deep enough for the equipment and

rated to support the combined weight of all the equipment. 

What kind of equipment can you install in a rack?

IT Production Equipment

IT production equipment is the servers, storage, network switches, routers, telecommunications hardware and other devices

that fulfill the primary purpose of the IT installation. (Racks also have applications that may not employ typical IT equipment,

such as manufacturing, but the same general principles apply.)

Racks are designed to hold all standard 19-inch rack-mountable equipment, as long as it isn’t too deep for the cabinet or

too high to fit in the available rack spaces. The equipment often includes removable mounting brackets or “ears” that fasten

to the vertical rack rails with screws. Heavier equipment may also include horizontal rails or shelves that mount in the rack to

provide extra support. If the equipment isn’t made for rack installation, you can install a rack shelf to hold it. Just make sure

the equipment isn’t too big or heavy for the shelf.

IT Infrastructure and Rack Accessories

Racks also accommodate IT infrastructure and rack accessories that support the operation of the production equipment. This

includes equipment like UPS systems (uninterruptible power supplies), PDUs, cable managers, KVM switches, patch panels

and shelves. Although most equipment is designed to fit in standard rack spaces, vertical PDUs and cable managers mount

to vertical rails without using any rack spaces. Equipment that installs this way is referred to as 0U (“zero U”). 

How do you choose the right rack  size?

Height

The height of the rack is one of the most important considerations because it determines how many rack spaces are

available for your equipment. In fact, the height of the rack is expressed in the number of available rack spaces. (You can

also find the external measurements to make sure the rack will fit through the doors in your facility.) Common heights for

floor-standing racks and rack enclosures are 42U, 45U and 48U, with custom sizes up to 58U available for some high-density

data center applications. You may also wish to leave extra space for horizontal cable managers, future expansion or other

purposes, so keep that in mind when determining the height.

Width

Since the width of the rack’s mounting rails and the rack equipment itself is standardized, that part is simple—just make sure

your equipment and your rack both follow the EIA-310-E standard. But you also need to consider the external width of the

rack. The standard width for rack enclosures is 24 inches or 600 mm, which corresponds to the standard for removable floor

tiles in a raised-floor data center. Extra-wide rack enclosures (30 inches or 750 mm) are also available, with or without extra

depth. Enclosures with extra width have side channels that accommodate PDUs, high-density cabling and cable managers

without obstructing airflow. This is especially helpful for network switches that have side-to-side airflow and forward-facing

ports. Extra-wide rack enclosures may also have modified vertical mounting rails that include built-in cable channels. Some

extra-wide racks can even be customized with extra rack spaces at the sides to fit as much equipment as possible.

Depth

The rack’s depth is important because you need to make sure it will be deep enough for your equipment, including any

cabling that extends past the equipment cabinet. You can usually adjust the depth of the vertical mounting rails in a fourpost

rack or rack enclosure, but only within certain limits. The standard depth for a rack enclosure is around 42 inches,

but you can get extra-deep (48 inches) rack enclosures for deeper servers and blade chassis. The extra depth ensures that

there’s enough room for cabling, PDUs, cable managers and other accessories without blocking airflow or compromising

serviceability. There are also racks that are shallower than the standard depth to save space. These are available as middepth

(36 inches) and shallow-depth (32 inches) enclosures. Wall-mount rack cabinets are usually shallower than their floorstanding

counterparts, with usable mounting depths around 12, 16, 20 or 32 inches.

Load Rating

The load rating (or weight capacity) of the rack is how much weight it can safely support. Remember that you also need to

make sure that your facility’s floor can support the weight of the rack and any equipment installed inside it. If you’re installing

a wall-mount rack, both the wall and the fasteners need to be able to support the combined weight. Racks usually list two

different load ratings: a stationary/static load rating for when the rack is installed in place and a rolling/dynamic load rating

for when the rack is rolled from place to place on its casters. (The stationary/static and rolling/dynamic load ratings may be

equal if the rack has heavy-duty casters.)