How to select a cabinet.

Consider your requirements one at a time.

The sheer number of available options and variations can make choosing a data or server cabinet a daunting task. But, by considering your requirements one at a time, you can zero in on the enclosure that fits your needs best.

Size.

Before you choose a cabinet size, you need to determine what equipment you need to house. This list can include CPUs, monitors, keyboards, modems, servers, switches, hubs, routers, and UPSs. Consider the depth of your equipment —servers require extra depth and may require a cabinet that’s at least 42 inches deep. Lastly, remember to allow space for cabling and power provisions.

A standard full-sized cabinet is 84" high. This is the height that makes the most efficient use of floor space in rooms with standard-height 8-foot ceilings.

For tight spaces, you can choose a smaller cabinet or even a wallmount cabinet, but unless space is limited, it’s better to go larger rather than smaller—there always seems to be just one more thing that needs to go into a cabinet, plus extra space is always useful for cable management.

Rails.

A main cabinet component is one or two pairs of vertical rails with mounting holes to which you attach your equipment or shelves. The most common spacing between rails is 19 inches with hole-to-hole centers measuring 18.3 inches. 19-inch rails actually predate modern computers, having originated with racks made to hold railroad signaling equipment. A less common standard is 23-inch rails. Most rackmount equipment is made to fit 19-inch rails but can usually be adapted to fit the larger 23-inch rails.

A cabinet usually has two pairs of rails — one in front and one in back — so equipment is supported at all four corners. The two pairs of rails may be adjustable front to rear to accommodate different equipment depths.

Cabinet rails have holes for mounting equipment. Although these holes can be 12-24 or 10-32 tapped holes for use with a bolt, the M6 square hole has become the dominant standard for cabinet rails.

Because of the danger of stripped threads, tapped holes are commonly used only with equipment that is rarely changed.

Square M6 holes are adapted for use with bolts through the use of snap-in cage nuts, so stripped threads caused by frequent equipment changes are not a problem. M6 holes are also used for boltless mounting of servers designed to hook onto these holes.

Cabinet rails and the equipment that fit in them are measured in rack units, abbreviated “U.” One rack unit (1U) is 1.75 inches of usable vertical space. So, for example, a rackmount device that’s 2U high takes up 3.5 inches of rack space. A rack that’s 20U high has 35 inches of usable space. This measurement is marked on the rails.

Because rail width is standard, the amount of vertical space is what determines how much equipment you can install. Remember that a cabinet’s measurement in rack units is smaller than its external height.

Doors.

The first decision you need to make about cabinet doors is whether you need them at all. Doors offer protection to equipment and can make cabinets look neater and more attractive, but within a protected data center, they often serve little use and just get in the way.

If you choose to have cabinet doors, there are many options. Common cabinet door choices are perforated, mesh, plexiglass, polycarbonate, and solid metal.

Because ventilation is important, choose perforated or mesh doors for air movement. If, however, you’re using an enclosure blower or other ventilation scheme that moves air from the bottom to the top of the cabinet, you’ll require solid doors. Doors are also available with built-in fans to help move air through the cabinet.

In areas with limited space, choose split doors, which require less clearance to open.

If cabinet doors are there for security, be sure you select an appropriate lock. Key locks are standard, but can be problematic because of lost keys and the ease with which keys can be duplicated. Because of the key issue, combination locks are preferable for cabinets that must be accessed frequently yet kept secure. The most secure lock option is a biometric lock that requires a finger scan.

Side panels.

Like doors, side panels are optional and may be omitted in protected data centers, except where they’re needed to control ventilation.

Without doors or sides, a cabinet starts to look a lot like a four-post rack. The difference is that you could add doors or sides to the cabinet should they become needed, but you cannot add them to a rack. Also, a cabinet usually has adjustable rails whereas a four-post rack typically does not.

Moisture and dust resistance.

When you’re housing delicate electronic components outside of a protected data center —for instance on a factory floor—look for a cabinet with a NEMA rating.

The National Electrical Manufacturers’ Association (NEMA) specifies guidelines for cabinet certifications. Common NEMA ratings for data cabinets include NEMA 12 and NEMA 4X.

• NEMA 12 cabinets are constructed for indoor use to provide protection against contaminants including falling dirt; circulating dust, lint, and fibers; and dripping or splashing liquids. Protection against oil and coolant seepage is also a prerequisite for NEMA 12.
• NEMA 4X cabinets are weather-proof enclosures suitable for indoor or outdoor applications offering protection from windblown dust, rain, or hose-directed water. NEMA 4X cabinets are also corrosion resistant.

Assembled or not.

Order a preassembled cabinet if you need to set up your installation in a hurry — you can load your equipment as soon as the cabinet arrives.

A flat-pack cabinet, on the other hand, requires some time and effort to assemble, but can be maneuvered through narrow doors and spaces that may not accommodate a full-sized assembled cabinet.

Power provisioning.

Cabinets need provisions for powering enclosed equipment and for managing the cables associated with the equipment.

Rackmount power strips mount either vertically or horizontally with a wide range of outlet counts, types, and spacing.

Power distribution units (PDUs) are high-end power strips that include surge protection and often have additional capabilities such as remote management and metering. PDUs may be rackmounted or mount behind the rails—in which case they don’t take any rack space. When choosing a vertical (0U) PDU, make sure it’s compatible with your cabinet for ease of mounting.

Uninterruptible power supplies (UPSs) are an important part of power provisioning. If you’re planning to use a rackmount UPS, it’s important to allot space when planning the cabinet.

UPS units are typically mounted in the bottom of a the cabinet because of their weight. Alternatively, several UPS units may be housed in a separate cabinet in the data center.

Cable management.

Although most cabinets come with cable management features, you will still need to plan for some additional organizers and raceways to keep cables under control.

Plotting your connections in advance helps you to decide how to organize the cables. Knowing where the connectors are on your equipment tells you where it’s most efficient to run cables horizontally and where it’s better to run them vertically.

The important thing is to have a plan — if you let your cables get away from you now, you’re sure to pay for it down the road.