Choosing a rack cabinet: U height, depth, wall-mounted or floor-standing

What is a U unit, and how is height calculated?

Network and server cabinets have mounting rails 19 inches wide, and height is measured in U units. One U is about 4.45 centimeters. A patch panel or a small switch is usually 1U, many servers are between 1U and 2U, and a rack-mount UPS takes 2U or more depending on the model. The value written on the cabinet, for example 9U or 42U, is the total device height that will fit inside.

For the calculation, each device is listed one by one and their heights are added up. Cable organizers placed between patch panels, shelves, fan modules and power distribution units also take up space and are added to this list. On top of the total, space is left for devices to be added later.

A common mistake is choosing the cabinet to fit today's devices exactly. A year later there is no room for a switch that is added or a second recorder, and the device is placed on top of or beside the cabinet. This spoils the cable layout and ventilation.

Wall-mount or floor-standing?

Wall-mount cabinets hang on the wall, take up less space and suit small installations. In a shop, a small office or a floor distribution point, a wall-mount cabinet is enough for a few patch panels, a switch and a modem. Their depth is limited, and the weight they can carry depends on how sturdy the wall and the mounting are.

Floor-standing cabinets sit on the floor and are deeper and taller. They are used for systems housing servers, a rack-mount UPS, large recorders and many switches. Heavy devices go at the bottom of the cabinet and light ones on top. This keeps the cabinet's center of gravity low and reduces the risk of tipping over.

Trying to fit heavy equipment such as a recorder and UPS into a wall-mount cabinet makes mounting difficult and leaves no space behind the devices for cables and air.

Depth and width

Cabinet depth is chosen according to the deepest device that will go inside. Switches and patch panels are shallow, but servers and rack-mount UPS units are deep. The bend allowance for the power and data cables at the back is added to the device's depth. A cabinet that will house servers must be noticeably deeper than one that will house only switches.

There are two common choices for width. In narrow cabinets little space is left between the rail and the side panel, and the cables have to pass in front of the devices. In wide cabinets there is room at the sides for a vertical cable channel, and many patch cables are carried neatly. In a cabinet with several 48-port switches and patch panels, a wide model makes the cable layout easier.

Expert note: Before buying the cabinet, look at the technical datasheet of your deepest device and choose the cabinet size accordingly. Add to the depth stated on the datasheet the allowance for cables to curve at the back; with the exact size, the cabinet's rear door will not close.

Ventilation and door type

The devices inside the cabinet generate heat. Air needs to enter at the front of the cabinet and leave at the back; most devices are cooled in this direction. Perforated front and rear doors make airflow easier. In cabinets with glass doors and closed side panels airflow is limited, and top fans or a fan module are needed.

In small cabinets a top fan may be enough, but the room the fan pushes hot air into also needs to be cool. In a Şanlıurfa summer, adding a fan to a cabinet in a room without air conditioning only stirs up the hot air inside. Cabinet selection is considered together with room cooling.

Empty U spaces in the cabinet are closed with blanking panels. Open gaps let the hot air behind the devices circulate to the front, so the devices draw in their own hot air again.

Load capacity, floor and fixing

The load a cabinet can carry is stated by the manufacturer, and a full cabinet can be heavy. Rack-mount UPS units and their batteries are the heaviest parts of the cabinet. If the cabinet comes on wheels, its feet should be lowered and fixed after installation. A full cabinet standing on wheels can slide or tip over when pushed.

The points to check when choosing are these:

• The cabinet's static load capacity
• Rails with adjustable depth
• Shelves or sliding rails for heavy devices
• Leveling feet and the option to fix it to the floor
• Top and bottom openings for cable entry
• Grounding connection points

Lock and access

The cabinet door lock prevents unauthorized people from reaching the devices and cables. This is especially important for cabinets in shared areas such as schools, hotels and public institutions. An unlocked cabinet in a corridor can bring down the whole network in the hands of a curious student or a cleaner who pulls a plug by accident.

Good practice is to have separate locks for the front and rear doors, side panels that can be removed from the inside, and a record of who holds the keys. In critical server rooms, the cabinet lock forms a layer of security together with the room's own access control.

Where to put the cabinet

Enough space should be left in front of and behind the cabinet for maintenance. Changing cables is very hard in a floor-standing cabinet pushed against a wall whose rear door cannot be opened. The cabinet should not be placed under water pipes, in front of a window or against a wall that gets sun.

The location and size of the cabinet are planned together with the cable routes in the building. Floor distribution cabinets are placed where the farthest point on each floor stays within the 100-meter cable limit.

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