What is the difference between nominal cabinet depth and usable internal depth?

The nominal cabinet depth describes the overall depth class of the enclosure — for example 300mm, 450mm, 550mm or 600mm.

The usable internal depth is the space available inside the cabinet after allowing for the front door, rear structure and other parts of the enclosure. It is therefore smaller than the nominal depth.

Approximate usable internal depths are 250mm for a 300mm cabinet, 400mm for a 450mm cabinet, 500mm for a 550mm cabinet and 550mm for a 600mm cabinet.

Usable internal depth should not automatically be treated as suitable equipment depth because connectors, cable bends, rail position and front-door clearance also require space.