Reserved length for entering and exiting the distribution box

The reserved length for a distribution box should be approximately 20 mm larger than the box dimensions, with depth accounting for the box plus wall plastering.

Reserved Hole Dimensions

When preparing a wall for a distribution box, the reserved hole should exceed the box's length and width by about 20 mm to allow for proper fitting and adjustments during installation. The reserved depth should equal the thickness of the distribution box plus the thickness of the plaster or wall finish covering the hole. This ensures the box sits flush with the finished wall surface and allows for secure mounting.

Box Depth and Conductor Clearance

For boxes enclosing devices supplied by 12 or 10 AWG conductors, the minimum internal depth is 30. 2 mm (13/16 in.). If the equipment projects rearward from the mounting plane by more than 25 mm (1 in.), the box depth must be at least the equipment depth plus 6 mm (1/4 in.).

For conductors entering the center portion of the rear wall opposite the equipment, the minimum clearance increases to 13 mm (1/2 in.) to prevent insulation damage during installation. Side-entry conductors must also maintain a minimum clearance of 13 mm (1/2 in.) from the box wall.

Installation Considerations

The box should be installed firmly, horizontally, and vertically, with vertical deviation not exceeding 3 mm. There should be no gaps around the box; edges should be close to the wall, and contact surfaces coated with anti-corrosion paint. When buried in masonry, the box can be fixed by filling concrete between the box and wall.

Proper spacing ensures safe conductor bending, prevents overcrowding, and allows heat dissipation, reducing fire risk and maintaining insulation integrity. By following these reserved length and depth guidelines, the distribution box will be properly aligned, accessible, and compliant with electrical installation standards.