A dry lining box only has cable entries top bottom and back not side. How much wider than the box height can a safe zone be to allow a horizontal cable run to enter the box at the top or bottom. The "extra" only needs to be the width of the cable. It is not possible to come into the back of the box as the wall is a sound reducing wall constructed with resilient bars and insulation filled cavity. The gap between the studding and plasterboard created by the resilient bar is convenient for the cable and makes it possible to ensure the cable is kept clear of the insulation. Coming into the back of the box would mean coming through the insulation. Thanks