Metal boxes which are installed flush in a wall to allow a socket, switch or other accessory to be fitted.
Metal backboxes are used where accessories such as switches and sockets are flush mounted in a wall.
There are two main sizes, generally referred to as 'single' and 'double', from the type of socket outlet that can be fitted to each. However switches, fused connection units, cooker controls and many other items can also be fitted to them.
Some manufacturers make non-standard sizes for specific accessories such as triple socket outlets or media plates which combine several items into a single plate.
Standard depths of metal backbox are 16mm, 25mm, 35mm and 47mm.
16mm are only suitable for standard light switches. They are too shallow for other items including dimmer switches and some types of decorative flat plate switch. Also known as 'plaster depth' boxes their only advantage is not requiring a recess in the wall. However as very little will fit into them, they are not recommended.
25mm are the standard depth and can accept most normal sockets and switches.
35mm are required for 'flat plate' accessories and devices such as USB socket outlets where more depth is required. They are also useful where walls are to be lined with plasterboard, as the holes for wiring are set back from the front edge by approximately the thickness of standard plasterboard (12.5mm).
47mm are normally only used where larger size wiring is required such as a cooker switch or shower isolator.
Most boxes have 20mm diameter holes partially cut, which can be knocked out with a heavy object. Conduit can be attached to these holes using suitable adaptors, as can most other types of 20mm cable gland.
Where cables will enter directly (such as twin and earth cable), PVC grommets should be fitted to prevent the sharp metal edges causing damage to the cables.
Some 47mm depth boxes have 25mm knockouts in addition to the 20mm ones.
Two threaded lugs are provided to secure the accessory to the box. These are metric thread, 3.5mm diameter, 0.6mm pitch. Screws are normally supplied with the accessory, but can be purchased separately particularly if longer screws are required.
Older boxes from many decades ago may have 4BA threads which are not compatible with modern M3.5 screws.
Some old boxes have 4 fixing lugs, 2 on each longer edge. These were used with sockets from the same time period. Boxes and sockets like this haven't been available for decades, however most of the boxes from that era also had the 2 lugs either side so modern accessories can be fitted to them.
Extension studs are available for situations where the box is deeply recessed and the accessory fixing screws are too short.
Clip on flanges can be used to mount a metal box into a plasterboard wall - however purpose made plastic boxes are available for that situation.
If the thread in an existing box has been damaged, it can be repaired using a rethreading tool. This avoids having to replace the box.
Extension frames are an alternative to using longer fixing screws.