How to choose the right electrical back box

Helpful video reference. John Ward of jwflame covers the full range of plastic pattresses and boxes used in UK electrical work in "Plastic Pattresses and Boxes for UK Electrical Accessories". John is a Dorset-based qualified electrician who has been publishing UK electrical guides on YouTube since 2014. He handles each type of box and explains when to use it, which makes this video a practical primer before you go to the wholesaler.

1. Identify the wall construction

Tap the wall firmly. A hollow sound with a slight bounce means you are on plasterboard over timber or metal studs, sometimes called a partition wall or dry-lining. A dull solid thud means brick, block or concrete behind the plaster. The two types need different boxes and different fixing methods.

Some older houses have a solid outer skin but hollow internal walls. Do not assume all walls in a house are the same type. The wall at the back of a chimney breast, for example, is often a different construction to the walls on either side of it.

2. Surface or flush?

A surface-mounted box (pattress) screws directly to the face of the wall. The box itself is visible, sticking out by 25-47 mm. Surface mounting is faster because there is no chopping, and it is reversible because you have not damaged the wall. It is the right choice for garages, workshops, utility rooms, loft conversions, and anywhere where looks are secondary to speed and practicality.

A flush-mounted box sits inside a recess cut into the wall so the faceplate finishes level with the plaster. It is the standard for living areas, kitchens and bedrooms. In solid masonry, you cut a chase with a cold chisel and bolster, or an angle grinder with a dust shroud, to the depth of the box. In plasterboard you cut a rectangular hole and use a dry-lining box that grips the board from behind.

3. Gang size

Gang describes the number of switch or socket positions side by side on the faceplate:

• Single gang - one standard 86 x 86 mm unit. Used for one socket outlet, one switch, one data point.
• Double gang - one 146 x 86 mm unit. Fits a double socket, twin switch, or two data ports side by side.
• Grid systems - modular plates where you choose how many modules fit in a frame. Used mainly in commercial work but sometimes specified in modern kitchens for a cleaner look.

UK accessories use a standard 86 mm centre spacing for fixing screws on single gang plates. European accessories do not share this standard, so a continental socket will not fit a UK back box without an adaptor plate, and vice versa.

4. Depth

Depth is where most mistakes happen. The standard depths for UK flush boxes are 16 mm, 25 mm, 35 mm and 47 mm. Most 13A single or double socket outlets need 25 mm. Some accessories need more:

• Sockets with USB-A or USB-C charging ports: usually 35 mm, sometimes 47 mm. Check the manufacturer's data sheet.
• Shaver socket outlets: often 35-47 mm because of the transformer inside.
• TV coaxial plates and RJ45 data sockets: usually 25-35 mm, but check for angled patch through designs that may need more.
• Metal-clad industrial accessories: 25 mm is usually adequate, but check.

If the back box is too shallow, the faceplate will not sit flat against the wall. Forcing it will either crack the plastic or put tension on the wiring. If the box is too deep, the conductors will rattle inside and the earth tail may not reach the earth terminal on the faceplate.

5. Plastic pattress or steel back box?

For surface work, plastic pattresses are standard. They are light, cheap, easy to drill out for cable entry, and do not corrode. For flush work in solid walls, steel back boxes are the professional choice: they are more rigid, they do not crack when the cable is under tension, and they provide a better earth continuity path when used with metal-clad accessories.

If you are fitting a metal-clad socket or switch, you should earth the back box itself via a short earth tail from the box earth terminal to the earth terminal on the accessory. A plastic box cannot do this, though some plastic boxes have a separate earth terminal fitted; check if yours does.

6. Dry-lining boxes for plasterboard

Dry-lining boxes are designed specifically for plasterboard walls. After you cut the rectangular hole, you push the box through from the front, then tighten the screws on the lugs. The lugs rotate and grip the back face of the plasterboard, pulling the box tight against it. No plugs, no chiselling, no mess compared to solid walls.

Two important rules: first, cut the hole neatly to the size specified in the box instructions - a slightly too-large hole means the lugs overlap the gap and do not grip. Second, never fit a heavy-duty socket with thick, stiff cables into a dry-lining box if there is room to put a timber noggin behind the board first. A noggin gives you a solid fixing and prevents the box working loose over time as the cable is plugged and unplugged.

When to call us

Changing a like-for-like back box on an existing socket or switch is within most DIYers' capability once the circuit is isolated. The wiring side, adding a new circuit or extending a ring, needs a registered electrician. If you are in any doubt about what you are looking at inside the wall, Richard is based in Sandwich and covers east Kent.