Electrical Tips & Common Pitfalls for Builders, Other Contractors & DIY Enthusiasts

Whether you're a contractor or a DIY enthusiast, there are some basic principles and rules you need to be aware of if you're doing electrical work. All electricians are familiar with certain issues that come up time and time again; follow these guidelines and you'll avoid some of the major pitfalls and mistakes we see all the time, which are often both dangerous and costly to rectify.


Note that all domestic electrical work done in the UK must be carried out according to the current amendment of BS7671. The easiest way to achieve this is by hiring an electrician to do the work for you. This page is in no way intended to help non-experts understand the complex and detailed regulations. The information here should be taken as helpful advice and will not apply to every situation or circumstance. If in any doubt at all, consult a specialist.

Cable Ratings & Fault Protection

In the UK it's common to use Twin & Earth cable in domestic use. This has an insulated line (live) and neutral cable, along with an earth wire (usually thinner), all contained within sheathing. The cable is rated by the size of the conductor. For example, 2.5mm² Twin & Earth (T&E) cable has line and neutral wires with a cross-sectional area (CSA) of 2.5mm², and an earth (CPC) wire CSA of 1.5mm².


In the event of a fault condition, it's important that all wiring is protected by a device which will disconnect the supply before the wiring is damaged or catches fire. Normally this will be a fuse or a Type B MCB, both of which protect against short circuit faults.


If the wiring is too small for the fuse/MCB, or the fuse/MCB is too big for the wiring, then the safe time to disconnect the supply will not be achieved, which may lead to fire and other damage. Consequently, every piece of wiring must be protected by an appropriate upstream fuse or MCB.


In general, the following fuse/MCB values should be used as a baseline. In many situations, LOWER MCB values should be used; installation methods and derating are outside the scope of this advice.


  • 1.0mm² and 1.5mm² T&E cable: should use maximum 6A MCB (or 5A fuse). Generally used in lighting circuits.
  • 2.5mm² T&E cable: should use a 16A MCB (or 15A fuse). If in a ring configuration (see below), can use a 32A MCB (or 30A fuse).
  • 4.0mm² T&E cable: can be used on a 32A MCB (or 30A fuse).

In the UK it's common to install 32A ring mains: that is to say, the cable forms a ring from the MCB to socket to socket to socket and ultimately back to the same MCB. This allows 2.5mm² cable to be used, as the current has two alternative routes around the ring.


If you are extending a 32A ring, you have the following choices:

  1. Insert a new socket between two existing adjacent points in the ring, extending the ring. Your new socket will have two 2.5mm² cables, joining it to the two points either side of it.
  2. Spur off the 32A ring at any point, but using 4.0mm² cable.
  3. Spur off the ring using 2.5mm² cable, in which case it must be protected by a fused connection unit; the fused connection unit should either be part of the ring itself, or if spurred off the ring, 4.0mm² cable should be used to supply the fused connection unit, with 2.5mm² cable used downstream of this.

You should not use 2.5mm² cable to spur off a 32A ring unless a fused connection unit is between the 2.5mm² cable and the ring. (Yes, there is provision in the regulations for using 2.5mm² cable for a limited spur, but in our view this causes a lot of problems and should be avoided.)


Finally, be aware that accessories such as switches, sockets and light fittings are designed to be protected in the same way as the wiring. Check the manufacturer's specification and ensure the correct fuse or MCB is protecting the device. For example, most light fittings should be protected by a 6A MCB or 5A fuse; if you installed such a fitting on a 32A ring main, you would have to do so through a fused connection unit fitted with a 5A fuse.


In a nutshell, you must ensure that every cable and device attached to it is protected by the correct value fuse or MCB between it and the point of supply.


Note that use of an RCD in no way changes the fuse/MCB requirements to protect cables and devices. An RCD protects people in the event of current leakage, whereas a fuse or MCB protects cables and devices in the event of short circuits. Read more on the differences here.

Common Mistakes & Pitfalls

  • Boxing in junctions. All junctions between cables should be accessible and identifiable. If you join two cables and then bury the join beneath a floorboard or behind a wall, it's neither identifiable nor accessible. The exception is with certain types of maintenance-free connector, which in some circumstances do not need to be accessible.
    • This also applies to situations where sockets & switches are fitted in such a way they can't be removed. Never grout or seal a socket or switch to a wall. When an electrician needs to access the device for testing, they'll have to chisel out the grout, which will cost you money and make a mess.
  • Permitted cable routing zones. To avoid risk of electrocution, hidden cables must only ever be run around a room in certain permitted routes. Broadly speaking, these are:
    • Vertically or horizontally from a visible surface accessory (e.g. switch, socket or wall light fitting)
    • Within 150mm of a ceiling
    • Within 150mm of an angle formed between two adjoining walls (i.e. a corner)
    • Freely above a ceiling providing they are 50mm from the surface, or laid loosely on the upper surface with no movement constraint
    • See this visual guide from the FIS for further information. The regulations do allow for these permitted zones to be ignored providing the cable is protected in another way according to the regulations (for example in earthed metallic conduit), but the conditions are often practically difficult to achieve.
  • Earth sheathing. Twin & Earth cable comes with an earth (CPC) wire which is not sheathed. Whenever it is terminated, for example inside a socket or light fitting, green & yellow sheathing (which can be purchased separately) should be used to shroud the earth wire up to the point of connection.
  • Cable types. Flexible cable (multi-strand flex, normally white and round) is usually used where the cable is not fixed, for example between a plug and an appliance, or from a fuse to an immersion heater, because it is designed to be flexed without breaking. Single-strand wires such as those found in Twin & Earth cable (normally grey and flat) should only be used where the cable is fixed in place, as it is not designed to move repeatedly without breaking. So T&E cable should never be connected to a 3-pin plug, for example.
  • Class 1 (earthed) devices & accessories. Many metallic light fittings and switches are designed as Class 1 devices, which means they must be earthed. You must not fit a metallic light fitting to a circuit which does not have a tested earth (CPC) conductor, unless it is specifically stated to be a Class 2 device (which will be built with insulation in such a way that the metallic parts do not need to be earthed).

Case Study - Electrocuting Light Switch

After writing the start of this page, literally the very next day we did a job which highlights the danger of many of these pitfalls.


The client had reported that they frequently got substantial electric shocks from a light switch in a barn conversion.

When we attended, the first thing we found was that the earth wire in the light switch was not sheathed. Unnecessary exposed sections of live wire in the switch (see the black wire in the photo) were coming into contact with the earth wire, making the metallic box behind the switch live. This in turn was making the screws at either side of the switch live.


(Note that the black wire in the photo is actually a line wire, not a neutral as the colour suggests - tape should have been used to denote this is a switched line (live) wire.)


This in itself was a problem as it was giving potentially fatal 230V shocks to the householder. However, we knew the whole house was RCD protected, which means the RCD should have tripped immediately in the event of a large earth leakage like this; that's what they're there for. Further investigation showed that the earth wire (CPC) on this circuit wasn't actually earthed at all...


The lights this switch was connected to were metal (Class 1), and were earthed to the CPC, but the CPC wasn't connected to earth, and in fact was live. So not only were the screws in the switch live, but the metallic light fittings were also live, and within easy reach of anybody using the bed directly below them.


The final nail in the coffin of this installation was that the junction box for the lighting had been buried behind a stud wall, meaning it was not only inaccessible, but would be extremely difficult even to locate in order to drill a hole in the wall to access.


Apart from sheathing the earth wire, testing the CPC and not burying the junction box, the installer should have done R1+R2 testing to confirm the CPC was connected to earth, and Zs testing to confirm that the disconnection time would be met. Clearly neither of these tests had been carried out, otherwise these dangerous and potentially fatal problems would have been identified immediately!

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