Selecting the correct wire gauge for a given electrical project is essential. Whether you’re looking to add a new socket, install lighting, wire an electric shower, or run a cooker circuit, getting your cable size right will be crucial for avoiding such risks as overheating, voltage drop, or tripped breakers. In the worse-case scenario, the wrong size of cable could even lead to fire.
The UK Context of Deciding on the Suitable Wire Gauge
We’re writing this guide for the benefit of UK readers, which is why you’ll see us referring to the IET Wiring Regulations. Also often referred to as BS 7671, this is the standard that UK electrical installations must comply with. It dictates such aspects as current-carrying capacities, voltage drop limits, and protective device ratings.
So, let’s take you through the essentials of deciding on the appropriate wire gauge for your installation in the UK, presuming a common domestic scenario.
Just to add as a quick note: much electrical work in the UK is notifiable. Part P of the Building Regulations requires most work to be undertaken or certified by a qualified, registered electrician. So, if you aren’t competent, you should always hire a professional.
What Do We Mean By ‘Wire Gauge’, Anyway?
The term “wire gauge” refers to the measurement of a wire’s physical diameter or thickness. This, in turn, directly determines the electrical capacity, resistance, and weight of the wire.
However, while some countries use American Wire Gauge (AWG), it is customary in the UK to use metric sizes based on the cross-sectional area (CSA) of the copper conductor in square millimetres (mm2).
This difference between national practices can cause confusion for someone in the UK who is following a tutorial from a US source. This is because while AWG works on the basis of higher numbers meaning thinner wire, metric sizing is intuitive; the higher the number, the greater the current the wire can handle.
If, then, you are a person in the UK consulting a US-based online guide, it can be a good idea to bookmark an AWG-to-metric conversion calculator to help you along. For example, 14 AWG is roughly equivalent to 2.5mm2.
3 Steps to Make the Right Choice of Wire Gauge for Your Installation
Going through the below processes will enable you to decide on the appropriate sizes of electrical wires for every installation you take on:
- Match The Cable to The Circuit
A commonly made mistake among people undertaking an electrical project, is using a cable that cannot handle the “load” (amps). Under-sizing can lead to overheating and even fires.
So, here are some of the cable sizes you might consider for different circuits:
- 1.0mm2 to 1.5mm2 are standard measurements for lighting circuits, usually protected by a 6A or 10A breaker.
- 2.5mm2 is the go-to for standard 13A socket outlets (ring or radial circuits).
- 4mm2 to 6mm2 is the required range for higher-power appliances such as electric ovens or hobs.
- 10mm2 is essential for heavy-duty applications like electric showers.
- Account For Voltage Drop
In the event of your cable run being particularly long (for example, running power to a garden shed), the resistance of the wire will cause voltage to “drop” by the time it gets to the end.
This may have implications for your chosen cable size:
- Be mindful of the 4% rule. You may have read that according to the BS 7671 regulations, for a standard 230V voltage supply, the voltage drop should not go beyond 4% (approximately 9.2V). However, 4% is an approximate “shorthand” rule; to be more specific, BS 7671 sets a maximum voltage drop of 3% for lighting circuits, and 5% for other circuits like sockets and showers.
- This means than if your run is long, you must upsize the cable to compensate for such resistance. For example, you may decide in favour of 4mm2 instead of 2.5mm2.
- Consider The Environment
A wire’s “current-carrying capacity” changes depending on how it is installed.
- Insulation: if a cable is buried in thermal loft insulation, it won’t be able to dissipate heat as easily. This will mean it needs to be “de-rated” (in other words, treated as having lower capacity).
- Grouping: the cooling ability of cables will also be reduced if they are run together in the same conduit.
Never Underestimate the Importance of Cable Size
Remember: selecting the right size of wire isn’t merely about what “fits”. That’s because the decision you make will very much affect your electrical installation’s safety, compliance, and futureproofing.
By enlisting the services of a Part P registered electrician for your project, you can greatly help to stay safe, while avoiding potentially expensive (and dangerous) mistakes.
