Installing a fused connection unit (FCU)

LAST UPDATED: 11th January 2023 by The Editorial Team

Electrician wiring a FCU

Summary: Step by step guide and video showing how to prepare and wire a fused connection unit (FCU).

How to wire a fused connection unit (FCU)

* Notifiable project requiring Building Control approval if installation is located in a kitchen, bathroom or other special location.

A FCU is a socket with an integral fuse used for connecting high-wattage appliances, such as cookers, extractor fans, central heating timers and heaters, to a circuit. You can connect a FCU to a circuit by replacing an existing socket outlet with a FCU, or by running a spur from a socket outlet on a ring main circuit or from a radial circuit. FCUs can be flush-mounted which allows the cable to be concealed in the wall, or wall mounted with the cable covered by trunking. 

A FCU is fitted with a fuse holder in the face-plate that takes a 3 amp fuse for appliances using up to 700 watts and a 13 amp fuse for appliances using higher wattage. They may also have a neon indicator light. FCU units can come with or without a switch but if it is not fitted with a switch, the appliance it supplies must have an on/off switch.

A fused connection unit.

Preparation

  • Cut off the power supply to the relevant circuit from the consumer unit.
  • Remove the fuse or switch off the MCB protecting the circuit you will be working on. All appliances connected to the circuit should be unplugged.
  • If the FCU is replacing an existing socket outlet, remove the face-plate of the socket and release the cores from the terminals.
  • Alternatively, if the FCU is being fitted on a new spur, plan the cable route to where the FCU mounting box is to be positioned. Fit the FCU at a convenient height e.g. above a worktop, so it is easily accessible.
  • If the cable from the FCU to the appliance is to be concealed in the wall, use two-core-and-earth cable and not flex.
  • The mounting box for a FCU is the same type used for socket outlets.
  • Prepare the end of the cable, flex and cores.
  • If any Earth cores are bare, cover the exposed end with a green-and-yellow sheath.

Wiring a FCU

A FCU is fitted with two sets of terminals. The Live and Neutral terminals will be marked Load or Out and Mains or In. Be aware that the Mains terminals can also called Feed or Supply.

  • Feed the flex from the appliance into the FCU mounting box.
  • Connect the brown (live) core from the flex to the Live terminal marked L and Load (or Out).
  • Connect the blue (neutral) core from the flex to the Neutral terminal marked N and Load (or Out).
  • Connect the yellow and green core from the flex to one of the Earth terminals. Some FCUs may only have one Earth terminal in which case delay connecting the earth core from the flex until you are ready to connect the earth core from the cable, and connect them at the same time.

Now connect the spur cable to the FCU.

  • Feed the spur cable into the FCU mounting box, pull enough cable through to connect it to the face-plate.
  • Prepare the ends of the cable and the cores.
  • If any earth cores are bare, cover the exposed end with a green-and-yellow sheath.
  • Connect the red (live) core from the cable to the Live terminal marked L and Mains.
  • Connect the black (neutral) core from the cable to the Neutral terminal marked N and Mains.
  • Connect the earth core fitted with a yellow/green plastic sheath from the cable to the second earth terminal. If there is only one earth terminal connect the earth cores from both the cable and the flex to it. 
  • If you have a metal mounting box, ensure it is earthed by fitting a flying earth from the terminal in the face-plate to the mounting box using a small length of cable core covered in yellow/green sheath.
  • Carefully fold the cables into the mounting box.
  • Screw the face-plate to the mounting box.
  • Fit the appropriate fuse for the appliance (3 or 13 amp) into the fuse holder in the FCU face-plate.

Tools Needed

  • Insulated screwdriver
  • Wire cutters/strippers
  • Trimming knife

Materials needed

  • Cable OR flex (If wall mounted)
  • Green-and-yellow sheath
  • Flying earth for metal mounting boxes
  • A 3 OR 13 amp fuse

Important note

The colour-coding system for electric cable changed in 2006, make sure you read Electric cable and flex to view the new information.

How to install a FCU video

Author

The Editorial Team

The Editorial Team at lets-do-diy.com are a team of experts led by Mark Wilson. We have been creating DIY projects, articles, costs guides and product reviews since 2008. For more info see our about page.