In this article, originally written for IET Wiring Matters magazine, James Eade seeks to dispel some of the myths surrounding the use of flexible cables in fixed installations.
The content of this article was inspired by a comment posted on the IET EngX forum which stated: “Flexible cables are not allowed in fixed installations.” This myth is not uncommon.
Its origins are unclear but appear to date back to the 15th edition of the IEE’s Wiring Regulations (if not earlier) and are thought to have arisen from the types of insulating materials used for flexible cables in years gone by.
In the 16th Edition, there were definitions for flexible cables (i.e., a cable designed to be flexible in use) and flexible wiring (flexible cable with conductive cross-sectional areas (csa) of 4 mm² or less), but no reference to them is specifically prohibited.
Like a breakup, memories of some old demands linger. In the 16th edition (including AM2, 2004), Regulation No. 521-01-04 states:
“Flexible cable or flexible wire may only be used for fixed connections if the relevant provisions of the regulations are met.”
This requirement does not appear in a negative light, giving the impression that flexible cables can be used as a last resort. Interestingly, this requirement still exists today in BS 7671:2018+A2:2022 in Regulation 521.9 as follows:
521.9 Use of flexible cables
521.9.1 Flexible cable should only be used for fixed connections if the relevant provisions of BS 7671 are met. Flexible cables used for fixed wiring should be of the heavy type unless the risk of damage during installation and service, due to impact or other mechanical stresses, is low or It is reduced to a minimum or protection against mechanical damage is provided.
Note: Descriptions of the light, normal and heavy types are given in BS EN 50565-1.
Continuing the current edition, Regulations 521.9.2 and 521.9.3 require the use of flexes to connect equipment that may be moved during use. The definition of flexible wiring has also been deleted, leaving only the flexible cables listed in Part II. However, there is still no ban on their use.
The main differences between installation cables and flexible cables are the use of Category 5 stranded conductors instead of Category 1 (rigid) or Category 2 (semi-stranded), along with differences in insulating materials to allow flexibility.
These cables are used widely from construction sites to events (where they are almost exclusively used), as well as, of course, as final connections in lighting fixture installations or from fused connection units to equipment such as heaters, for example.
Having decided to connect your installation with a flexible cable, what are the “relevant provisions of BS 7671” referred to in 521.9.1? In addition to adhering to the general rules, there are some basic points to note in this:
• Flexible cable must have the appropriate mechanical strength for the application as set out in the note to the regulation referring to BS EN 50565-1. This standard describes different categories, summarized as follows:
1. The term “heavy duty” describes the stresses that can be expected, for example, in industrial and agricultural buildings.
2. “Normal duty” describes the stresses that can be expected, for example, in domestic, commercial and light industrial applications.
3. “Light duty” applications may include flexible cabling for small devices.
4. “Very light duty” applications are, for example, very small devices such as electric shavers, mobile phone chargers, etc. Where this protection cannot be achieved using the cable alone, appropriate additional mechanical protection must be provided.
● The temperature rating of the flexible cable must be suitable for the connected wiring accessories (Regulation 526.4).
● Wire ends may need to be terminated with a suitable termination method as required by Regulation Group 526.9. Note 3 to the IET Directive contains details of suitable wiring accessory terminals and their markings in Table 2.1.
● In general, all load current capacity and voltage drop tables in Appendix 4 are for Class 1 and Class 2, except Tables 4F1 to 4F3. Correction of current carrying capacity and voltage drop may be needed if schedules for other cables are used, such as PVC cables complying with the insulation and temperature requirements in Table 4D2. Information is available in Section 2.4 of Appendix 4.
Apart from being prohibited, flexible cables are required in some parts of BS 7671:2018+A2:2022 examples include:
● Regulation 422.3.201 allows them to be used in locations where there is a certain risk of fire, although there are some requirements for their construction or protection as detailed in Regulation 521.9.
● Regulation 413.3.4 includes special requirements regarding their use in parts of the facility with double protective measures or enhanced insulation.
● Regulation 418.3.6 for electrical disconnection of more than one item of equipment requires flexible cables that include a protective conductor.
● Required for use in equipment using suspended current (Regulation 522.7.2).
● They are required for installations in flexible structures, or where the structures are to be moved (Regulation 522.15.2).
● Minimum cross-sectional areas for flexible cables for “any other application” are given in Table 52.3.
● Flexible cables are required to connect equipment that may be moved regularly or occasionally during use (Regulations 521.9.2 and 521.9.3).
● Flexible cables are required as a means of connecting equipment with high protective conductive current as detailed in Regulation 543.7.1.202.
● Some Part 7 special locations specify specific wiring systems in which flexible cables are options, or required as stated in Regulation 522.15.2: (Regulation 704.522.8 Group, Regulation 711.52, Regulation 717.411.3.1.2, Regulation 717.52 Group, Regulation 721.521 .2, Regulation 740.521.1, Regulation 740.55.1.1).
As always, it’s worth reviewing the basic principles in Part 1. Relevant here are Regulations 132.6 and 132.7 for the cross-sectional area of conductors, types of wire and installation methods, as well as the set of Regulations 133 for the selection of equipment.
Flexible cables can tick all the boxes, and if the extra cost over more rigid types isn’t a concern, there’s no reason why they can’t be used.
With thanks to Graham Kenyon for his contributions to this article, originally published in May 2023 for IET Wiring Matters.
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