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Whilst the current-carrying capacity of a cable is dependent on a number of factors, this article looks at the impact of thermally insulating materials on the current carrying capacity of flat twin and earth cables installed in domestic premises.
A cable that is run in a space where thermal insulation is likely to be applied, such as in the loft of a dwelling, should preferably be installed so that contact with the thermal insulation is avoided. Otherwise, the operating temperature of the conductors may be exceeded (70˚C for thermoplastic (pvc) insulated conductors), possibly leading to a reduction in the service life of the cable or damage to the insulation of the conductors or adjacent material (Regulation 523.9 of BS 7671 refers).
However, it is common practice for thermally insulating materials to be installed in the walls and ceilings of domestic and similar premises to satisfy the energy efficiency and noise requirements of Building Regulations. Consequently, as shown in Fig 1, contact with thermal insulation is generally unavoidable for most twin and earth cables installed in domestic and similar premises.
So that cables installed in such locations will remain sufficient to meet the requirements of BS 7671, the cross-sectional area of conductors may need to be increased to compensate for any reduction in the current-carrying capacity due to thermal insulation.
Two different cases that need to be considered are:
Where a cable is to be totally surrounded by thermal insulation for less than 0.5 m, the current-carrying capacity of the cable must be reduced appropriately, depending on the size of cable, length in the insulation and thermal properties of the insulation. The rating factors (Ci) shown in Table 52.2 of BS 7671 should be applied, as appropriate, for conductor sizes up to 10 mm<sup>2</sup> in thermal insulation having a thermal conductivity (λ) greater than 0.04 Wm<sub>-1</sub> K<sub>-1</sub>.
For cables installed in a thermally insulating wall or above a thermally insulating ceiling - the cable being in
contact with a thermally conductive surface on one side - it is not necessary to apply a rating factor for thermal insulation from Table 52.2.
For such purposes, tabulated current carrying capacities are given in Appendix 4 of BS 7671 based on the Installation Method used. The electrical installation designer must decide which of the methods listed in Table 4A2 of Appendix 4 is the most appropriate. Installation Methods 100 – 103 (typicallynassociated with new domestic and similar premises) are specifically for flat twin and earth cable in thermal insulation. Where cables are installed using these Installation Methods, the current-carrying capacity of the cables should be determined using Table 4D5 of Appendix 4 (70ºC thermoplastic insulated and sheathed cable with protective conductor).
In Fig 2, the current-carrying capacity of a 2.5 mm<sup>2</sup> flat twin and earth cable is shown in brackets for each of these Installation Methods. The values shown illustrate the impact of these Installation Methods on the current-carrying capacity of the cable. Particularly, considering a 2.5 mm<sup>2</sup> conductor clipped direct (Reference Method C) has current-carrying capacity of 27 A (Table 4D5 of Appendix 4 refers).
It should be noted that where more than one Installation Method is applied for a circuit route, the method which provides the lowest current-carrying capacity (worst case scenario) should be used to determine the cable size.
For Installation Methods 100 – 103, Table 4A2 does not attribute a letter to indicate the Reference Method. Therefore, when compiling the electrical certificate, the Installation Method number should be recorded in the box entitled ‘Reference Method’ in the Schedule of Circuit Details (SOCD) for the particular circuit.
For more information on installation practices refer to the Site Guide for Electrical Installations publication.
For other guidance and publications please see the NICEIC website. For information about the NICEIC Approved Contractor or Domestic Installers schemes, visit www.niceic.com
Brand:
NICEIC
Topic:
Regulations and Legislation
Certification Schemes
BS7671
Cables Wiring and Wiring Accessories
MV & LV Cables & Accessories
Standards Relating to Cables
Thumbnail image:
Teaser:
This article considers the impact of thermal insulation on the current carrying-capacity of flat twin and earth cables.
Type:
Technical Articles
Main image(s):
Links
05/12/2017
Target group:
Voltimum
View the full article
A cable that is run in a space where thermal insulation is likely to be applied, such as in the loft of a dwelling, should preferably be installed so that contact with the thermal insulation is avoided. Otherwise, the operating temperature of the conductors may be exceeded (70˚C for thermoplastic (pvc) insulated conductors), possibly leading to a reduction in the service life of the cable or damage to the insulation of the conductors or adjacent material (Regulation 523.9 of BS 7671 refers).
However, it is common practice for thermally insulating materials to be installed in the walls and ceilings of domestic and similar premises to satisfy the energy efficiency and noise requirements of Building Regulations. Consequently, as shown in Fig 1, contact with thermal insulation is generally unavoidable for most twin and earth cables installed in domestic and similar premises.
So that cables installed in such locations will remain sufficient to meet the requirements of BS 7671, the cross-sectional area of conductors may need to be increased to compensate for any reduction in the current-carrying capacity due to thermal insulation.
Two different cases that need to be considered are:
- cables surrounded by thermal insulation, and
- cables routed in a thermally insulating wall or above a thermally insulating ceiling, the cable(s) being in contact with a thermally conductive surface on one side.
Where a cable is to be totally surrounded by thermal insulation for less than 0.5 m, the current-carrying capacity of the cable must be reduced appropriately, depending on the size of cable, length in the insulation and thermal properties of the insulation. The rating factors (Ci) shown in Table 52.2 of BS 7671 should be applied, as appropriate, for conductor sizes up to 10 mm<sup>2</sup> in thermal insulation having a thermal conductivity (λ) greater than 0.04 Wm<sub>-1</sub> K<sub>-1</sub>.
For cables installed in a thermally insulating wall or above a thermally insulating ceiling - the cable being in
contact with a thermally conductive surface on one side - it is not necessary to apply a rating factor for thermal insulation from Table 52.2.
For such purposes, tabulated current carrying capacities are given in Appendix 4 of BS 7671 based on the Installation Method used. The electrical installation designer must decide which of the methods listed in Table 4A2 of Appendix 4 is the most appropriate. Installation Methods 100 – 103 (typicallynassociated with new domestic and similar premises) are specifically for flat twin and earth cable in thermal insulation. Where cables are installed using these Installation Methods, the current-carrying capacity of the cables should be determined using Table 4D5 of Appendix 4 (70ºC thermoplastic insulated and sheathed cable with protective conductor).

In Fig 2, the current-carrying capacity of a 2.5 mm<sup>2</sup> flat twin and earth cable is shown in brackets for each of these Installation Methods. The values shown illustrate the impact of these Installation Methods on the current-carrying capacity of the cable. Particularly, considering a 2.5 mm<sup>2</sup> conductor clipped direct (Reference Method C) has current-carrying capacity of 27 A (Table 4D5 of Appendix 4 refers).

It should be noted that where more than one Installation Method is applied for a circuit route, the method which provides the lowest current-carrying capacity (worst case scenario) should be used to determine the cable size.
For Installation Methods 100 – 103, Table 4A2 does not attribute a letter to indicate the Reference Method. Therefore, when compiling the electrical certificate, the Installation Method number should be recorded in the box entitled ‘Reference Method’ in the Schedule of Circuit Details (SOCD) for the particular circuit.
For more information on installation practices refer to the Site Guide for Electrical Installations publication.
For other guidance and publications please see the NICEIC website. For information about the NICEIC Approved Contractor or Domestic Installers schemes, visit www.niceic.com
Brand:
NICEIC
Topic:
Regulations and Legislation
Certification Schemes
BS7671
Cables Wiring and Wiring Accessories
MV & LV Cables & Accessories
Standards Relating to Cables
Thumbnail image:

Teaser:
This article considers the impact of thermal insulation on the current carrying-capacity of flat twin and earth cables.
Type:
Technical Articles
Main image(s):

Links
- The installation of Band I and Band II circuits This article aims to provide clarity on the requirements of BS 7671 concerning Band I and Band II circuits.
- Electric boilers for wet central heating This article discusses the factors that a contractor should consider when installing an electric boiler.
- Maximum prospective fault current This guide considers the measurement and calculation of the prospective short-circuit current and prospective earth fault current at the origin and at other relevant points in the installation.
05/12/2017
Target group:
Voltimum
View the full article