Mark Bormann asks this question, which concerns cable run between external walls, and whether it needs a service void or not? It has been answered by BASEC (Theme - Cables, Wiring & Wiring Accessories):
Question: The following refers to the attached drawing (PDF document - use the link near the bottom of this web page), which shows an external wall detail and is a typical design produced by architects in order to achieve the required U-values.
We have carried out cable calculations based upon the cables being run within the external walls between the 140mm insulation and Celotex insulated plasterboard, as shown on the attached drawing.
Our calculations show the following:
- Lighting CCCTs - Using a 6A RCBO and 1.5mm 90 degree thermosetting twin and earth cables (*complies with BS7671)
- Ring CCCTs - Using a 32A RCBO and 2Nr 2.5mm 90 degree thermosetting twin and earth cables (*does NOT comply with BS7671).
- Cooker Circuit - Using a 32A RCBO and 10.0mm 90 degree thermosetting twin and earth cable (*complies with BS7671).
- Shower Circuit - Using a 40A RCBO and 10.0mm 90 degree thermosetting twin and earth cable (*does NOT comply with BS7671).
*Note that circuits are based on Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface), and also not being grouped with other cabling.
We are advising main contractors/clients that to satisfy the above scenario and BS7671:2008 (2011), they will have to provide a service void. The feedback we are getting is that other electrical contractors are not installing cables in this way, but in the ways described above, which we have found do not comply with BS7671:2008 (2011), unless they are using larger cables, i.e. 6mm cables for ring circuits etc, which we very much doubt.
Can you please advise?
Answer: BASEC cannot provide detailed interpretation of BS 7671 in individual cases, but can make the following comments:
If the installation is intended to meet BS 7671, then calculation of cable ratings needs to follow BS 7671, including the requirements for de-rating when mounted in insulation.
Has the possible effect on the cable of mounting it between the Celotex PIR insulation and the loft roll been taken into account?
Also, can the Celotex PIR cope with 90 degree cable contact? Has this been checked with the insulation manufacturer?
Note that Reference Method 103 refers to Table 4D5, which is for 70 degree cable, while you are proposing to use 90 degree thermosetting cable.
The best approach would probably be to install suitable conduit, to ensure that the cable could be replaced easily without the need for extensive relining.
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