Top tips when installing cables covered by thermal insulation to satisfy Regulation 523.9
It is common practice for thermally insulating material to be installed into buildings to reduce heat losses or improve sound attenuation.
However, if thermally insulating material covers a cable, such as is shown in Fig 1, this reduces the cable’s current-carrying capacity (the maximum current a cable can carry without its steady-state temperature exceeding the rated value, such as 70°C for general-purpose thermoplastic insulated conductors).
A problem can arise if the thermal insulation reduces the current-carrying capacity of a cable to a value less than is necessary to suit the rated current of the overcurrent protective device of the circuit, or, if the current is limited by the characteristics of the load, to less than the load current.
This could result in the cable’s conductor temperature exceeding the maximum admissible value under load or overload conditions, possibly leading to damage to the insulation and sheath of the cable and adjacent materials. The amount by which the current-carrying capacity of a cable is reduced by thermal insulation can be seen from some of the tables of current-carrying capacity in Appendix 4 of BS 7671. For example, Table 4D5A shows that for a flat twin-and-earth PVC/PVC cable installed above a ceiling (method 101) or within a wall (method 103), the presence of thermal insulation reduces the current-carrying capacity by as much as 35% or 50%, respectively, compared with if the cable was clipped direct to a surface and open (Reference Method C).
Other tables of current-carrying capacity in Appendix 4 give similar comparisons. Also, Table 52.2 and Regulation 523.9 give rating factors for cables totally surrounded by thermal insulation, according to the length of cable surrounded.
Requirements of BS 7671
Regulation 523.9 requires that a cable shall preferably not be installed in a location where it is liable to be covered by thermal insulation. Where a cable is installed in such a location, the same regulation requires that the cable shall preferably be fixed in a position where it will not be covered by thermal insulation.
Where this is not practicable, the regulation requires the conductors of the cable to be adequately sized so that their effective current-carrying capacity – taking account of (amongst other things) the reduction in current-carrying capacity caused by thermal insulation – meets the requirements of Chapter 43 (Protection against overcurrent).
Regulation 523.9 permits the nature of the load and diversity to be taken into account. However, this is not applicable where the maximum current is sustained for long periods, such as in a circuit supplying a night storage heater. In such cases the cable and protective device ratings must be adequate for the maximum current. The implications of Regulation 523.9 as regards the cross-sectional area required for the circuit conductors are discussed below for radial circuits and ring final circuits.
Radial circuits
Regulation 433.3.1(ii) allows overload protection to be omitted for circuit conductors which, because of the characteristics of the load, are unlikely to carry overload current. However, the conductors are still required to be protected against fault current (short-circuit and earth fault) in accordance with Section 434.
An example of a circuit unlikely to carry overload current is one supplying only luminaires or heating elements, if the conductors are adequately sized to carry the full load current continuously.
Socket-outlet circuits, however, invariably require overload protection, as users can easily add load (possibly too much load) by plugging in current-using equipment.
For a circuit for which overload current protection is required, the lowest rated conductor used for the circuit should have an effective current-carrying capacity (Iz) of not less than the rated current (In) of the overload protective device of the circuit, or, if the device is a semi-enclosed fuse to BS 3036 not less than In ÷ 0.725 (Regulations 433.1.1 and 433.1.202 refer).
The tabulated current-carrying capacity (It) for the size of conductor that meets the above requirement can be found by using the formula given in Appendix 4 Section 5.1. The value of It determined should be located in the appropriate table of currentcarrying capacity in Appendix 4 of BS 7671, and the corresponding cross-sectional area of conductor identified directly.
For a circuit for which overload current protection is not required, the conductors used should have a tabulated currentcarrying capacity (It) not less than the current (Ib) to be carried by the circuit in normal service (Regulation 523.1 and Section 5.2 of Appendix 4 refer). The tabulated current-carrying capacity (It ) for the size of conductor that meets the above requirement can be found by using equation 5 given in Section 5.2 of Appendix 4. This value of It (or the next highest value) should be located in the appropriate table of current-carrying capacity in Appendix 4 of BS 7671, and the corresponding cross-sectional area of conductor identified directly.
Ring final circuits (supplying 13 A accessories to BS 1363)
For a conventional 30 A or 32 A ring final circuit, as shown in Fig 2, Regulation 433.1.204 requires (amongst other things), that the live conductors of the circuit cable are of sufficient size to give an effective current-carrying capacity (Iz) of not less than 20 A, subject a minimum size of 2.5 mm2 copper, or 1.5 mm2 copper for two-core mineral insulated cable. The tabulated current-carrying capacity (It) for the size of conductor that complies with Regulation 433.1.204 can be found by using Equation (1).
This value of It (or the next highest value) should then be located in the appropriate table of current-carrying capacity in Appendix 4 of BS 7671, and the corresponding cross-sectional area of conductor identified directly.
Existing cables subsequently covered by thermal insulation
A problem sometimes encountered is that thermal insulation has been installed around or over the cable of an existing circuit at some time after the circuit was originally designed and installed.
This is becomes a problem if the thermal insulation has reduced the effective current-carrying capacity (Iz) of the cable to less than that needed the meet the relevant requirements of BS 7671, referred to earlier.
However, in some cases it may be found that there are no signs of thermal damage having been caused to the cable or its surroundings as a result of the thermal insulation being added.
Where this is the case, a careful assessment of the design current (lb) carried by the circuit cable in normal service (taking account of the nature of the load and diversity, where applicable) might reveal that Ib does not actually exceed Iz.
In these circumstances, a possible solution might be to replace the overload protective device of the circuit (where applicable) by one having a suitably reduced rated current (In), provided this would not result in unwanted operation of the device, such as due to inrush current under starting conditions.
Applying this approach to a ring final circuit, for example: suppose that, as a result of the installation of the thermal insulation, the reduced value of current-carrying capacity (Iz) for the circuit cable was 10 A (which is 50% of the 20 A minimum required by Regulation 433.1.204). In that case, a protective device having a rated current not exceeding 15 A or 16 A (that is, 50% of 30 A or 32 A) would be appropriate.
