XLPE Twin & Earth - Appendix 4 Table - Method of install

Question

Hi there, I have a general technical query regarding installation methods and Appendix 4 tables for Twin & Earth cables. Table 4D5 is applicable to PVC/thermoplastic Twin & Earth. In this instance, with the cable installed in accordance with Reference Method 103 &ndash; within a stud wall with thermal insulation, with the insulation not touching the inner wall surface &ndash; a 2.5 mm&sup2; cable has a tabulated current-carrying capacity of approximately 13.5 A. This can also be verified within ElectricalOM, where selecting Flat Twin 70&deg;C PVC with CPC gives 13.5 A for 2.5 mm&sup2; under the relevant installation conditions. However, when selecting an XLPE Flat Twin & Earth cable (90&deg;C thermosetting), ElectricalOM identifies the cable against Table 4E2. The software also provides an option to &ldquo;Run to 70&deg;C&rdquo;. When this is selected, and Reference Method A No. 3 is selected, it gives a current-carrying capacity of 21 A for 2.5 mm&sup2;. My question is therefore whether this is an appropriate application of Reference Method A No. 3 for XLPE Twin & Earth installed within a thermally insulated stud wall. In particular, where the installation is not relying on the 90&deg;C operating temperature of the XLPE cable, is it permissible to use the 4E2 installation/reference-method arrangement but limit the conductor operating temperature to 70&deg;C, resulting in the 21 A rating? Or, when XLPE Twin & Earth is installed in this type of thermally insulated construction, should the 70&deg;C PVC/thermoplastic values from Table 4D5 be used instead, despite the cable itself being a 90&deg;C thermosetting/XLPE cable? 
 
 The particular installation I am considering is 2.5 mm&sup2; 6242B XLPE Twin & Earth, installed within a 90 mm stud wall with approximately 50&ndash;70 mm of thermal insulation surrounding the cable. The circuit has a design current of 11.26 A and is protected by a 20 A protective device. I would appreciate clarification on which Appendix 4 table and reference method should correctly be applied in this situation, and whether the 21 A value obtained from ElectricalOM is an appropriate basis for the cable selection.

AJJewsbury · Accepted Answer

My issue is that people are taking the 4D5 derating table Reference 103 and showing this as only 13.5Amps. 
 
 That's because it's a different situation - 103 is for a cable totally surrounded by thermal insulation - Method 3 (ref Method A) is for a cable in contact with the plasterboard one side and so is only partly covered with thermal insulation. If you cable is totally surrounded by insulation the 103 is correct - and if you're using 4D2 you'd probably need method C plus a correction factor for the insulation (likely 0.5) - e.g. 27 * 0.5 = 13.5 (see reg 523.9). If your cable is only partly surrounded by insulation and can loose heat through the plasterboard, then Method A would be correct, which would correspond to method 102 (rather than 103) in table 4D5. 
 (as a bit of background, some years ago there wasn't a table 4D5 - we just used 4D2 for T&E just like for other sheathed cables. As thermal insulation became more common though, people were having trouble justifying using 2.5mm&sup2; T&E on ring circuits even though it seemed fine in practice. A bit of research confirmed that the shape of T&E (thinner sheath & larger surface area) meant it did have a somewhat higher current carrying capacity for the same c.s.a. compare with the round section cables 4D2 was designed for - and that small improvement often made the difference between a cable being satisfactory or just too small to meet the regs. Hence table 4D5 was called into creation.) 
 - Andy.

gkenyon · Answer

Reference Method A is in a conduit in a thermally insulating wall. It is NOT equivalent to Method 103# ... they are two different columns in Table 4D5. So, for 2.5 sq mm in Table 4D5, Reference Method A I get 20 A ... this seems to be very similar to the 21 A Electrical OM is giving you. So, I don't think you're comparing like-for-like. Are you installing the cable in a conduit in the wall, or not? If not, then you can't use Reference Method A you selected in Electrical OM ...

XLPE Twin & Earth - Appendix 4 Table - Method of install

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