Sizing cables from TX to LV Panel

Question

Hi all, 
 Is it a common occurrence for ICPs/DNOs to not consider grouping factors when sizing supply conductors (TX to LV panel)? 
 To my understanding, if the supply circuit fell within the scope of BS 7671:2018, as per note 5 of Table 4C1 in Appendix 4, I&rsquo;d assume that the applicable grouping factor for a circuit consisting of 3 single core cables per phase, installed direct in ground (hypothetical example), would be 0.70. 
 Thanks, 
 Bh

Jam · Accepted Answer

They should be taking into account the cable installation conditions, including grouping. However, do note that they may not be using BS7671 Appx 4 to do this*. For example, ERA 69-30 or DNO's design rules. 
 *Which, being an informative appendix, does not necessarily mean the design doesn't conform to BS7671... And furthermore if it's the DNO side of the meter it doesn't need to anyway.

mapj1 · Answer

Further to that, BS7671 cable ratings tend to be very conservative in many ways. Starting with the 30C ambient which is unusually hot for the UK, and then allowing only a 40C rise, passing through the idea that the full load and highest temperature are present all the time, and that we want a near indefinite cable life. The DNO will 'know better' from similar installations that they can rely on the maximum load not being 100% all the time, and that short flights of a few hours overheating a year do not suddenly cause the particular cables they use to fail. 
 If you want to contrast this with the ratings of cables used in cars, where brief 110C episodes for PVC are considered normal, the apparent ratings and grouping for the same wire gauge are quite significantly higher, and indicators can be wired in thinner than brake lights despite being the same wattage, as they are only on half the time.. 
 Mike.

John Peckham · Answer

All the jobs I have done I have calculated the CSA of the AWAs from the panel to the Tx and the LV contractor installs them and presents them to the DNO for connection. 
 See my adjacent post on CT chambers. 
 JP

AJJewsbury · Answer

To my understanding, if the supply circuit fell within the scope of BS 7671:2018, as per note 5 of Table 4C1 in Appendix 4, I&rsquo;d assume that the applicable grouping factor for a circuit consisting of 3 single core cables per phase, installed direct in ground (hypothetical example), would be 0.70. 
 
 Also depends on the layout - e.g. the spacing between individual cables in the ground - as that, as well as the differing thermal behaviour of soil compare with air can make quite a difference. BS 7671 seems to side-step the issue, there being no method number for single core cables buried direct in the ground and no method D listed in tables 4D3A, 4E3A, 4H3A, 4J3A etc. So while 4C1 might be thought to apply, it's difficult to see in BS 7671 terms what it would apply to. So I suspect buried single core cables is a method that BS 7671 simply doesn't cover and, as others have already mentioned, you'll have to fall back onto other approaches. 
 - Andy.

Sizing cables from TX to LV Panel

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