Cables in buried duct - Table 4C3.

There is an example for multiple cables in ducts in section 4.3.5 (specifically, page 54) of the Electrical Installation Design Guide, 4th Edition.


Grouping factor is applied as per item 5.1.2 in Appendix 4 to BS 7671:2018 (page 378), but note you should take into account the following correction factors using the expression if Equation 2 in item 5.1.2 of Appendix 4

• C_a for the Assumed ambient temperature

• C_s for the particular soil thermal resistivity

• C_d for the specific depth

• C_f if a semi-enclosed fuse is used

• C_c (0.9 for a cable laid in the ground where overload protection is required).

Thanks Graham - I'll have to get a copy...

I have just had a look at the EIDG example but all I can find is "Four 4-core copper armoured cables are laid in touching ducts beneath a road"... Which to me reads like 4 ducts each containing a single cable, whereas the OP asked for a method for multiple cables in a single duct. Apologies if I've missed it.


Personally I'd treat single cores from the same circuit in one duct the same as the equivalent multicore, since the heat being dissipated is the same. But most of the time BS7671 doesn't give ratings for singles buried in ducts (would that it were) so other methods are needed anyway.


For multiple circuits in a single duct I'd default to table 4C1 row 1 (i.e. bunched and enclosed) if pushed, but for more valuable circuits or for large numbers of simulataneous loads or those close to the calculated rating I might be inclined to check via another method, which might be consultation with a manufacturer or calculation to BS IEC 60287.

Apologies, yes you are correct, the wording is as you say, and I admit I missed that when I was reading it.


I agree, that an approach might well be to apply 4C1, on top of 4C3 should there be more ducts in the vicinity, noting that it's still necessary to apply correction factors using the expression of Equation 2 in item 5.1.2 of Appendix 4

• C_a for the Assumed ambient temperature

• C_s for the particular soil thermal resistivity

• C_d for the specific depth

• C_f if a semi-enclosed fuse is used

• C_c (0.9 for a cable laid in the ground where overload protection is required).

So, basically, follow the example in the EIDG, but apply 4C1 on top in any ducts where there is more than one circuit being served.

In my experience BS7671, the clue is in the title "Building regs".It only is useful as a quick guide to underground cabling.

In the good old days, the "go to" reference was the CEGB standards. I don't know if anyone is keeping these going but the manufacturers may have much better guides to the rating of underground cabling.

Anyone know?