Well to start with you appear to be looking for a grouping factor for the cables that make up a single circuit. Grouping factors apply to multiple circuits not the conductors in a single circuit.

ERA 69-30 part 3 - This report deals with cables to BS 6346 and looks at two main factors:
1) current rating;
2) grouping of circuits (not conductors);

All this for two core and three core circuits in copper or aluminium at various voltages.

So for current rating, taking a typical example of a buried direct circuit.

Cable = 3 * single core, 300mm2 copper, 600/1000 volt

Depth of laying 0.5 metres
Thermal Resistivity 1.2 Km/W (a lot less than BS 7671)
Ground Temperature 15 degrees
Air Temperature 25 Degrees
Maximum Conductor Temperature 70 Degrees

If the three cores are laid flat touching the rating is 521A, for spaced it is 515A - the values for spaced cables up to 185mm2 are more than touching cables, but after that they are less (cable inductance - someone will be along soon to explain that!).

I don't know what the spacing is (it's probably in the report somewhere) but one cable diameter is a good starting point.

Another table in the report deals with grouping - this is group of circuits - not conductors

Typical values - 3 circuits (so 9 single core conductors) 0.15 metre spacing between circuits grouping factor is 0.7, at 0.6 metre spacing it is 0.86.

When all is said and done, your first action should be to select a cable make and then ask the manufacturer!

Regards
Geoff Blackwell

Well to start with you appear to be looking for a grouping factor for the cables that make up a single circuit. Grouping factors apply to multiple circuits not the conductors in a single circuit.

I think it's fair enough to consider 3 parallel cables on the same circuit in the same way as three circuits - the fusing won't make any difference to the current carried by individual conductors in normal service or (in particular) the heat the conductors generate as a result - and as I read it that's what NOTE 5 to table 4C1 is trying to tell us.

   - Andy.

Bs 7671 and the ERA report both apply grouping factors to groups of circuits.  Applying them to cores in a circuit, particularly of this type, can produce errors as the factor simply deal with thermal constraints and do not consider inductance.

Regards

Geoff Blackwell

I'm sure I'm missing something obvious here, but I still can't see how the circuit arrangement makes a difference. Cable layouts yes, certainly (trefoil, or flat, spaced or touching can make a very significant difference to both thermal and inductive characteristics) - likewise how the armours are bonded (both ends or single point) - but couldn't t the same layout be chosen for both parallel conductors and several independent circuits?

   - Andy.

I'm sure I'm missing something obvious here, but I still can't see how the circuit arrangement makes a difference. Cable layouts yes, certainly (trefoil, or flat, spaced or touching can make a very significant difference to both thermal and inductive characteristics) - likewise how the armours are bonded (both ends or single point) - but couldn't t the same layout be chosen for both parallel conductors and several independent circuits?

   - Andy.

'I'm sure I'm missing something obvious here'

No probably not, but there are many complications where single core AWA cables are used and we have an added complication that these cables are buried.

After reading your latest post I thought I best re-read the original post, always a good idea, because the older I get the more I miss!

I now see that I assumed you were applying your 'grouping factor' to the conductors that make up the circuit, i.e. between each phase. I had not realized that you were addressing 3 cables in parallel per phase. This changes things - but not everything - so apologies for that.

First I standby my statement that grouping factors apply to circuits. As to Note 5 - I am not sure what it means, but it applies to 4C1 which does not address buried cables, we need 4C2 for that. Is it reasonable to do what you suggest - yes but only for thermal constraints, because it might produce erroneous results if other factors are involved.

Regulations 523.1, 523.7 and 523.201 apply.

Further reading - Paul Cock - Commentary sections 9.6 and 9.8.

An interesting report https://www.jduntonassociates.co.uk/issues-with-single-core-cables/

All this indicates that you need to be very careful when designing circuits that use these cables. I avoid them like the plague if I can get away with it.

It must be said that in the particular application being consider here the typical circuit lengths are probably too short to produce many of the problems.

I have produced a quick sample design using Trimble (Amtech) software - 1000 kVA transformer to switchboard - loaded at 1250A per phase, 30 metres run. This gives 3 * 400mm2 cables per phase. It uses BICC data for ratings etc as BS7671 does not provide data for this type of circuit. My aim was to try and see if I could extract correction factors from the various reports that are produced. Not much luck - reports indicate that this is one circuit so grouping factor = 1, but they also include a factor of 0.74 which looked as if it could be the factor they use for the three parallel cables per phase - but I cannot match it to the cable size given.

Note that this software solution does not address any of the possible problems indicated above.

Regards

Geoff Blackwell

GeoffBlackwell said:

As to Note 5 - I am not sure what it means, but it applies to 4C1 which does not address buried cables,

I'm not quite sure about that. The Bunched in air, on a surface, embedded or enclosed arrangement (row 1 of Table 4C1) applies to reference methods A to F, which includes reference method D, which as we know is either direct in ground or in underground ducts.

thanks for attaching the report, it is an interesting read.

-Bh.