Cables buried direct in ground...

I gave this much thought in the context of my self-build chum's distribution circuit to the house and we did discuss it in here. In the event, he took the view that 25 sqmm was not much more expensive than 16 sqmm and perhaps his cautious nature overcame his parsimony which would make a Yorkshireman appear to be the soul of generosity. (I cursed him a bit when it came to terminating the larger size.)

The property is in the South Lakes and at the bottom of a hill with a spring elsewhere on the plot, so I have no doubt that the thermal resistivity of the soil is low. However, without being able to measure it, I felt obliged to comply with BS 7671.

So an interesting question and perhaps it is like diversity: a range of opinions will exist.

Also, in the case of short cables the extra cost of a larger size is not that great.

On a long run when cable cost is more significant, then a larger size might be needed anyway for voltage drop reasons. And for a long hour load, larger cable may be worthwhile to reduce the energy cost of cable losses.

“Remember Auckland” One of the worst ever power failures in a major city in a first world country. Unduly optimistic assumptions about thermal resistivity of soil was a major factor.

A high-pressure, oil-filled, pipe-type cable system used for underground electrical power transmission is made up of several cables enclosed in a steel pipe. ... Chilled oil is circulated through the pipe, and most of the heat generated in the cable and insulation is absorbed by the oil.

Z.

96A vs. 152A on something as small as a 25mm 4c

(I think the 152A is for single phase 90 degrees C)

Presuming the cable comes above ground to be terminated - there's also the (usually) clipped direct and 70-degree conductor temperature limitation - say 110A for 25mm2 4C - which might limit any benefits from improving the CCC of the buried section.

        - Andy.

broadgage: 
 

Also, in the case of short cables the extra cost of a larger size is not that great.

On a long run when cable cost is more significant, then a larger size might be needed anyway for voltage drop reasons. And for a long hour load, larger cable may be worthwhile to reduce the energy cost of cable losses.

“Remember Auckland” One of the worst ever power failures in a major city in a first world country. Unduly optimistic assumptions about thermal resistivity of soil was a major factor.

Absolutely, not really thinking for short runs. 

I am thinking along the lines of better engineering rather than skirting compliance, ERA methods still comply with Bs7671, they just use different figures for correction and lower thermal resistivity values, we just need to be able to support such figures when designing. 

Not too sure on Auckland 60 & 30 year old cables at the time, the older lot failed stressing the ‘newer’ ones I believe, I thought they decided it was poor maintenance of cables that ought to have long been replaced?

Zoomup: 
 

A high-pressure, oil-filled, pipe-type cable system used for underground electrical power transmission is made up of several cables enclosed in a steel pipe. ... Chilled oil is circulated through the pipe, and most of the heat generated in the cable and insulation is absorbed by the oil.

 

Z.

Copper might not be that expensive yet! 
 

AJJewsbury: 
 

96A vs. 152A on something as small as a 25mm 4c

(I think the 152A is for single phase 90 degrees C)

Presuming the cable comes above ground to be terminated - there's also the (usually) clipped direct and 70-degree conductor temperature limitation - say 110A for 25mm2 4C - which might limit any benefits from improving the CCC of the buried section.

        - Andy.

 

Both figures were ’run at 90’ 3ph, the only difference being installation method. 
 

Clearly we cant run at 90 for most standard switchgear though…

I have 25 4c run to 70 in ground as 82A to standard method…

Martynduerden: 
Not too sure on Auckland 60 & 30 year old cables at the time, the older lot failed stressing the ‘newer’ ones I believe, I thought they decided it was poor maintenance of cables that ought to have long been replaced?

It is self-evident that if one of a pair of cables in parallel fails, the other will be overloaded.

I sincerely hope that cables last more than 30 years. Our service cable is at least 39 years old; the transformer dates from 1959; and there is a map of the cables in the Hampshire Archives which is pre-war.

you can run at 90 into line taps in a box and go up a size for the last 3 feet if you want. In a well sized enclosure splitting the cable into singles already has a significant cooling of the ends.

Mike.