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Topic Title: another urgent help request! :)
Topic Summary: Laying too many SWA cable side by side!!!
Created On: 01 November 2013 08:18 PM
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 01 November 2013 08:18 PM
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MuratCypriot

Posts: 19
Joined: 28 June 2013

Hi All!!

I am designing a projet which main backbone of the infrastructure consist of 19!!! 4x240mm2 cables (feediing from 2x1,5MVA Generators + 1.5 MVA Transformer). They are n2xh cables (XLPE and ARMORED) in order not too loose too much current carrying capacity, i left 12.5cm between cables, ohhh by the way they are directly burried to ground. But if i lay them down side by side, i mean all of them, i will need 4 meter wide trench!!! which is not.... well not very good. I am thinking of burrying them to the same trench up and down, like 3 layer of them consisting 6-7 in a row!!! But i dont know how it will effect the current carrying capacity since there is no rating factor for such option. All i have is rating factors at varrying depth ( from BS 7671). Well, my depth is 0.8 meters but i am thinking of making 3 rows with 12.5 cm between each row, thus first row at 0.6 meter, other is 0.75m, and last one is 0.9 meter. In that case how my current carrying capacity will be effected? Any ideas for different solutions??

Thanks and BEST regards!
 01 November 2013 11:10 PM
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HarryJMacdonald

Posts: 254
Joined: 15 May 2002

The ERA issued tables that were much better than anything else available, but I would consult the supplier. As a general principle the heat is being conducted away through the soil so the larger the perimeter of the trench the better rating, i.e. a 4m wide trench will be much better than a trench with a square cross section.

This is a really specialist area and factors arise which tables can never allow for. I remember once trying to get a design for a large current and the guys at BICC came back and said that my design would carry the current continuously for 2 weeks but not indefinitely. All to do with the likelihood of the ground drying out..

Can you say that the cables can never all be fully loaded at the same time? That would help if they are not.
 05 November 2013 01:25 PM
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MuratCypriot

Posts: 19
Joined: 28 June 2013

Thanks for guidance, i was also thinkig to take an advice of cable producer, but i wanted to as if there is such table in regulations.

From where can i get this ERA report?

They are designed (c.s.a of cables) according to full load situation so they shouldnt carry all of the load at the same time. But be aware that most of my load consist of resistive electrical concevtors (panel radiators) so in winter situation i am pretty confident load can be as high as 0.8 x full load capacity.

Best Regards
 05 November 2013 02:39 PM
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Avatar for OMS.
OMS

Posts: 18942
Joined: 23 March 2004

Originally posted by: MuratCypriot

Hi All!!

I am designing a projet which main backbone of the infrastructure consist of 19!!! 4x240mm2 cables (feediing from 2x1,5MVA Generators + 1.5 MVA Transformer). They are n2xh cables (XLPE and ARMORED) in order not too loose too much current carrying capacity, i left 12.5cm between cables, ohhh by the way they are directly burried to ground. But if i lay them down side by side, i mean all of them, i will need 4 meter wide trench!!! which is not.... well not very good. I am thinking of burrying them to the same trench up and down, like 3 layer of them consisting 6-7 in a row!!! But i dont know how it will effect the current carrying capacity since there is no rating factor for such option. All i have is rating factors at varrying depth ( from BS 7671). Well, my depth is 0.8 meters but i am thinking of making 3 rows with 12.5 cm between each row, thus first row at 0.6 meter, other is 0.75m, and last one is 0.9 meter. In that case how my current carrying capacity will be effected? Any ideas for different solutions??



Thanks and BEST regards!


If you go for layers of 6 or 7 and then stack up the layers, then Table 4C2 is still relevant.

Based on your 0.125m vertical and horizontal spacing,then you could assume a derating of 0.6 across the whole group.

You could asume a derating of zero for the average depth of lay of 0.7m

The biggest effect will be to determine the ground thermal resistivity - if it's closer to 1.2 K.m/W than 2.5K.m/W then you will see approx a 40% increase in current carrying capacity (backfilling the whole group in sandy material would easily achieve that) - you could expect that to wipe out of the equation a lot of the 0.6 derating for grouping leaving you with:

Tab CCC x 0.6 x 1.4 = 0.85 x TabCCC.

So effectively, you need to derate each cable by 0.85 at worst, which seems to sit above your predicetd worst case load factor of 0.8.

That's a fisrt approximation of course - a canter through BS 7769 would help refine the design, but at the moment, you should be able to deliver circa 4500A per phase into the system (approx 3.2MVA) - not sure if your gensets and supply TX are synchronised parallel sources and I'm assumin that you don't actually have 19 cables in parallel - more that you have a number of sub mains exiting the source switchboard to discrete locations or loads

Regards

OMS

-------------------------
Failure is always an option
 06 November 2013 01:31 PM
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MuratCypriot

Posts: 19
Joined: 28 June 2013

OMS that was realy helpfull!!!! after all, logically i should be able to use same table for vertical seperation!!!!

I have synch. panel from double 1.5 mva genset, they are working all togerher at once, i have 19 cables from synch panel to 3 seperate site distribution board, that go all together for 60-70 meters than they drop to 12 then 6.
 06 November 2013 06:32 PM
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OMS

Posts: 18942
Joined: 23 March 2004

OK - then you should be able to safely deliver at least 3.2MVA into the system loads if you triple stack, but maintain at least 0.125m seperation in both horizontal and vertical axis.

You really do need to understand the impact of ground thermal resitivity however - and that might well need knowledge of where the water table level is at any time - and just how much imported backfill you are prepared to use in those trenches - personally speaking I'd want some assurance that they remain soaking wet and that you have a really good surround of stone dust or sand material.

As I said, it's was a first approximation - I suspect you need to delve deeper into this to get some design certainty

Regards

OMS

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Failure is always an option
 07 November 2013 10:21 AM
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MuratCypriot

Posts: 19
Joined: 28 June 2013

Thanks! i may able to dig a trench wide enough to lay them as single layer, its not certain yet, i will discuss that topic with a site contractors.

For soil thermal resistivity, i take it 2.5 at first then dropped to 1.2 due to extremely high c.s.a. Probably we will use sand backfill there and soil is humid and watery there if its any help to drop soil thermal resistivity.
 07 November 2013 12:28 PM
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OMS

Posts: 18942
Joined: 23 March 2004

OK - but you just can't pick a soil condition to help your cable sizing - you need to know what it is (or approximates to) - for either the flat single layer or the stacked design solution

Regards

OMS

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Failure is always an option
 13 November 2013 02:46 PM
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MuratCypriot

Posts: 19
Joined: 28 June 2013

Originally posted by: OMS

OK - but you just can't pick a soil condition to help your cable sizing - you need to know what it is (or approximates to) - for either the flat single layer or the stacked design solution



Regards



OMS


Yeah you are definatelly right i know, but i just couldnt get soil conditions even for earthing!!! But as i said, i did not used diversity for cabling sizing, i assumed full loads, so it should be ok!! And from the soil report, i had the idea soil thermal resistivity should be less than 2,5 and backfill should also help!! But as you suggested nothing can replace good engineering with all the factors known...
 13 November 2013 02:57 PM
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msandoz

Posts: 11
Joined: 13 November 2013

In reality you wont be able to install these cables with any guaranteed separation. They will be there for months installing them.

Full grouping required.

Go BICC buried direct in ground, for best cable ratings.
 13 November 2013 04:21 PM
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OMS

Posts: 18942
Joined: 23 March 2004

Originally posted by: msandoz

In reality you wont be able to install these cables with any guaranteed separation. They will be there for months installing them.

Don't be silly - I've put seperated cables direct to ground in trenches that are many hundreds of metres long and have upwards of 40 cables in them.

Full grouping required.

How so - seperating cables is a perfectly legitimate way to optimise a system design - it's supported by robust standards and is common in the industry

Go BICC buried direct in ground, for best cable ratings.

Indeed - but you still need to know the arrangements of the groups and most importantly that the actual ground conditions are at least approximate to the BICC data references for ground conditions.


Regards

OMS

-------------------------
Failure is always an option
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