Underground cables in Concrete Encased PVC ducts 1

[Adam1980]

Dear members,
for the above mentioned installation conditions for underground cables a thermal calculation is usually recommended. However in case we want to make a rough estimation of the cable amapcity derating what is the recommended factor to be used? According to available references from textbooks and manufacturer catalouge this factor varies between 0,8 and 0,9.
Any recommendations or suggestions based on experience?

Please notice that the subject here is a factor which is additional for the other factors of ground temperature, grouping, soil thermal resistivity.

Thank you.
Adam

 

[Adam1980]

Hello again,
i uploaded the calculations sheets i made i hope it is possible for you to check them. I made the calculations for 300 mm.sq. with cables active in ductbank from duct 1 to duct 9 where duct 5 is always empty.
i used in N for Teaerth of T'''4 as 3xno. of active cables and i used n in the main equation the same.
for one cable and two cables i am getting a very high rating which i cant understand!
hope that you have some notes for me.
Thank you.

 

[7anoter4]

I think Fk is wrong.The vertical column for LI calculation has to be:
LI81 LI82 LI83 LI84 LI86 LI87 LI89
3000 3000 3000 3165 3165 3330 3330
Te is wrong -N=8 [not 24!]

 

[Adam1980]

you are correct the FK was wrong:
the values are:
LI81, LI82, LI83 = 330 + 2 x (1000 + 80) = 2490
LI84, LI86 = 165 + 2 x (1000 + 160 + 50 + 80) = 2745
LI87, LI89 = 2 x (1000 + 160 + 50 + 160 + 50 + 80) = 3000

the 1000 mm is from ground to top of the first duct which is 75 mm from the duct bank border

if i am using N = 8 in Te then will get 2.337 and using n = 3 x N = 24 in the main equation i am getting for 8 cables operation a current of 152 A which is close to the result you got earlier

but the thing that i am totally not understanding is for 1 cable in operation:
N = 1 , i.e. one cable in operation in the ductbank the bottom one for worst case, so Te is 0.292
FK = 1 so Tc = 0.577
and n = 3 x N = 3 in main equation i am getting 882 A which is definitely wrong! the result should be around 350 A

where is my mistake!

Thank you.

 

[7anoter4]

I’m sorry, but a few data it is still not clear for me.
1)the distance between ducts it is 160+50=210 mm[not 165 mm].That is logical since no concrete can be hold between ducts if the thickness is less than 50 mm[2 inches approx.].
2) in this case the duct bank dimensions will be:
2*210+160+2*75=730 mm [indeed!].
3) The upper row center line will be 1000+160/2=1080 mm under the ground level.
4) Then vertical of LI81 =2*1080+2*210=2580 mm [for instance].
In this case the ampacity for 8 loaded ducts- 3 cables per duct-will be 185.9 A.
This it is ,of course, still a “virtual” value since the 90 oC cable insulation rating usually is not the equipment connected with [terminals] rating. So you may have to reduce it to 75 degrees.
Also all the dimensions-including earth temperature and thermal resistance- could be different. Usually a derating factor of 0.9 may be required.

 

[Adam1980]

you are correct i was consdering the 50 mm as 5 mm!
however still for only ONE cable in operation FK doesnt affect the calculation and i cant find my mistake! any thoughts from your side? or am I using the wrong section of the standard?

 

[7anoter4]

Something has to be wrong. There are two possibilities:
1)The same duct of 3*3 ducts but only one loaded[no.8]
Calculated- in the same way as for 27 cables-in excel- the result is 392 A. Calculated in VB6- according to IEC 60287-387 A. and according to Neher and Mc Grath 383 A.
2)Only a single duct located in lower most row [in no.8 duct position].
Since in the first case the concrete was about 50% of the surrounding material the average thermal resistance was lower. Now is elevated.
Results:
In excel 368 A, in VB6 IEC 366 in VB6 Neher and McGrath 360 A.

 

[Adam1980]

first, i really appreciate your support.
second these are the values i have for one cable case (8th cable in 3 x 3 ductbank)
T1 = 0.2 K.m/W
T2 = 0 (no armour)
T3 = 0.109 K.m/W
T'4 = 0.361
T''4 = 0.062
Te = 0.292
Tc = 0.577
T4 = 1.292 K.m/W
lambda 1 = 0.1 (Assumed)
lambda 2 = 0 (no armour)
deltatheta = 313.15 (assuming 90°C maximum operating temperature and 50°C as ambient temperature of the air surrounding the cable in the duct)
n = 3 ( 1 cable with 3 single-core conductors)
R = 8 x 10-5 ohm/m (from manufacturer catalouge)

any parameter seems wrong?

 

[7anoter4]

deltatheta=90-40=50 [ambient it is the earth not the air in the duct!] in Celsius degrees not Kelvin[ 40+273.15=313.15 oK ].

 

[Adam1980]

in 287-1 section 1.4.1.1 deltatheta "is the conductor temperature rise above the ambient temperature (K)" so in case considering the ambient as 40°C then deltatheta would be 90-40 + 273.15 = 323.15°K which leads to a higher current value!

is it possible to get a picture of the excel calcualtions you had for the one cable case!

 

[Adam1980]

ok you are very correct and i am very wrong it is the temperature rise!! so it should be 50 in the nominator!
now with 50 as deltatheta and taking n = 3 for all cases from one cable active till eight cables i get the following results:
one cable 365A
two cables 299A
three cables 259A
four cables 234A
five cables 215A
six cables 202A
seven cables 191A
eight cables 182A
the results are very logical now. I think i still need to fix n in the equation as you explained earlier in some cases it is 3 for the conductors and in other it is multipled by number of cables active x 3.
Thanks again.