Underground cables in Concrete Encased PVC ducts
Underground cables in Concrete Encased PVC ducts
(OP)
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
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






RE: Underground cables in Concrete Encased PVC ducts
concrete and earth thermal resistivity[RHO],concrete duct bank dimensions, duct diameter, duct depth-from the surface-earth temperature, number of loaded ducts in the duct bank and other.
An average concrete thermal resistivity could be 100 oC.cm/w [1 K.m/w] as per IEC 60287-2.NEC uses 55oC.m/w and IEEE 835 -60.RHO[earth] from 60 to 120 [IEEE-835] and up to 250 according to IEC.[it could be more].
RE: Underground cables in Concrete Encased PVC ducts
thank you for the information.
My question is in case we have two cable arrangments which are at similar installation conditions (earth resistivity, grouping, depth, temperature) and one of those arrangements is in PVC duct and the other is directly buried what would be a rough estimation of the de-rating factor. In one catalouge from ABB it is mentioned that this factor is 0,9 while a textbook "Transmission and Distribution Electrical Engineering" states that the factor is 0,8?
i started to do the calculations according to IEC 287-1-1 and 287-2-1 hoping that i arrive to a satisfactory answer but any help is appreciated!
Thanks.
RE: Underground cables in Concrete Encased PVC ducts
recommended when laying cable in conduits.
However, if you see, the factor f1[Table 13-54] for ground temperature depends also on earth thermal resistivity[from 0.7 to 2.5] and on load factor[0.5-1].This factor could be from 1.24 to 0.68 but it is referring to cable laying underground at 20 dgr.C 0.8-1.2 m depth.
If you would be more specific I could –may be-more helpful.
RE: Underground cables in Concrete Encased PVC ducts
The system is 60 Hz 13,8 kV 300 mm.sq. single-core unarmoured cable. The cables are laid in trefoil in 160 mm PVC duct in a 9-way duct bank. the center of the bottom ducts (worst case) is 1500 mm underground with 40°C ground temperature and 2 K.W/m soil thermal resistivity.
Thanks
RE: Underground cables in Concrete Encased PVC ducts
The distance between ducts[vertically and horizontally]will be 250 mm[center-to-center].In this case the ampacity will be 166 A/cable. If the cables are laid directly in the ground keeping the same distances and arrangement the ampacity will be 184 A [It is equal to the ABB tables also:
3*300 sqr.mm copper XLPE insulated 12/20 kV-buried cables in trefoil in 40 dgr.C and 2 K.m/w will be 599*0.68=407.3 A and for 9 groups will be:
407.32*0.45=183.3 A/each].
RE: Underground cables in Concrete Encased PVC ducts
so if the rating for cable in ducts is 166 A and for directly buried is 184 A the factor in between is around 0,9 (assuming all other conditions are the same). for a rough estimation is this valid?
for the duct in question it is a 3 x 3 configuration vertically and horizontally there is 50 mm spacing between the ducts (165 mm center to center)and 75 mm from the concrete encasing border to the duct order also vertically and horizontally.
i believe i have all data i need to calculate the ampacity using the equations in 287-1-1 and 287-2-1 but i am not getting a logical result. I am doing calcualtions only for one cable (3x1 300 mm.sq. in PVC duct) to see what is the ampacity and compare with the one given by the manufacturer for laid direclty in ground.
Finally, you said that you are using information from ABB for the 9 cables arrangement but is it valid to use the factors from IEC 502 when considering 3 x 3 configurations, the factors are for horizontal arrangement only! correct?
Thanks
RE: Underground cables in Concrete Encased PVC ducts
RE: Underground cables in Concrete Encased PVC ducts
Finally is it possible to explain more how to calculate the 3 x 3 arragnement from the IEC equations? we can do that on private email if you prefer?
Thank you.
RE: Underground cables in Concrete Encased PVC ducts
RE: Underground cables in Concrete Encased PVC ducts
RE: Underground cables in Concrete Encased PVC ducts
RE: Underground cables in Concrete Encased PVC ducts
Thanks
RE: Underground cables in Concrete Encased PVC ducts
is there a way according to IEC 287 to calculate the maximum capacity of a cable in a concrete duct bank while other cables are loaded equally. For example, if i have same to the above calculations 9 cables each with 150 A when all cables are operating, for 8 cable operating can i have 7 cables operating at 150 A and the 8th at 175 A?
Thanks
RE: Underground cables in Concrete Encased PVC ducts
The first way result was 166 A [exactly 165.5 A
RE: Underground cables in Concrete Encased PVC ducts
RE: Underground cables in Concrete Encased PVC ducts
thank you for youe previous assistance. the sheets you sent helped me a lot in understanding the calculations. I made the calculations and have some questions. I hope it is not a problem to ask you.
1. when i am considering non-armoured cables lambda 2 in 1.4.1.1 from 287-1-1 and T2 in 2.1.2.1 from 287-2-1 are zero since there is no armour
2. when considering single-core cables in duct banks:
2.a we have the parameter n in 1.4.1.1 from 287-1-1 which is number of load-carrying conductors. in this case n should be 27 in case i have 9 ducts which have cables each having 3 loaded conductors for which we calcualted T1, T2, T3, R and all the other variables.
2.b we have the parameter N in in 2.2.7.3 from 287-2-1 this should be the number of ducts containing acables in service, example for a fully loaded 3 x 3 duct bank i will have N = 9
3. should i always consider Wd in 2.2 from 287-1-1 in table 3 it is mentioned that this could be ignored for operating voltage under 63 kV for XLPE! for 13,8 kV and 24,5 kV operating voltage this could be neglected correct?
4. in your calculations you considered the eddy currents calculations as in 2.3.6.1 from 287-1-1 for single bonded or cross-bonded systems. However isn't it more critical to calcualte current capacity for bonding at both ends? using 2.3.1 the eddy currents could be ignored. However it wasnt clear for me the process of iterative calculation in 2.3.1 and i assumed losses of 10% coming from circulating current.
5. just to clarify a final point if i am calculating for the same cable and same ductbank for different number of loaded cables i only have to change the N in 2.2.7.3 from 287-2-1 and n in 1.4.1.1 from 287-1-1 and the d'pk/dpk product in 2.2.3.2 from 287-2-1 to correspond to the distances between the expected worst condition cable and the other cables.
Thank you very much!
RE: Underground cables in Concrete Encased PVC ducts
2)Correct. If in 1.4.1.1 formula you’ll take n=3 then in 2.2.7.3 N=9.
3)Wd may be neglected, indeed.
4)I considered only eddy current but if the shield is grounded both ends then a circulating current will flow through it. In this case you may neglect eddy current, of course.
5)Since the current are different you have to follow 2.2.3.1 Unequally loaded cables and fk factor will not be useful. But you may split the temperature drops in two groups :
Dp=1/2/pi().W1.LN(d’p1/dp1)+1/2/pi().W2.LN(d’pk/dpk*…*d’pn/dpn)|k=2 to n-except k=p[k<>p]
New Fk= d’pk/dpk.….d’pn/dpn|k<>p,k<>1
W1=total losses in cable no.1 [main conductor losses+shield losses+wd(if any)+armor losses(if any) and W2 is the same sum of losses in the rest of cables.
RE: Underground cables in Concrete Encased PVC ducts
one last question:
above you indicate n = 3 and N = 9 is there any place where we will use 27? that is 9 ducts and in each duct a cable with 3 conductors.
RE: Underground cables in Concrete Encased PVC ducts
DT= (I^2*R + ½ Wd).T1 + [I2R (1 +lambda1) + Wd].n.T2 + [I2R (1 +lambda1 +lambda2) + Wd].n.(T3 + T4)
This is the sum of thermal resistances multiplied by losses-thermal flow passing through.
T1=Ri insulation thermal resistance of one cable only I^2*R=main conductor losses
T2=Rj jacket thermal resistance [for single-core cable one cable losses = main +shield]
T3 = external serving th.res.[for single-core cable one cable losses = main +shield+armor]
T’4=temperature drop from bunch of 3 cables to inner part of the duct [n=3 cables total losses]
T”4=temperature drop through the duct thickness [n=3 cables total losses]
T’”4=Tconcrete+Tearth
Tconcrete summarizes the thermal flux from all around ducts reaching the “one” then will be from all 8 remaining ducts. But you have to multiply by n=3 since each duct contains 3 cables.
Tearth –it refers to all cable together that means thermal flow = total losses of 3[n]x9[N] =27 cables.
RE: Underground cables in Concrete Encased PVC ducts
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.
RE: Underground cables in Concrete Encased PVC ducts
LI81 LI82 LI83 LI84 LI86 LI87 LI89
3000 3000 3000 3165 3165 3330 3330
Te is wrong -N=8 [not 24!]
RE: Underground cables in Concrete Encased PVC ducts
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.
RE: Underground cables in Concrete Encased PVC ducts
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.
RE: Underground cables in Concrete Encased PVC ducts
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?
RE: Underground cables in Concrete Encased PVC ducts
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.
RE: Underground cables in Concrete Encased PVC ducts
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?
RE: Underground cables in Concrete Encased PVC ducts
RE: Underground cables in Concrete Encased PVC ducts
is it possible to get a picture of the excel calcualtions you had for the one cable case!
RE: Underground cables in Concrete Encased PVC ducts
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.