IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
(OP)
Dear all,
concerning the current carrying capability of three-single core cables in trefoil in 9 way (3 x 3) duct bank and following the procedure in IEC 287 - 1 -1 and 287 - 2 - 1
when we are calculating T4 = T'4 + T"4 + T"' 4
T'4 is according to 287-2-1 2.2.7.1
T"4 is according to 287-2-1 2.2.7.2
T'" 4 is according to 287-2-1 2.2.7.3 and 2.2.3.2 considering the mutual effect of the other cables laid in the other ducts
The question is for T'"4 should we use 2.2.4.3.1 instead of 2.2.3.2?
that is use: (1.5/pi) x rho concrete x [ln(2u x (d'p/dp...)) - 0.630] instead of (1/(2 x pi)) x rho concrete x ln[(u - squareroot(u^2 - 1)) x (d'p/dp...)]
Thank you,
Adam
concerning the current carrying capability of three-single core cables in trefoil in 9 way (3 x 3) duct bank and following the procedure in IEC 287 - 1 -1 and 287 - 2 - 1
when we are calculating T4 = T'4 + T"4 + T"' 4
T'4 is according to 287-2-1 2.2.7.1
T"4 is according to 287-2-1 2.2.7.2
T'" 4 is according to 287-2-1 2.2.7.3 and 2.2.3.2 considering the mutual effect of the other cables laid in the other ducts
The question is for T'"4 should we use 2.2.4.3.1 instead of 2.2.3.2?
that is use: (1.5/pi) x rho concrete x [ln(2u x (d'p/dp...)) - 0.630] instead of (1/(2 x pi)) x rho concrete x ln[(u - squareroot(u^2 - 1)) x (d'p/dp...)]
Thank you,
Adam






RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
if we considered 2.2.3.2, which is a variance of the single isolated cable, how are we accounting for the mutual heat exchange between the three cables inside the duct itself. After all we are calculating the current carrying capabiltiy for one conductor of the three single-core conductors inside the PVC duct.
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
It is indicated for pipe-type cable indeed, but it is employed for 3 single-core cables also.
The Neher and McGrath article indicated the same procedure [see D’s diameter formula 41].
In ch.1.4.1.1 formula you have to multiply this thermal resistance T4 by number of cables per duct[n].
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
the 3 single-core cables are touching while the PVC ducts are not touching!
i am convinced that using the section with touching is not correct! but i cant find a clear argument that i can use section 2.2.3.2 for a three single-core cable. is there any literature or publication that can support that
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
2) The single-cores cables in a duct or pipe are always in "touching" position then you don't need to
add nothing to your calculation.
3) You could see also in IEEE-835/1994 [for instance]: in page Intro-23 A2-Calculation examples:
A.2.1 Example 1: 3-1/c 350 kcmil aluminum, 600 V cables installed in a 3 inch PVC conduit in the earth.
Cable Dimensions:
O.D. over conductor: 0.681 in ; O.D. over insulation: 0.871 in ; * Circumscribed diameter = 2.15 x 0.871 = 1.873 in
Cable to conduit:
Rsd=n'A/(1+ (B+CTm)Ds') TOF (Equation 41-Neher&McGrath)
Rsd=3 * 17/[1+ (2.3+0.024x70) (2.15 x 0.871)]= 6.033 TOF [Thermal Ohm-Feet].
4) You may try also Google "IEC 60287 Calculation Examples"
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
the challenge that i have is that i am getting a comment that for part 2 of T'"4 i should use 2.2.4.3.1 along with the FK muliplier in order to account for the single-core cable touching inside the PVC duct.
for me it is clear that this request is not clearly written in the standard, however having section 2.2.4.3.1 entitled "3-single-core cables in trefoil" is making my arguement weaker. I appreciate any advice!
Thanks
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
now for my understanding:
for singel-core conducter
T1 is the heat from the insulation
T3 is the the from the PVC jacket
T'4 is the heat from the inside ambient of the Duct
T"4 is the heat from the Duct thickness
T'" 4 is the mutual heat from the other cables in the duct bank (thats why we multiply by 3).
i fail to see here the effect of the other two cables within the PVC duct.
if we considered 2.2.4.3.1 in any of the above calculations i believe we shouldn't multiply by 3 which leads to very unrealistic results. however i still miss the arguement for not using a section which is called three single-core cables in trefoil
can you advice!
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
ch.2.2.4 it is for “Groups of BURIED cables (touching) equally loaded”. Your cables-if you agree-are in "air" in the duct. Then could be ch. 2.2.1.1 “Cables protected from direct solar radiation”.
In this case for I=203 A and 8.02E-5 ohm/m resistance you'll get 4.5 degrees drop for "free air" and 3.6 dgr. in duct [according to 2.2.7.1].
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
However, i would like first to get your opinion that the standard is not very clear especially when it comes to single-core cables and thats why many interpertations could be possible.
as a result i would like to add to the calculation that i sent to you earlier an additional component to T4 and considering the worst case situation i will consider this component coming from 2.2.4.3.1.
in this case all the understanding we discussed before stay the same but for T4 i add a component from 2.2.4.3.1 which is accounting for three single-core cables which are touching.
in this case T4 = T'4 + T"4 + T'"4 (from earth) + T'"4 (from concrete and mutual effect of external not touching cables in other ducs) + T4new (mutual effect of cables touching within the duct).
the T4new is not multiplied by 3 since this is accounted for in transition from regular 1/(2 x pi) to 1,5/pi which is already multiplication by 3. this is case is very pessimistic since i am considering a rho of concrete where actually i am in free air inside the duct.
based on the above explanation for the calcualtion i sent to you earlier:
T4 becomes 5.062 and the current is 193 A
tell me your opinion!
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
Before, if you would introduce the calculated T1, T2 [actually your T3 is T2 as per 2.1.2.1 and T3=0] and T4 in formula 1.4.1.1:
Δθ = (I2R + ½ Wd) T1 + [I2R (1 + λ1) + Wd] n T2 + [I2R (1 + λ1 + λ2) + Wd] n (T3 + T4)
taking I=203 A- you would get Δθ=90-40=50 oC.
Now, if you will put instead of T4=4.586 the new T4 =4.586-0.361+0.1254=4.3504 then you’ll have to
enhance the current to 208.3 in order to keep the same 50 dgr.C.
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
is it possible to provide some links for examples that utilizes the approach we have here. you told me to go to google and search for IEC calcaution examples but i couldn't find much examples to support that this approach is the one practiced.
Thank you
RE: IEC 287 calculations for three single-core cables in trefoil in 9 way (3 x 3) duct bank
If you’ll put D’e=overall diameter=39 mm you’ll get Ipermissible=187 A.
In my opinion it is a conservative value but I accept it.
In the formula 41 used in IEEE-835/1994 –it is a very close formula to 2.2.7.1-D’s=2.15*overall diameter-and it seems to me more realistic.
Good luck!