Duct bank

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Electriman

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Greetings,

The cable ampacity is greatly reduced when the cable is running in duct bank i.e. a 750 Kcmil per table 310.15 is 535 at 95 degree but when it is in a ductbank next to 20 other cables per calculation it gets near 200 A.
So if a large feeder i.e. 4500 that has above ground and underground portion then the required number of cable for above ground and under ground is different for instance in this example for AG 9 run vs UG 23. Running 23 cables above ground is difficult and not economical.
I am trying to find solutions. I was thinking to use cable tap box or using trench instead of ductbank.
I was hoping I can get some advice in this case. Is there any body experience similar thing and what they have done?
 
You will have the same issues with a trench, as the reduced ampacity is a result of the mutual heating and the insulation effect of the earth.
Larger spacings between the ducts can reduce these effects.
Use of more thermally conductive back fill can also help.
Most of the electrical software suites of calculation programs can run these calculations.
 
don_resqcapt19 said:
You will have the same issues with a trench, as the reduced ampacity is a result of the mutual heating and the insulation effect of the earth.
Larger spacings between the ducts can reduce these effects.
Use of more thermally conductive back fill can also help.
Most of the electrical software suites of calculation programs can run these calculations.
Click to expand...
Thanks for the reply. What about cable tap box? Do you recommend that?
 
You would run 9 runs above ground to tap box and 23 runs from tap box underground. This way you just have 23 runs underground but when you are above ground you run 9.
 
You can try to take advantage of 110.14(C)(2) to utilize 90 deg rating of certain conductors, but remember that the circuit breakers in their enclosures almost always have a 75 degree wire termination that you must consider and use the 75 degree column for ampacity when terminating in a breaker enclosure.

UG conductors will always be derated. More than 2X is pretty intense. Which table are you using for the UG reference to get the 750kcmil at 200A? I think Annex B Table B.310.7 per Detail 3, 750's with the worst dirt (rho = 120) and 100% load factor still have an ampacity of 301A.
110 14 C 2.jpg
 
ron said:
You can try to take advantage of 110.14(C)(2) to utilize 90 deg rating of certain conductors, but remember that the circuit breakers in their enclosures almost always have a 75 degree wire termination that you must consider and use the 75 degree column for ampacity when terminating in a breaker enclosure.

UG conductors will always be derated. More than 2X is pretty intense. Which table are you using for the UG reference to get the 750kcmil at 200A? I think Annex B Table B.310.7 per Detail 3, 750's with the worst dirt (rho = 120) and 100% load factor still have an ampacity of 301A.
View attachment 2553847
Click to expand...
The UG rating is very dependent on the number of conductors in the ductbank. When there is only one the ampacity is higher. But when I have 23 cable and spacing is only 3” between conduits 200 is about what I get.
 
Electriman said:
The UG rating is very dependent on the number of conductors in the ductbank. When there is only one the ampacity is higher. But when I have 23 cable and spacing is only 3” between conduits 200 is about what I get.
Click to expand...
What table are you using, or are you calculating that with software?

The UG rating is dependent on # of conductors in the duct, ducts in the ductbank, spacing between ducts, depth of ducts, rho of the surrounding material. Many things within the designers control
 
According to my calculation [on my Visual Basic 6 program] using the same duct bank data as per NEC Annex B, 750 MCM copper conductor XHHN single core cables in concrete duct bank of 6*7 ducts [42 cables-14 per phase] for 90oC insulation, Earth 20oC 90 RHO, the ampacity was 326 A. That means 14*326=4564 A per phase
 
Julius Right said:
According to my calculation [on my Visual Basic 6 program] using the same duct bank data as per NEC Annex B, 750 MCM copper conductor XHHN single core cables in concrete duct bank of 6*7 ducts [42 cables-14 per phase] for 90oC insulation, Earth 20oC 90 RHO, the ampacity was 326 A. That means 14*326=4564 A per phase
Click to expand...
That is good. How have you verified the program?
 
One program is based on Neher & McGrath theory and another according to IEC 60287.
The cable dimensions are very important.
conductor diameter:23.06 mm[compact]
cable overall diameter:27.2mm [Okonite catalogue]
d.c.resistance at 20oC: 0.0463 ohm/km
 
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