Effect of conduit size on cable derating factor
Effect of conduit size on cable derating factor
(OP)
Hi everyone,
I am pretty new to the field, and I am wondering if anyone can advise me on this. In relation to underground distribution cable, if ducted installation is to be performed, would the conduit size/ duct size being used affects the derating factor of the cable to be installed. If it has, then what is the main cause for it? i.e. heating?
Thanks in advance.
I am pretty new to the field, and I am wondering if anyone can advise me on this. In relation to underground distribution cable, if ducted installation is to be performed, would the conduit size/ duct size being used affects the derating factor of the cable to be installed. If it has, then what is the main cause for it? i.e. heating?
Thanks in advance.






RE: Effect of conduit size on cable derating factor
(a) Thermal Resistivity through cable air from cable surface to duct inside surface - mostly (but not totally) independant of duct size
(b) Thermal Resistivity through duct wall - decrease as the ratio of the duct's outside to inside diameter decrease (i.e. increase the raduis but keep wall thickness the same) - more area (smaller resistivity) is available for heat flow as the raduis is increased. The thermal resistivity will increase if the wall thickness is increased.
(c) Thermal Resistivity from duct to surounding material (backfill/concrete) - decrease as duct size increase (larger duct area is in contact with surrounding material and therefore easier heat transfer)
You will need to use IEC 287 or finite element analysis packages if you want to calculate the exact influence for your application.
RE: Effect of conduit size on cable derating factor
RE: Effect of conduit size on cable derating factor
First of all, we cannot change THERMAL RESITIVITY of materials as this is one of their characteristics. We can change thermal RESISTANCE.
(a) item - It suggests that if you increase conduit/duct size the circuit ampacity increases (sorry if I misunderstood). If we increase distance and air volume inside the duct the thermal RESISTANCE increases as thermal resitivity of air is approx. 33.8 [K*m/W] !!! which is many times bigger than i.e. water 1.65 [K*m/W] or even PVC 6.0 [K*m/W]. This is why a material called bentonite is pumped into the ducts to expell the air.
Using "derating factors" on buried cables is only approximation. I would suggest using IEC60287 standard or Neher-McGrath calculation which includes cyclical loading. In more complicated arrangements the mentioned finite element method is the best practical solution.
RE: Effect of conduit size on cable derating factor
T'4=n'·A/[1+0.1·(B+C·Tm)·D's]
where D's is the equivalent diameter of the cable. The simplified equation is independent of the duct diameter, as stated by KJvR.
The thermal resistance of the duct wall and from the duct wall to surrounding earth is lower for larger ducts as stated by KJvR.
RE: Effect of conduit size on cable derating factor
Thank you as well to jghrist, KJvR and cgrodzinski for providing me with the document and equation used to calculate this. I will try to do further research on this.
RE: Effect of conduit size on cable derating factor
RE: Effect of conduit size on cable derating factor
L30N,
if you have a project that involves more that one circuit in close proximity to each other I would suggest asking for help an engineer specilizing in u/g T&D circuits. Pleae note that once the circuit is buried we have very little control over it beside de-rating. This, in turn, could be learned in the costly way sometimes. The only way to monitor its performance is to install a system called DTS which would measure the cable temperature along the entire length of the circuit. A sensor for this device is common fiber optic cable which costs a fraction of a power cable.
RE: Effect of conduit size on cable derating factor
I wouldn't use this to increase the cable rating factor.
RE: Effect of conduit size on cable derating factor
ht