Ampacity Ratings of Urd Power Cable
Ampacity Ratings of Urd Power Cable
(OP)
In trying to determing the ratings of underground cable for my company I determined that the manufacturers vary widely in their ratings, i.e., for 25kV 4/0 Copper 1/3 neutral 260 Mil insulated cable buried in ducts: Southwire = 287 Amps, King Wire = 320 Amps, and Perreli = 314 Amps. The ratings are all for 90 DegC, continuous current, in ducts.
My questions are:
1. Does anyone know why they vary so widely?
2.Which one is correct or is there a better document or calculation to get accurate ampacities?
Thanks.
My questions are:
1. Does anyone know why they vary so widely?
2.Which one is correct or is there a better document or calculation to get accurate ampacities?
Thanks.






RE: Ampacity Ratings of Urd Power Cable
RE: Ampacity Ratings of Urd Power Cable
I've seen some cable data that provides ducted thermal ratings but is based on 3 single core cables laid in trefoil in a single duct where as others can be based upon single core cables in seperate ducts.
This is the only thing I can think of other than the construction of the cable varying between manufacturers and some applying different de-rating factors for the thermal resistivity of the specific materials used.
RE: Ampacity Ratings of Urd Power Cable
RE: Ampacity Ratings of Urd Power Cable
In addition, there are other external factor and operating conditions assumed for the ampacity calculation such as:
ambient soil temperature(typ 20 oC), depth of duct bank, max. conductor operating temperature, earth thermal resistance (RHO), concrete RHO, duct RHO, duct diameter/thickness, load factor, number of circuit in the duct bank, cable arrangement, grounding points: Single vs. multiple, etc.
RE: Ampacity Ratings of Urd Power Cable
RE: Ampacity Ratings of Urd Power Cable
RE: Ampacity Ratings of Urd Power Cable
Thanks.
RE: Ampacity Ratings of Urd Power Cable
One factor that makes a big difference is the earth thermal resistivity. It varies with soil moisture, so just checking it at one point in time is not good. We don't measure it here (we are a small cooperative) and I think most people dont bother to try to measure it, but it can make a big difference in the allowable ampacity. This is obovious if you think about it as the earth carries away the heat generated.
Also, your cable is probably rated at 90 degrees C
K2ofKeyLargo