Ampacity Ratings of Urd Power Cable

[joozu6]

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.

 

[joozu6]

Correction, Rated at continuous current @ 90 DegF

 

[GiSH]

Do the companies give details of the ducting used, if this is with or without bentonite filling etc?
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.

 

[joozu6]

Some were more specific than others, but for most of them it is one cable per duct. No filling was mentioned.

 

[cuky2000]

I guess the variation in ampacity depend on the overall cable thickness, insulating material characteristics and cable construction that may be different among manufacturers.

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.

 

[BJC]

I'd would ask someone at the local utility if you can. Soil conditions can vary by area and seasonally. Around here the cables are essentially in mud except August of every third year when they think they are in Arizona.

 

[WoodrowJWeen]

Could you be looking at two different insulation types? XLP vs. EPR maybe.

 

[joozu6]

No. They are all XLPE.

Thanks.

 

[k2panman]

The companies that sell the cable should all have computer programs that can calculate allowable currents for your particular installation. They are based on the duct configuration, depth of burial, earth ambient temp, load factor, wether the shield/neutral is connected at one end or both, what the earth thermal resistivity is (big factor but most people just use the standard Rho=90). I would discuss this all with your cable manufacturer - you can contact Okonite (epr insulated cable - all we use) and they have an engineering data bulletin #ehb-98 that gives a lot of info on this subject along with tables of ampacity data for different configurations.

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