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Thermal Resistivity and Underground Cable Sizing

Thermal Resistivity and Underground Cable Sizing

Thermal Resistivity and Underground Cable Sizing

(OP)
I'm sizing the underground cable in direct buried conduits with 2 x 8 arrangement using ETAP software.The thermal resistivity (RHO) based on the geotechnical report indicates around 40 cm degC/Watts which is considered very low. The IEEE 835, and NEC basically list the soil RHO at 60, 90 , and 120 and don't mention if we can use Soil RHO that is less than 60.

My question is would it be acceptable to use the thermal resistivity at 40 cm degC/Watts to avoid oversizing the cable? I was using soil RHO at 60 and ended up need to increase the cable size from 250 kcmil to 750 kcmil in order to avoid the cable to be overheated when the cable is loaded at 100%.

Your inputs will be greatly appreciated.

Thanks.

RE: Thermal Resistivity and Underground Cable Sizing

That is really your decision if you are doing the design. Personally, I would not be comfortable doing this unless using controlled, select backfill such as thermal sand. The main problem with the Neher-McGrath calculations is all of the thermal constants required. If I was considering taking credit for that low value, I would want to review this with the geotechnical engineer to see how competent they are and how confident they are with that number.

You might look at using direct-buried CLX-type metal-clad cable. This might increase the ampacity a little. Okonite will probably calculate this for you.

RE: Thermal Resistivity and Underground Cable Sizing

The NEC allows for engineered calculations. For conductors rated 2001 to 35,000 Volts, there is an equation under 310.60(D). For conductors rated 0-2000 Volts, there is an equation under 310.15(C). Both equations allow you to calculate the ampacity of the conductor with your own value of conductor temp, ambient temp, dc conductor resistance, Yc, and thermal resistivity.

RE: Thermal Resistivity and Underground Cable Sizing

Soil tests are done prior to construction. During construction a lot of dirt could get moved and replaced. How confident are you that the same soil will be around and on top of the duct bank and that the duct burial depth will be close enough to your calculations? What is the rho of the backfill material? Or the rho of the asphalt and gravel base where the duct bank crosses the road?

How good is your loading estimate? Will it really be 100% load factor? If it is 80% with rho 60 or 90 what size cable do you need?

My view that we as engineers look at all of these variables and make an informed decision about the cable's current carrying capability under all conditions. No one answer works, unless we agree to oversize everything.

RE: Thermal Resistivity and Underground Cable Sizing

I am not sure I understood the issue. There are 2*8 conduits[metallic or PVC?] filled with cables[single-core or three-cores]
how many circuit are? If there is a neutral then could be 4 circuits?
If I did not wrong calculation the approx.[ 100% LF, 40 Rho, 25oC, 250 mcm copper conductor, 600 V ] 285 A per each conductor
and for 60 Rho 300 MCM will do the same job.

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