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.
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
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
RE: Thermal Resistivity and Underground Cable Sizing
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
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.