soil rho for duct bank

[tgammon]

Does anyone know how to estimate soil resistivity for duct bank?

If the conduit is packed in sand which has been compacted with water, and some of the conduits are known to be infiltrated with water, what is a good estimate for rho?

120 -- very dry soil (rocky or sandy)
90 -- average soil
60 -- damp soil

 

[charlie b]

Since you are an engineer, it will be you (or the PE signing the design drawings) who makes the call. I have used a RHO value of 60 for installations in my area (Seattle). I think you could use 90 without being questioned, in most of the country. I don't think you need to go with 120, unless you are talking about rural Nevada or a similar area.

 

[kingpb]

If in doubt use the RHO that requires the most strict design. But I would agree with CEB, that 90 is pretty standard.

 

[bob]

In annex B of the NEC it states that Average soil (90% of the USA) has a Rho pf 90. Okonite lists its UG cable ampacity using Rho = 90.