RHO thermal resistivity

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frenchcomp

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I note thermal resistivity for concrete is 55?C.cm/W in B.310.15(B)(2) NEC 2005 but in figures B.310.3 B.310.4 and B.310.5 that Rho concrete = Rho Earth-5 (minus 5).
As Rho Earth can vary up to 120 (and more?), I believe it'll be more adequate to consider 120-5=115 instead of 55 when cabling in duct bank backfill with concrete.
Right?
 
Re: RHO thermal resistivity

Why would you want to do that? The higher the value of RHO, the lower your ampacity.
 
Re: RHO thermal resistivity

In this topic my concerne is not only to be safe, I don't try to get lower ou higher ampacities but I need to consider applicable data of the project.
If ducts runs in concrete, I'd like to know either I use Rho=55 or Rho=Rho earth-5.
 
Re: RHO thermal resistivity

Have their been any soil borings taken? That would tell you for sure (I think). But that might be going too far depending on the project size.
 
Re: RHO thermal resistivity

The topic is RHO of the concrete, not the soil. I would stick with 55, and use 90 for the soil. Those are given numbers; you could not be questioned or criticized for that choice. Anything else would require an engineered justification. You would have to show why you picked a different number, and why that number is valid for the situation.

But here again I have to ask: If you have a valid, justifiable number (55), why would you seek to use a different number that would only serve to make the installation more costly?
 
Re: RHO thermal resistivity

If Charlie is right, it is clear I will use lower Rho value to minimize cost.
But I wonder if Rho=55 for concrete is not a value for concrete only and that when concrete is used as backfill in soil trench as per figure B.310.x, the Rho will be Rho soil -5: i.e. 90-5= 85.
Looking figures B.310.x there is only approx. 2in. concrete which separe the soil from the first duct, therefore the heating dissipation on that duct will be the result of Rho concrete but also Rho soil. Concrete is too thin to have a total influence (55) but will reduce a little bit the soil value (-5).
If I'm wrong, why is it written as design criteria of figure B. 310.3 that Rho concrete = Rho Earth-5 and not 55?
 
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