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Neher McGrath Typical Value for Concrete

Neher McGrath Typical Value for Concrete

Neher McGrath Typical Value for Concrete

(OP)
I think it is interesting that when doing underground temp calcs, that most use 55 for the Rho of concrete, because it is printed as a typcial value in the NEC.

The "father" of underground duct bank calcs wrote in his paper in 1957 (AIEE Paper 57-660, Table IV) that the typical value is 85.

Any ideas why it has changed?  Maybe a simple typo?  Has anyone had their concrete tested for thermal Rho?

RE: Neher McGrath Typical Value for Concrete

Not sure why, but I am glad it is... the derates would hurt.

RE: Neher McGrath Typical Value for Concrete

55 is not a safe number to use any more than 90 is a safe number for earth. We have concrete tested and it the RHO can range from 45-90.

Reality means you live until you die. The real truth is nobody wants reality.


RE: Neher McGrath Typical Value for Concrete

Thanks scott449, that's good info.

I take excepton to your signature, though. To paraphrase Pastis,

We're born, we die. But in between we got scotch, cigars, and donuts!

Best to you,

Goober Dave

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RE: Neher McGrath Typical Value for Concrete

I agree with scott449.It is very embarrassing. NEC Annex B "Typical values of thermal resistivity (Rho) concrete=55; IEEE Standard 835-1994 3.1.4.2 Duct banks Concrete: 60 °C cm/W; IEC 60287-2-1 Table 1 – Thermal resistivities of materials, Materials for duct installations Concrete =1 Km/W [100 °C cm/W].
The same problem with Earth Thermal Resistance. NEC= 90 C cm/W ;IEEE 835 60,90,120 C cm/W ;VDE 298 07-2.5 Km/W
I think NEC is so generous since it about domestic [or commercial or industrial facilities] where the actual load is super-estimated [and so the diversity].
For a steady state load- as in a Power Station- 100 % load factor one has to take a more elevated value.
And I agree with DRWeig, too .I doubt-at least for now- about cigar and donuts-but a good scotch it is a good reason to live!

RE: Neher McGrath Typical Value for Concrete

I'm not sure about the concrete, but so far e seem to have unanimous agreement on this...

Quote (scott449)

The real truth is nobody wants reality.
Grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Neher McGrath Typical Value for Concrete

One of the biggest misunderstanding about RHO is that engineers get the soil tested and then say " look it is X Rho In-situ lets use X RHO" good paper here on this. http://www.neher-mcgrath.com/41/neher-mcgrath_FAQ....

Reality is present RHO is no guarantee of future RHO, The real truth is nobody wants reality.



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