Neher McGrath Typical Value for Concrete 1

[RonShap]

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?

 

[Gragas]

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

 

[mrcheese]

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.

 

[DRWeig]

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

Haven't see the forum policies? Do so now: Forum Policies

 

[7anoter4]

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!

 

[waross]

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

The real truth is nobody wants reality.

Grin

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

 

[mrcheese]

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.html

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