Cable Derating 3

[Chuk1t]

I am trying to calculate cable derating factors for underground Medium Voltage EPR (15kV)cable using the
Neher-McGrath formula. Can anyone recommend a good handbook
with calculation examples?? The cables are assumed to be in concrete encased duct bank.

Any help would be appreciated

 

[dpc]

I'm not aware of a good reference book on Neher-Mcgrath method.

The original Neher-McGrath paper should still be available through IEEE, I believe. It was originally published by AIEE in October, 1957.

You might check with Okonite and Southwire. They both used to have Cable Installation guides that may have some information on de-rating procedure and equations. I haven't looked at their reference material in a while, however.

Okonite will do a calculation for you if you provide them with your configuration.

The real issue is coming up with an accurate value for the thermal resistivity of the soil.

Maybe someone else will be able to provide more references.

 

[rbulsara]

You may wish to try ampcalc, a software program. Google search for it. No idea how good or bad this software is.

It probably won't give you how it calculates..

 

[BJC]

Look up thread 238-44759.
I was having trouble with the ampacity charts in the back of the NEC and had the people help me.
USe the chart to adjust for for various rho values and load factor. You can use rho = 55 for concrete.

The Okonite websiet

http://www.okonite.com/

has ampacities for medium voltage cabels in one, three and six duct configurations. Look in their "engineering technical center".

rbulsara
Ampcalc is good and I have used it. It's a pretty expensive program.

 

[busbar]

 
There is some basic shielded-cable-in-ductbank derating info at

http://www.kerite.com/catalog/catalogfiles/ampacities_power.htm

 

[rbulsara]

bjc:

Thank you. Good to know it.

 

[jghrist]

I'd recommend Rating of Electric Power Cables, Amapacity Computations for Transmission, Distribution, and Indstria Applications, George J. Anders, IEEE Press/McGraw-Hill, 1997. This book covers both Neher-McGrath and IEC 287 methods of calculation and has lots of examples.

 

[cuky2000]

The Neher-McGrath formula is not easy to follow and you will be surprised of the amount of data and assumptions to be made. Example of this data required is soil thermal resistivity, loading profile (load factor), insulation max. temperature, geometry of duct bank, etc.

A practical way to calculate unusual configuration not included in the NEC is to use commercially available and validated software.

To see a partial view of the Neher-McGrath, see the enclose sites.

http://www.electrician.com/articles/ampacity.htm

http://www.elect.mrt.ac.lk/IEE2002powercables.pdf

http://www.vtt.fi/pro/tutkimus/tesla/julkaisut/rep66.pdf