Neher-McGrath calculations 1

[kingFrank]

Looking at the Neher-McGrath formula
I = sqrt((Tc - Ta)/(Rdc*Rca)), alot of factors must be considered. It is very hard to find any documentation of written examples, if anyone has ever worked on these calcs and has notes or anything to help with please let me know.

frank.m.gonzales@sce.com

 

[busbar]

Shot from the hip, but it's likely that most is entered into ready-made software, {maybe} included in loadflow, etc packages.

http://www.google.com/search?hl=en&q=Neher-McGrath+software

 

[busbar]

http://www.calcware.com/cwnmcalc2.htm

http://www.southwire.com/tech/pubs/tdu/98/tdu1098.htm

http://www.me.gatech.edu/me/publicat/brochures/rb/Black.html

http://www.prc.gatech.edu/publications/th.htm

An IEEE search would probably be a goldmine {or minefield}

 

Recommending this book "Rating of Electric Power Cables Ampacity Computations for Transmission, Distribution, and Industrial Applications" by George J. Anders IEEE Press

 

[jghrist]

I recommend:

Rating of Electric Power Cables, Ampacity Computations for Transmission, Distribution, and Industrial Applications, George J. Anders, IEEE Press, 1997.

 

[cuky2000]

Dear Mr. Gonzales:

Engineering application sometime are not easy but, there are a plenty opportunities for satisfactions building projects and applying new knowledge.

The formula that you suggested is a simplify form. There are other terms to be considered as follow:

i) I=SQRT{[T_c-( T_a+Delta.T_d)]/R_dc(1+Y_c).R’_ca}

ii) R’_ca=Ri+Rj+Re.

_{Where:
Ta = Ambient temperature (oC)
Tc = Conductor Temperature(oC)
Delta(Td) = Temp. Rise due to dielectric losses (oC).
1+Yc = Skin & proximity effect
R’ca = Total thermal resistance. In Thermal Ohm Feet (TOF)
Ri = Insulation thermal resistance (TOF)
Rj = Jacket thermal resistance (TOF)
Re = External resistance (Air, duct bank, etc).}

In many applications there are not incentive to expend a lot of time searching, learning in depth or investing in software. However, for HV or special LV & MV applications the selection and sizing the proper cable system requires from the design engineers more detail work and knowledge in this matter.

If you would like to start in this area, I recommend the IEEE publication "Cable Ampacity" Section 3 (Cable Parameters) and Annex A-2 (Calculation Samples).

A common and dangerous mistake is to be proficient in software without understanding the principles involved in the parameter selections process.

Good luck.