Cable Derating
Cable Derating
(OP)
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
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






RE: Cable Derating
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.
RE: Cable Derating
It probably won't give you how it calculates..
RE: Cable Derating
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.
RE: Cable Derating
There is some basic shielded-cable-in-ductbank derating info at
www.ke
RE: Cable Derating
Thank you. Good to know it.
RE: Cable Derating
RE: Cable Derating
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:/