Underground Raceway Calculations

[timm33333]

I am looking for a solved example which shows step by step calculations of underground cable ampacity using Neher McGrath method. The original Neher McGrath paper (published in 1957) has some examples, but these are not clear. NEC Annex B has tables, but it does not have example. Maybe IEEE-835 has examples of Neher McGrath method, but I don't have access to IEEE-835. Is there some solved example available on internet which describes more clear calculations of underground cable ampacity using Neher McGrath method? Thanks.

 

[7anoter4]

In IEEE 835/1994 there are only 4 calculation examples. No one is concerning cable ducts:

 

[timm33333]

Thanks. As cable ducts have more heat loss than direct buried cables, so the whole cable has to be sized for cable duct. Is there some document which has examples of calculations of cable ducts?

 

[FacEngrPE]

You can try this link

http://www.neher-mcgrath.net/calc.html

 

[timm33333]

Thanks. This link has an example of calculations of a 230 kV underground cable, which is good. I think we can use the same example calculations for low voltage cable as well. But as the low voltage cable does not have metallic sheath, so we can set all the metallic sheath factors to zero, and then use the same equations which are used for 230 kV cable. Is it correct?

 

[timm33333]

Also when we do underground raceway calculations using software like ETAP, several duct banks can be modeled together. What should be the spacing between adjacent duct banks? Thanks.

 

[7anoter4]

Theoretically in order to not be considered at all, the distance has to be infinite. In my opinion a distance equal to the maximum duct bank width it will be enough. As I calculated for 9*3 cables of 500 MCM copper for 1 width distance between two duct banks the ampacity will be only 82% of the ampacity calculated for a single duct bank and if the distance will be twice the duct bank width the ampacity will be 85%.