Underground Duct Bank Ampacity Calculation

[OCPD]

Hello All,


I will be using software (etap) for underground duct ampacity calculations. I was wondering what all information for cable is required to use Neher-Mcgrath equation. I will be requesting information for cables from the cable manufacture, want to make sure I request the correct items.

 

[ron]

The cable itself has relatively little to do with it.

Generally it is the terminations (75 deg for instance is common for 600V or less) that impose the biggest limitation, along with how it gets installed in the concrete/dirt and what kind of concrete/dirt.

 

[kingpb]

One of the most, if not the most important is to know the proper Rho of the soil and the concrete, i.e. surrounding medium (if conduits are encased in concrete). Using the default values can lead you a stray. The information can be gotten by having a soil test report done but you have to specifically ask for it, as it is not part of a "normal" study. On projects with large foundations this is a common thing done.

Also, the depth has a huge factor. Automatically you think deeper is better, but not true in actuality.

 

[Dale001289]

Hello All,


I will be using software (etap) for underground duct ampacity calculations. I was wondering what all information for cable is required to use Neher-Mcgrath equation. I will be requesting information for cables from the cable manufacture, want to make sure I request the correct items.

In addition Rho factor and depth, spacing criteria between ducts plays a role also.

 

[Julius Right]

I agree with kingpb .If we take the thermal resistance chain and we have Ri as thermal resistance through insulation, Rj through jacket, Rsd cable to duct, Rc thermal resistance of the duct itself and Re combined resistance of the duct bank and Earth, the last resistance represents approx.50% of the total resistance.
However, the shield [and armor] losses could reduce the ampacity of the cable. The shield/armor circulating current [if this will be grounded in multiple points -or at the both ends] for instance, it is very important.
Usually, high voltage cables (over 35 kV) are not recommended to run through ducts but directly buried.
IEC standard cable is simpler and the Etap [I don't use this program but I think so] could identify it according to cable symbol.