Neher-Mcgrath Calculation limitation

[Andrew445]

Hi,

I'm using ETAP to model underground raceways. I am having trouble modeling my situation because of certain geometric limitations with the Neher-Mcgrath method. I want to model DC cables in conduit buried below 4" of concerete. The concrete would be flush with the surface of the earth. The analysis software is telling me that the raceway is too close to the surface. How else am I supposed to model this situation? Is there some variation on this method I can use, or can I add some variance factor to make it work out? I'm thinking I might just have to model a "worst-case scenario" and go with it but I was wondering if anyone else has run into this problem.

Thank you for your time,

Andrew

 

[charlie b]

It has been a while since I used ETAP, so I don't know what its limits are with regard to modelling an installation. But I wonder if the installation you describe is even legal. Take a look at table 300.5. I believe it requires a minimum amount of dirt to be above the concrete, unless you are directly under a building.

Welcome to the forum.

 

[Andrew445]

It is legal, in accordance with Column 3, Row 4 of Table 300.5.

 

[jghrist]

Ampacity will be lower with soil over the concrete than with the concrete extending to the surface because the thermal resistivity of concrete is lower than soil. If you tell SKM that there is some soil you will get a conservative answer.