NEC 310.15(C) Engineering Supervision Calcs

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J.D.D.

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Hi Everyone, I am relatively new to the forum and this is my first post, hopefully someone can help.

I am employed as an Electrical Engineer for a Consulting firm and I am creating an Excel sheet to calculate the permissable ampacities for multiple underground conductors. The formula can be found easily in the code under 310.15(c). However there are certain constants in the formula, such as dielectric loss(DeltaTD), effective thermal resistance(RCA), etc that aren't given in the code as far as I know.

Does anyone know of a credible source where I can find these values?

Thanks,
J.D.D.
 
First of all, welcome to the forum. Secondly, you picked a particularly difficult question with which to begin your participation.

I don?t think there are any good resources. I also don?t think that formula is useful. Setting up the problem of calculating ampacity for underground ducts requires more than a simple spreadsheet with look-up tables of key parameter values. It is a system of ?a thousand unknowns and a thousand equations.? I believe it can only be solved using iterative methods.

My company has purchased a software package that is designed to perform these calculations. Among the parameters that I must give to the program, in order for it to perform a calculation, are the number and sizes of ducts, the geometry of the installation (e.g., a 3 x 4 array with 7.5 inches on center), the number and sizes of conductors in each duct, the type of fill material (i.e., concrete or dirt) around the ducts, the type of dirt above the ductbank, the depth of the ductbank, and the temperature limits we wish to impose. That degree of complexity is well beyond the simple looking formula contained in the NEC.
 
The reason I asked is due to a DCA comment that came back requesting to see the calcs on a drawing. I presumed that it would be a relatively simple fix, but now it is becoming clear that I may have a lot more work on my hands.

Thanks & Much Appreciated,
J.D.D.
 
Then to make it more complex, you can have external heat sources, such as a steam pipe running parallel that adds heat. This also needs ot be accounted for.
 
Ieee

Ieee

:grin: Another place to look is the IEEE Brown Book for cable ampacities it add to the confusion and difficulty. More formulas and variables.
 
J.D.D. said:
The reason I asked is due to a DCA comment that came back requesting to see the calcs on a drawing.
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This tells me that a design has been completed, or at least nearly completed. So I suspect that the layout of the ductbank is known, and a set of conductor sizes has been chosen. If you are now being asked to produce a calculation to justify the conductor sizes, then how were the conductors (i.e., the ampacities) selected in the first place?
 
The depth of burial also needs to be considered as well as the soil type. The soil dissipates heat very slowly so the deeper you embed your raceway the lass ampacity the cable can carry. Use a software package to calculate the ampacity.
 
I'd say the company you work for needs to pony-up the money to buy a software program. That will only cost them $10k.

If it's something you won't need to do again for a while, it may be a lot cheaper/quicker to sub it out to another consultant to do for you.
 
Neher-McGrath

Neher-McGrath

I do thermal calcs all the time. You need to get a copy of the Neher-McGrath paper and study it. It is the basis for ALL ampacity calculations, including IEC.

The equivalent thermal resistance is a thermodynamic engineering question that is quite complex. I use 2D and 3D finite element modeling to perform these calculations.
 
kingpb said:
I'd say the company you work for needs to pony-up the money to buy a software program. That will only cost them $10k.
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The one I use cost my company closer to $2K. But it will only do ampacity calculations. The ones that cost $10K will do a variety of other calculations, such as voltage drop, short circuit, fault coordination, and load flow.
 
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