Electrical Duct Bank - Required Distance

[DMG_1]

I have an electric duct bank design that is presently has 6 circuits:

(2) 22.9 kV Lines - 350 MCM
(4) 4.16 kV Lines - 750 MCM

Six circuits would classify this, according to NEC 310.60, as Detail 3. I am thinking about splitting this up into two seperate electrical duct banks so that each would be classifed as Detail 2. The difference in conductor ampere rating between Detail 2 and Detail 3 is approximately 20%. I am looking for information indicating the distance of seperation between the two electrical duct banks so that they can be considered seperate entities. Can anyone give me some insight on this?

Thanks.

 

[bob]

Did you actually make the Neher-McGrath calculations or are you just taking the values from the table? The problem that I find
with the table are the choice of Rho and the Load factor. If your conditions do not match the choices then the table does not
apply. I am not looking at the table but the LF choices are 100% and 50% if memory is correct. I would like to see a 75% factor added.
I found this information that may assist you.
http://neher-mcgrath.com/Heating_Calculations.pdf

 

[DMG_1]

Bob.

Sorry for the delay in my response. I did not make the Neher-McGrath calculations. I was thinking of splitting the duct bank into a 22.9 kV line and a 4.16 kV line and use the a standard configuration in the NEC using a Load Factor of 100%. The problem is that in both instances we are running the feeders over 200 feet and I do not want to oversize the design because the price of copper is ridiculous. I am going to ask the customer if they can do a 24 hour load study in an effort to determine the actual Load Factor. Once we do this I will then do the calculation. I referenced the .pdf that you sent me and I can see that the Load Factor makes a big difference. Thanks for your help.

Dave

 

[charlie b]

I am looking for information indicating the distance of seperation between the two electrical duct banks so that they can be considered seperate entities.

The code does not provide an answer to this question. I would think that three feet is enough. But I have no technical basis for that number. My defense for choosing that distance, if I were to be questioned by an AHJ, would be that I need to have enough dirt between the two ductbanks to allow heat to dissipate to the other dirt that surrounds them both.

 

[charlie b]

I am going to ask the customer if they can do a 24 hour load study in an effort to determine the actual Load Factor. Once we do this I will then do the calculation.

Oh yea? May I ask exactly what formula you will be using? While I am at it, how would you define "load factor," given that the NEC does not define that term? :happyno:

 

[DMG_1]

charlie b.

See page 2 of the attached. http://neher-mcgrath.com/Heating_Calculations.pdf

 

[charlie b]

See page 2 of the attached. http://neher-mcgrath.com/Heating_Calculations.pdf

That is the same formula that appears in one of my old college textbooks. I know of no other, or better, way to define that term. But how do we know that that was the basis for the NEC tables?

Please note that in order to get below 50% LF, the building has to be nearly completely shutdown for half the day. The pdf claims that 41.6% is typical for a commercial building. Their calculation assumed (essentially) that the load remained at its peak value for 10 hours, and was zero for the remaining 14 hours. That does not seem reasonable to me. I would be curious to see the results of your calculation.

 

[kingpb]

Since you are running the circuits 200ft, do you know if you have any point sources of heat, or steam lines nearby? These will all affect the design.

I think your best solution is to utilize a computer program for your design. The NEC, IMO, only provides rough order of magnitude type direction for underground duct banks. There are far to many variables involved to adequately perform the number of iterations that may be required. After all you are talking about MV distribution lines, not some circuit out to a remote pump house.

 

[bob]

DMG check your messages.

 

[DMG_1]

Thanks Bob.