Underground Raceway Calculations (NEC)

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Julius Right said:
I’ll try to copy and transmit one of calculation example from IEEE 835 based on Neher & McGrath theory.
First a.c. resistance calculation
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Thanks for the example calculation. Unfortunately it did not download very well and seemed partial. If you have time to send again to me, i would be grateful. I find the IEC and the derating tables supplied by the metric cable manufactures very helpful. I would really like to be able to use the Neher McGrath formula and compare results. Thank you in advance if you can share your experience and knowledge. I really would like to be able to master this formula.
 
The example calculations which are attached above are for single-circuit/single-ductbank. If we have multiple-circuits/multiple-duct banks in parallel, then what derating factors would be required?
 
When we use three single conductor cables to form a single 3-phase circuit, it actually has three (single conductor) cables. So even if it is a single three-phase circuit, still it comprises of three (single conductor) cables. Do we need to do derating for this?
 
JEFF MILLAR said:
Thanks for the example calculation. Unfortunately it did not download very well and seemed partial. If you have time to send again to me, i would be grateful. I find the IEC and the derating tables supplied by the metric cable manufactures very helpful. I would really like to be able to use the Neher McGrath formula and compare results. Thank you in advance if you can share your experience and knowledge. I really would like to be able to master this formula.
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If possible can you email with attachment.
 
In my last pdf file are 9 ducts and 3 single core cables per duct.
 

Attachments

  • 9 ducts 3 single core cables in each duct.pdf
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I have a similar situation, we need to swap out a 1MVA transformer for 1 2.5MVA. As the transformer has 480V secondary the ampacity is 3010A.
I've been looking through the code, and ended up in Annex B. Problem is that Annex B, is not part of NEC, so do I just use table 310.16. Which is for direct buried. To say I'm confused is an understatement.
 
Chunkie said:
I have a similar situation, we need to swap out a 1MVA transformer for 1 2.5MVA. As the transformer has 480V secondary the ampacity is 3010A.
I've been looking through the code, and ended up in Annex B. Problem is that Annex B, is not part of NEC, so do I just use table 310.16. Which is for direct buried. To say I'm confused is an understatement.
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Aren't you under the CEC? Tables are similar but not sure they are the same
 
If you are talking about derating underground circuits in duct banks due to mutual heating between circuits, the NEC does not require derating of LV circuits unless something has changed quite recently. Derating due to mutual heating is required for higher voltages. Neher-McGrath method is used for this. This assumes that the LV conductors are sized based on applicable NEC load calculations. (Other derating factors such as ambient temperature and more than three conductors in a raceway must be applied for LV feeders. )

AmpCalc software has been around for a long time and provides a full-featured implementation of the Neher-McGrath ampacity calculation. Another option is to have a cable supplier do it for you. The difficulty in the Neher-McGrath method is determining the heat transfer properties and constants for all of the materials involved.
 
David Castor said:
If you are talking about derating underground circuits in duct banks due to mutual heating between circuits, the NEC does not require derating of LV circuits unless something has changed quite recently. Derating due to mutual heating is required for higher voltages. Neher-McGrath method is used for this. This assumes that the LV conductors are sized based on applicable NEC load calculations. (Other derating factors such as ambient temperature and more than three conductors in a raceway must be applied for LV feeders. )

AmpCalc software has been around for a long time and provides a full-featured implementation of the Neher-McGrath ampacity calculation. Another option is to have a cable supplier do it for you. The difficulty in the Neher-McGrath method is determining the heat transfer properties and constants for all of the materials involved.
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i don't have the soil data, so are you saving that I can use table 310.16? I'll be using 3C-750MCM.
 
If you are using direct burial then yes. 2017 NEC
Table 310.15(B)(16) (formerly Table 310.16) Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts,
60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth
(Directly Buried), Based on Ambient Temperature of 30°C (86°F)*

What code cycle
 
This is from the 2020

310.16 Ampacities of Insulated Conductors in Raceway, Cable, or Earth (Directly Buried).



The ampacities shall be as specified in Table 310.16 where all of the following conditions apply:
  • (1)
    Conductors are rated 0 volts through 2000 volts.
  • (2)
    Conductors are rated 60°C (140°F), 75°C (167°F), or 90°C (194°F).
  • (3)
    Wiring is installed in a 30°C (86°F) ambient temperature.
  • (4)
    There are not more than three current-carrying conductors.
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Chunkie said:
i don't have the soil data, so are you saving that I can use table 310.16? I'll be using 3C-750MCM.
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For low voltage, NEC does not require derating for multiple circuits in an underground duct bank. These requirements are in Article 311 and only apply to medium-voltage cables. All of the derating requirements in Article 310 would apply.
 
As mentioned, Annex B is not technically part of the Code, but is referenced in Informational Notes in several spots and is helpful when considering Code Sections such as 310.14(A)(3) Temperature Limitation of Conductors.
No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a with respect to type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded.
In my experience, if you are designing to Code minimums, you generally do not have to consider low voltage UG feeder derating (although an overzealous AHJ might invoke 310.14(A)(3) to force it. When designing for reliability, most Engineers will use Neher McGrath, in which Annex B Tables are prescriptive typical applications of those calculations.
 
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