# Thread: Underground Duct Ampacity 310.15(B)

1. ## Underground Duct Ampacity 310.15(B)

Where in the NEC does it mandate that we need to use another table for ampacity other than 310.15(B)(16) for underground feeders, such as those found in Annex B for low voltage?

Mutual heating would imply Neher McGrath Calcs or the tables in Annex B, but I can't find the requirement.

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Originally Posted by ron
Where in the NEC does it mandate that we need to use another table for ampacity other than 310.15(B)(16) for underground feeders, such as those found in Annex B for low voltage?

Mutual heating would imply Neher McGrath Calcs or the tables in Annex B, but I can't find the requirement.
There is nothing in the NEC that requires anything else other than the 310.15 calculation for ampacity of a standard circuit in an underground conduit, even if it is a multiple conduit ductbank underground. Nor is there any objective specification of how far apart the conduits have to be spaced, whether it is 1 centimeter or 2 inches. As far as the NEC is concerned, only the heat from wires within the same conduit are considered. Neighboring conduits do not affect one another, according to the NEC's calculation.

Does this match physical reality? Well that is another issue entirely.
Last edited by Carultch; 09-15-18 at 06:27 PM.

3. Originally Posted by ron
Where in the NEC does it mandate that we need to use another table for ampacity other than 310.15(B)(16) for underground feeders, such as those found in Annex B for low voltage?
That's not quite what it says. Rather, the top of that table says it applies to conductors in raceway, cable, or direct-buried in earth. A concrete encased duct bank does not fall into either of those three categories, so it (theoretically) cannot be used.

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Originally Posted by charlie b
That's not quite what it says. Rather, the top of that table says it applies to conductors in raceway, cable, or direct-buried in earth. A concrete encased duct bank does not fall into either of those three categories
How's that? The conductors in a duct bank are inside a raceway, so that fits the description in the table heading. Excluding duct banks would require additional language.

Cheers, Wayne

5. Originally Posted by charlie b
That's not quite what it says. Rather, the top of that table says it applies to conductors in raceway, cable, or direct-buried in earth. A concrete encased duct bank does not fall into either of those three categories, so it (theoretically) cannot be used.

Since it is in raceway, not sure that flies.

The MV version is so clear. 310.60(C) pushes directly to Tables which includes options for details on underground applications (with backfill, earth or concrete) starting with table (77) +

The LV forces to you an un-enforceable Annex. B-2 clearly says to uses the particular Tables in Annex B for underground applications, but Annex B is un-enforceable. Seems odd that MV is clear but LV applications are not.

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Originally Posted by wwhitney
How's that? The conductors in a duct bank are inside a raceway, so that fits the description in the table heading. Excluding duct banks would require additional language.

Cheers, Wayne
Exactly. Unless you use plumbing pipe to build the ductbank, which would violate the product's listing, it is "in a raceway".

You create the same situation when you run a group of EMT raceways through a wall interior, and backfill it with spray foam thermal insulation, yet that doesn't create any additional code concerns. Concrete and Earth conduct heat a lot better than foam insulation that is intentionally put there to minimize the conduction of heat.

7. Originally Posted by wwhitney
How's that? The conductors in a duct bank are inside a raceway, so that fits the description in the table heading. Excluding duct banks would require additional language.
Originally Posted by ron
Since it is in raceway, not sure that flies.
Originally Posted by Carultch
Exactly. Unless you use plumbing pipe to build the ductbank, which would violate the product's listing, it is "in a raceway".
Youse guys is gangun up on me!

I have always considered “raceway,” in the context of the top of that table, to mean conduit or cable tray open to the air. I can’t offer any code language that backs up that interpretation. However, subparagraph (3) under the Informational Note following article 310.15(A)(3) acknowledges that the material surrounding a conduit will impact the operating temperature. Dirt and concrete surrounding a conduit will definitely reduce the ability of heat to dissipate from the wires inside a conduit. I can say that every time I have run a Neher-McGrath calculation to obtain the ampacity of conductors inside a concrete-encased ductbank, the results were lower than the values in Table 310.15(B)(16). From that, I infer that the tabulated values were not intended to apply to a concrete-encased ductbank.

8. Originally Posted by charlie b

I can say that every time I have run a Neher-McGrath calculation to obtain the ampacity of conductors inside a concrete-encased ductbank, the results were lower than the values in Table 310.15(B)(16). From that, I infer that the tabulated values were not intended to apply to a concrete-encased ductbank.
I generally find the same, but a casual use of the Table B.310.15(B)(2)(7) with one duct in dirt (rho 90) (LF 100) provides higher ampacity than the regular 310.15(B)(16) table.

I really am disturbed that I can't find something mandating the use of Annex B tables or Neher Mcgrath calcs for essentially derated values for ampacity.

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