Has anyone ever investigated in detail underground installations, in particular how many motor circuits should be allowed in a single duct bank for branch circuits as "standard" design without having to derate based on Annex B. It is my understanding that the neher mcgrath calculations behave rather curiously when approching 15 motor circuits underground. There are so many variables to consider that affect the ampacity of underground cable temperature I am having a hard time buying into this for branch circuits.
Underground installations
- Thread starter ERTW
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Let me start by presuming that the ?derate? you are talking about is not due to having more than three current carrying conductors in a single duct.
You don?t ?derate based on Annex B.? Rather, that Annex gives ampacity for certain conductors in certain physical installations. These values are generally (if not always) lower than the values in Table 310.16. But that does not mean that Annex B is a method of derating a conductor from the ampacity numbers in 310.16. ?Ampacity? is defined as the current that a conductor can carry under conditions of use, without exceeding its temperature rating. What Annex B is giving is just a different set of ?conditions of use.?
My answer to the question that I think you are asking is that you could have one motor circuit in one duct in a ?one duct ductbank,? or you could have one motor circuit in each duct in a ?two duct ductbank,? or you could have one motor circuit in each duct in a ?fifty duct ductbank,? and you might be able to justify using the same ampacity for the conductors in all three cases. The restriction I would impose is that you can only use Table 310.16 to obtain the available ampacity of a conductor, if you started by calculating the required ampacity in accordance with Article 220.
You don?t ?derate based on Annex B.? Rather, that Annex gives ampacity for certain conductors in certain physical installations. These values are generally (if not always) lower than the values in Table 310.16. But that does not mean that Annex B is a method of derating a conductor from the ampacity numbers in 310.16. ?Ampacity? is defined as the current that a conductor can carry under conditions of use, without exceeding its temperature rating. What Annex B is giving is just a different set of ?conditions of use.?
My answer to the question that I think you are asking is that you could have one motor circuit in one duct in a ?one duct ductbank,? or you could have one motor circuit in each duct in a ?two duct ductbank,? or you could have one motor circuit in each duct in a ?fifty duct ductbank,? and you might be able to justify using the same ampacity for the conductors in all three cases. The restriction I would impose is that you can only use Table 310.16 to obtain the available ampacity of a conductor, if you started by calculating the required ampacity in accordance with Article 220.
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