National Electrical Installation Standards

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Thursday, December 21, 2023

Question:

Hello CQD, thank you for providing insights and answers to electrical code related questions!

 My questions are below:

(1)    Which ampacity Table needs to be utilized when there are 3-Phase medium voltage cables 5kV (MV-90/105) inside a conduit? The installation is above ground.

 (2)    Table 310.60(C)(77) indicates one/three/six circuit configuration for underground installations and refers it to Table 310.60(C)(3).

a. Does it mean they are (3) parallel sets and (6) parallel sets of same circuit OR can they be different circuits from the switchgear?

b. What if I need two/four/five sets of same circuit OR if I need only two/four UG conduits containing different circuits? What will be the ampacity of the cables in that case?

Please clarify. Thank you for the great work, Abhi P.
A

Answer:

Hi Abhi, thank you for the kind words, glad to hear you are enjoying the CQD forum. To unpack each of your questions:

 (1)    The applicable Tables would Table 310.60(C)73-(C)76, based on the cable material and cable type/configuration installed in conduit above ground.

(2)    Section 310.60(A) provides guidance on this. The NEC® provides two (2) options for determining the ampacity of conductors 2001V-35kV:

a. Use the table method per Section 310.60(C) applying the parameters specified in the heading of each of the Tables 310.60(C)(67) - (C)(86) and the any applicable duct spacing specified in the referenced detail of figure 310.60(C)(3). 

OR

b. Use the engineering supervision per Section 310.60(B) using the provided equation. This option can be used for those installations that do not necessarily fit into one of the Tables.

Also see Section 310.60(A)(1) and (A)(1) Exception for additional information on selecting the proper ampacity. It is also worth noting that as of the 2020 edition of the NEC® all of these cited Tables have been relocated to a new Article 311 for Medium Voltage Conductors and Cables.

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IMPORTANT NOTICE: Unless the question requests a response based on a specific edition, all answers are based on the latest edition of NFPA 70® National Electrical Code®.

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