I am looking for guidance on 2023 NEC requirements for de-rating the ampacity of more than one medium-voltage, 3/C cable installed in the same electrical duct, or in the same direct burial trench.
The voltages in question for the systems I am considering are 13.8kV to 34.5kV, so I deferred to guidance in Article 315 for Type MV cables. If this is incorrect, please let me know.
Section 315.60 provides ampacity requirements for medium voltage (Type MV) cables. Ampacity tables are provided for underground electrical ducts and direct burial, as described in 315.60(F). The ampacity tables for installation in underground electrical ducts are specific to one 3/C cable per duct, with multiple circuits requiring separate ducts with specified duct-to-duct spacing. Similarly, the ampacity tables for installation via direct burial is for a single direct-buried cable, with ampacities provided for two circuits consisting of two direct-buried cables separated by 24 inches. In both of these cases, ampacity adjustment (de-rating) guidance is not provided for scenarios in which either a) more than one 3/C cable is installed in the same underground duct, or b) direct burial cables are either touching or installed closer than 24 inches together. The only de-rating guidance provided is for cable shield grounding, ambient temperature adjustment, and burial depth.
The sort of de-rating guidance I am looking for is similar to what is provided in Article 310 (310.15(C)(1)) for more than three current carrying conductors. However, I don't see any reference / link from the applicable Article 315 back to Article 310, let alone the de-rating guidanec in 310.15(C)(1).
Question 1 - Does the NEC have specific ampacity de-rating guidance that applies to medium-voltage cabling installed via direct burial or in an underground duct when the number of conductors or cables installed in close proximity exceeds the standard assumptions baked-into the 315.60 ampacity tables?
Question 2 - Can I use de-rating guidance in 310.15(C)(1) to adjust the ampacities provided by Table 315.60(C)(14), for example?
Thanks!
The voltages in question for the systems I am considering are 13.8kV to 34.5kV, so I deferred to guidance in Article 315 for Type MV cables. If this is incorrect, please let me know.
Section 315.60 provides ampacity requirements for medium voltage (Type MV) cables. Ampacity tables are provided for underground electrical ducts and direct burial, as described in 315.60(F). The ampacity tables for installation in underground electrical ducts are specific to one 3/C cable per duct, with multiple circuits requiring separate ducts with specified duct-to-duct spacing. Similarly, the ampacity tables for installation via direct burial is for a single direct-buried cable, with ampacities provided for two circuits consisting of two direct-buried cables separated by 24 inches. In both of these cases, ampacity adjustment (de-rating) guidance is not provided for scenarios in which either a) more than one 3/C cable is installed in the same underground duct, or b) direct burial cables are either touching or installed closer than 24 inches together. The only de-rating guidance provided is for cable shield grounding, ambient temperature adjustment, and burial depth.
The sort of de-rating guidance I am looking for is similar to what is provided in Article 310 (310.15(C)(1)) for more than three current carrying conductors. However, I don't see any reference / link from the applicable Article 315 back to Article 310, let alone the de-rating guidanec in 310.15(C)(1).
Question 1 - Does the NEC have specific ampacity de-rating guidance that applies to medium-voltage cabling installed via direct burial or in an underground duct when the number of conductors or cables installed in close proximity exceeds the standard assumptions baked-into the 315.60 ampacity tables?
Question 2 - Can I use de-rating guidance in 310.15(C)(1) to adjust the ampacities provided by Table 315.60(C)(14), for example?
Thanks!