beanland
Senior Member
- Location
- Vancouver, WA
Existing Table Title:
Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60C Through 90C (140F through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F)
Proposed Table Title:
Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60C Through 90C (140F through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F), Where Not Installed in an Underground Bank of Raceways or Underground Multiple Cable Installation.
Discussion
The title of Table 310.15(B)(16) implies that the ampacity indicated in the table can be applied to underground duct banks or underground direct-buried cable banks where any number of cables can be installed. The IEEE Standard Power Cable Ampacity Tables, IEEE Std 835-1994, clearly indicates that there is a loss of ampacity when multiple parallel circuits are installed together in a duct bank. This loss of ampacity is comparable, though not identical, to the adjustment factors given in Table 310.15(B)(3)(a).
In industrial applications, it is not uncommon to parallel four, six, eight or more conduits and sets of power cables to achieve the desired rating. Failing to properly derate cables in these circumstances will lead to overheating of cables and their failure. The correct application of Table 310.15(B)(16) should include the use of adjustment factors in Table 310.15(B)(3)(a) wherever more than 3 current carrying conductors are installed underground (or encased), regardless of the number of encased cables or conduits.
Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60C Through 90C (140F through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F)
Proposed Table Title:
Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60C Through 90C (140F through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F), Where Not Installed in an Underground Bank of Raceways or Underground Multiple Cable Installation.
Discussion
The title of Table 310.15(B)(16) implies that the ampacity indicated in the table can be applied to underground duct banks or underground direct-buried cable banks where any number of cables can be installed. The IEEE Standard Power Cable Ampacity Tables, IEEE Std 835-1994, clearly indicates that there is a loss of ampacity when multiple parallel circuits are installed together in a duct bank. This loss of ampacity is comparable, though not identical, to the adjustment factors given in Table 310.15(B)(3)(a).
In industrial applications, it is not uncommon to parallel four, six, eight or more conduits and sets of power cables to achieve the desired rating. Failing to properly derate cables in these circumstances will lead to overheating of cables and their failure. The correct application of Table 310.15(B)(16) should include the use of adjustment factors in Table 310.15(B)(3)(a) wherever more than 3 current carrying conductors are installed underground (or encased), regardless of the number of encased cables or conduits.