Selection of Ampacity according to NFPA 70 - 310.60(B)(1)

[Dan Ground]

According to NFPA 310.60(B)(1) "Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used

"Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m(10ft) or 10 percent of the circuit length calculated at the higher ampacity, wichever is less"

I'm calculating the required conductor for feeding a MCC 100 m (328 ft) away from a power transformer. The cable run is divided in three sections. The first one starts in the transformer and is made on cable tray for 80 m., then the second section is made in bank duct for 8 m., and the last section is run on cable tray again. My question is if the 8 m. of run in the bank duct makes me select the lowest ampacity value, having in mind that 8 m is less than the 10% of the whole circuit length.

Thank you.

 

[hurk27]

Welcome to Mike Holt's NEC forum.

Being that this is for MV circuits, I believe that the NEC is allowing the last portion (3M's or 10') of the circuit to be allowed to use the calculated ampacity of the first part of the circuit when the last section calculates out to a little lower derated ampacity then what is in the duct bank and it is 10' or under, lets say your conductors in the duct bank come out to a 80 amps rating when applying all the derating then the last section comes out a little lower say 75 amps because of some added derating such as being exposed to sunlight or such, if this last section is lower then the first then you are allowed to use the first rating (the higher one) for the final section if it is under 3 meters or 10 feet.

Also it states that the distance allowed for this is 10% or 3 meters which ever is lower, so 3 meters is the lower of the two being that your circuit length is 100 meters, you would have to have a 30 meter or less run for the 10% to kick in

I may have this wrong so maybe others will chime in but again welcome to the forum.

 

[GoldDigger]

According to NFPA 310.60(B)(1) "Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used

"Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m(10ft) or 10 percent of the circuit length calculated at the higher ampacity, wichever is less"

I'm calculating the required conductor for feeding a MCC 100 m (328 ft) away from a power transformer. The cable run is divided in three sections. The first one starts in the transformer and is made on cable tray for 80 m., then the second section is made in bank duct for 8 m., and the last section is run on cable tray again. My question is if the 8 m. of run in the bank duct makes me select the lowest ampacity value, having in mind that 8 m is less than the 10% of the whole circuit length.

Thank you.

Since 8m is longer than 3m, it cannot be ignored even though it is less than 10% of the higher ampacity segment.