NEC 310.60 (B)(1) Selection of Ampacity

[Stan Millet]

I have a 3-phase circuit of 500 kcmil, 5kV cable, 5960 ft in length.
Sixty (60) ft of which is located inside an Electrical Room at a temperature of 20 deg C. and the rest of the cable (5900 ft) is outside at a temperature of 15 deg C.

Because of the different ambient temperatures, the ampacities are different. I don't understand the Exception in Section 310.60 (B)(1). Do I rate the whole circuit (cable) at 20 deg C or 15 deg C?

The ampacity I will use will be calculated using Section (C)(4) Ambients Not in Tables.

 

[charlie b]

The ampacity is going to be higher in the area with the lower temperature. That is the bulk of the run. The total run is 6020 feet. Ten percent of that is 602 feet. You can use the higher ampacity value (based on 15C temperature) for the area with the higher temperature, as long as the cable does not extend into that 20C area more than 602 feet. Since it only extends 60 feet, which is way less than 10% of the total run, you can use the 15C value for the whole run.

Welcome to the forum.

 

[Smart $]

The ampacity is going to be higher in the area with the lower temperature. That is the bulk of the run. The total run is 6020 feet. Ten percent of that is 602 feet. You can use the higher ampacity value (based on 15C temperature) for the area with the higher temperature, as long as the cable does not extend into that 20C area more than 602 feet. Since it only extends 60 feet, which is way less than 10% of the total run, you can use the 15C value for the whole run.

Welcome to the forum.

I don't think so!!!

The Exception says a distance of 10 ft or 10% of the circuit length figured at the higher ampacity, whichever is less. Ultimately, 10 feet is the maximum. 10% restricts the distancet to less than 10 feet for circuits where the higher ampacity portion is less than 100 feet in length.

 

[charlie b]

He's right. I missed the "whichever is less" part. You are stuck with the 20C value for the entire run.