KLeary
Member
- Location
- Bridgewater, NJ
This question is about ambient temperature derating of conductors in a cable tray.
The situation:
Generator room with two 1,150kW generators side by side. The feeder from one generator passes closely below the muffler of the other generator on its way to the switchgear, in uncovered ladder type cable tray. Distance from hot surface to conductors is about 14 inches.
The voltage is 480V three phase delta.
The conductors consist of 5 sets of three 500 kcmil, bundled with a gap between each set. Bundled A-B-C, meaning three phases bundled in groups in accordance with 392.8(D).
The conductors are rated 90 degrees C (194 degrees F) and are rated for cable tray, RHW-2 - RHH - USE-2.
The mechanical engineer gave me the worst case ambient temperature for that cable tray which is 125 degrees F and this only happens for the short distance under the muffler. Its a big flat pancake muffler so its about 7 feet across.
Each generator feeder terminates at a 2,000A circuit breaker.
My question is how to derate the ampacity of the conductors.
The cables will be installed in accordance with 392.10(A)(2). For the basic ampacity, I see that 392.11(B)(2) points me to Table 310.17, so for my 500 kcmil, 90 degree C, I have a starting point of 700A. That same section says the ampacity can't be more than 65% for uncovered tray.
But, I have the high ambient temperature of 125 degrees F to consider. Using the correction factors below Table 310.17, I would derate to 76%.
Since 392.11(B)(2) says the ampacity can't be more that 65%, I am assuming that governs unless I had a higher ambient temp that put me below 65% in the correction factors under table 310.17. Therefore each 500kcmil is good for 700A x 0.65 = 455A. That means the feeder is good. Or am I missing something?
The situation:
Generator room with two 1,150kW generators side by side. The feeder from one generator passes closely below the muffler of the other generator on its way to the switchgear, in uncovered ladder type cable tray. Distance from hot surface to conductors is about 14 inches.
The voltage is 480V three phase delta.
The conductors consist of 5 sets of three 500 kcmil, bundled with a gap between each set. Bundled A-B-C, meaning three phases bundled in groups in accordance with 392.8(D).
The conductors are rated 90 degrees C (194 degrees F) and are rated for cable tray, RHW-2 - RHH - USE-2.
The mechanical engineer gave me the worst case ambient temperature for that cable tray which is 125 degrees F and this only happens for the short distance under the muffler. Its a big flat pancake muffler so its about 7 feet across.
Each generator feeder terminates at a 2,000A circuit breaker.
My question is how to derate the ampacity of the conductors.
The cables will be installed in accordance with 392.10(A)(2). For the basic ampacity, I see that 392.11(B)(2) points me to Table 310.17, so for my 500 kcmil, 90 degree C, I have a starting point of 700A. That same section says the ampacity can't be more than 65% for uncovered tray.
But, I have the high ambient temperature of 125 degrees F to consider. Using the correction factors below Table 310.17, I would derate to 76%.
Since 392.11(B)(2) says the ampacity can't be more that 65%, I am assuming that governs unless I had a higher ambient temp that put me below 65% in the correction factors under table 310.17. Therefore each 500kcmil is good for 700A x 0.65 = 455A. That means the feeder is good. Or am I missing something?
