310.60 (C) (2) (B)
"Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per 1 ft increase in depth for all values of rho shall be permitted."
Looking at the electrical ducts in figure 310.60, the note says
"Maximum depth to the top of electrical duct banks shall be 30"...."
So if I bury a duct deeper than 30", does that mean I have to derate the ampacities given in table 310.77?
Example: 5KV, 500 KCM MV-90 copper cables in a single duct, buried 8' deep.
From table 310.77, the ampacity is 470 amps. Since I am 5.5' deeper than 30", I have to derate 5.5 * 6% = 33%.
So the cable ampacity would only be 470* 0.77 = 362 amps? Is that right?
The code section says a derating factor "shall be permitted." Does that mean it is required? Why would you permit a derating factor, and not require it?
Steve
"Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per 1 ft increase in depth for all values of rho shall be permitted."
Looking at the electrical ducts in figure 310.60, the note says
"Maximum depth to the top of electrical duct banks shall be 30"...."
So if I bury a duct deeper than 30", does that mean I have to derate the ampacities given in table 310.77?
Example: 5KV, 500 KCM MV-90 copper cables in a single duct, buried 8' deep.
From table 310.77, the ampacity is 470 amps. Since I am 5.5' deeper than 30", I have to derate 5.5 * 6% = 33%.
So the cable ampacity would only be 470* 0.77 = 362 amps? Is that right?
The code section says a derating factor "shall be permitted." Does that mean it is required? Why would you permit a derating factor, and not require it?
Steve