NEC Article 310.60(C)(2)(b) states that ??an ampacity derating factor of 6 percent per 300-mm (1-ft) increase in depth for all values of rho shall be permitted.?. Does the derating factor not decrease with an increase in depth to a point at which it becomes zero? If not, using this derating method, there is a point at which the conductor will have negative ampacity.
Example: 15kV 1/0 AWG shielded single conductor copper cable, no neutral (select smallest neutral size in table)
Using table in IEEE Std. 835-1994 (pp. 349):
75% load factor, 90 Rho, 90?C max. conductor temp. from table, 105?C desired conductor temp. rating, 25?C ambient temp. used in table, 21?C desired ambient temp., 0 for temp. rise due to dielectric loss, 0 for new temp. rise due to dielectric loss, 234.5 inferred temp. of zero electrical resistance, 48? burial depth to top of duct bank
Using formula from IEEE Std. 835-1994, Section 3.4.1 (pp. Intro-10): Adjusting for ambient temperature, the multiplier obtained is 1.03.
Using formula from IEEE Std. 835-1994, Section 3.4.2 (pp. Intro-10): Adjusting for maximum conductor temperature, the multiplier obtained is 1.08.
Derating for depth: 1.00 ? 1.5(.06) = .91
Ampacity rating for 1/0 cable with 1/6 neutral: 139A
I = 139*1.03*1.08*.91 = 140A (rounding down)
Same example with 240? (20-ft) burial depth to top of duct bank:
Derating for depth: 1-(20*12-30)/12*.06 = -.05
I = 139*1.03*1.08*-.05 = -7A (rounding down)
How can this be possible? Also, is there no reason to account for the difference in ampacity between conductors at the top of a duct bank and conductors at the bottom of the same duct bank?
Example: 15kV 1/0 AWG shielded single conductor copper cable, no neutral (select smallest neutral size in table)
Using table in IEEE Std. 835-1994 (pp. 349):
75% load factor, 90 Rho, 90?C max. conductor temp. from table, 105?C desired conductor temp. rating, 25?C ambient temp. used in table, 21?C desired ambient temp., 0 for temp. rise due to dielectric loss, 0 for new temp. rise due to dielectric loss, 234.5 inferred temp. of zero electrical resistance, 48? burial depth to top of duct bank
Using formula from IEEE Std. 835-1994, Section 3.4.1 (pp. Intro-10): Adjusting for ambient temperature, the multiplier obtained is 1.03.
Using formula from IEEE Std. 835-1994, Section 3.4.2 (pp. Intro-10): Adjusting for maximum conductor temperature, the multiplier obtained is 1.08.
Derating for depth: 1.00 ? 1.5(.06) = .91
Ampacity rating for 1/0 cable with 1/6 neutral: 139A
I = 139*1.03*1.08*.91 = 140A (rounding down)
Same example with 240? (20-ft) burial depth to top of duct bank:
Derating for depth: 1-(20*12-30)/12*.06 = -.05
I = 139*1.03*1.08*-.05 = -7A (rounding down)
How can this be possible? Also, is there no reason to account for the difference in ampacity between conductors at the top of a duct bank and conductors at the bottom of the same duct bank?