Free Air Rating for Conductors Table 310.15(B)(17)

[ron]

When properly designing a single conductor layout in cable tray per 392.80(A)(2)(c), we use Table 310.15(B)(17) to determine the conductor ampacity. This is a 2000A feeder, so there is significant paralleled set reductions.

When these conductors enter a disconnect, where the conductor spacing cannot be maintained, is there any code relief to still allow the free air rating to be used for that short run into the enclosure? If not, it essentially will require the cable tray conductors to be reduced and have "normal" 310.15(B)(16) ampacities. An external splice at the enclosure from five sets (310.15(B)(16)) to three sets (310.15(B)(17)) is not practical.

 

[Smart $]

310.15(A)(2) Exception for the short section.

However, terminal temperature limitations of 110.14(C) still applies. Unless you do a 75?C to 90?C terminal rating transition at some distance from the disconnect (and on the other end if necessary), you will always be relegated to a maximum of the 75?C ampacity under Table 310.15(B)(16).

 

[ron]

However, terminal temperature limitations of 110.14(C) still applies. Unless you do a 75?C to 90?C terminal rating transition at some distance from the disconnect (and on the other end if necessary), you will always be relegated to a maximum of the 75?C ampacity under Table 310.15(B)(16).

If we don't do a transition, wouldn't I be able to use the 75 column of the free air Table 310.15(B)(17)

 

[Smart $]

If we don't do a transition, wouldn't I be able to use the 75 column of the free air Table 310.15(B)(17)

Nope. 110.14(C)(1) refers you specifically to Table 310.15(B)(16).

 

[Smart $]

If we don't do a transition, wouldn't I be able to use the 75 column of the free air Table 310.15(B)(17)

One option is to get an engineer to sign off on it, under the allowance permitted by 310.15(C). It'd actually be a stretch. I suggest running it by the AHJ after EE sign-off, before planning... because 110.14(C) has no such allowance. EE would essentially have to establish that T_C under 310.15(C) does not exceed 75?C at terminations.

PS: Forgot you are EE...

 

[ron]

One option is to get an engineer to sign off on it, under the allowance permitted by 310.15(C). It'd actually be a stretch. I suggest running it by the AHJ after EE sign-off, before planning... because 110.14(C) has no such allowance. EE would essentially have to establish that T_C under 310.15(C) does not exceed 75?C at terminations.

PS: Forgot you are EE...

I definitely don't do that kind of thing. I am a chicken

 

[Smart $]

I definitely don't do that kind of thing. I am a chicken

I understand it is a daunting task, possibly an expensive one for a one-off using a software program...

http://forums.mikeholt.com/showthread.php?t=103451

You can 'preview' the book recommended by jghrist here:

http://www.scribd.com/doc/66009980/...putations-for-Transmission-by-George-J-Anders

 

[RobbieNelson]

Nope. 110.14(C)(1) refers you specifically to Table 310.15(B)(16).

Since all conductors have terminations, does 110.14(C)(1) render 310.15(B)(17) moot? If not, when could you use the free air rating?

Sorry for bringing this back form the dead. ⚰