Continuously Covered Cable Tray

[turtlepokerman]

How close can a ladder cable tray be to the ceiling before it is considered covered? I am looking for in the NEC for guidance and cannot find anything on it.

All help is greatly appreciated. Thank you.

 

[Smart $]

How close can a ladder cable tray be to the ceiling before it is considered covered? I am looking for in the NEC for guidance and cannot find anything on it.

All help is greatly appreciated. Thank you.

It is never considered continuously covered based on proximity to ceiling. However, it cannot be up against ceiling because access is required (IMO, reaching up through bottom is not considered accessible). There is no specified distance for accessibility.

 

[turtlepokerman]

Yeah sorry I thought about removing the continuous portion of it after I posted it. So in theory I could place 500kcmil single conductors cables with a spacing of 1 cable diameter in a ladder cable tray without a cover and place it 6" below the ceiling and not have to derate the cables? That doesn't seem right.

It is never considered continuously covered based on proximity to ceiling. However, it cannot be up against ceiling because access is required (IMO, reaching up through bottom is not considered accessible). There is no specified distance for accessibility.

 

[Smart $]

Yeah sorry I thought about removing the continuous portion of it after I posted it. So in theory I could place 500kcmil single conductors cables with a spacing of 1 cable diameter in a ladder cable tray without a cover and place it 6" below the ceiling and not have to derate the cables? That doesn't seem right.

6" isn't much room to work... but it is Code compliant. As a side note, your conductors will be on the bottom of the tray, so they will actually be more like 9-10" below ceiling if tray is 6" below (depends on rail width). Also remember your circuit ampacity is still limited to the terminal temperature limitation of 110.14(C), which is typically the 75?C column of 310.15(B)(16)... and you still have to correct for ambient temperature.

 

[turtlepokerman]

If I have 500kcmil single conductors with a spacing of 1 conductor in between, I should be able to base my ampacity off of 310.15(B)(17) according to 392.80(A)(2)(c). As long as I do not have a continuously covered cable tray which was my initial concern. Is this thought process correct?

Also thank you for being so responsive. I really appreciate it.

6" isn't much room to work... but it is Code compliant. As a side note, your conductors will be on the bottom of the tray, so they will actually be more like 9-10" below ceiling if tray is 6" below (depends on rail width). Also remember your circuit ampacity is still limited to the terminal temperature limitation of 110.14(C), which is typically the 75?C column of 310.15(B)(16)... and you still have to correct for ambient temperature.

 

[Smart $]

If I have 500kcmil single conductors with a spacing of 1 conductor in between, I should be able to base my ampacity off of 310.15(B)(17) according to 392.80(A)(2)(c). As long as I do not have a continuously covered cable tray which was my initial concern. Is this thought process correct?

Also thank you for being so responsive. I really appreciate it.

You can use Table 310.15(B)(7) allowable ampacity as the value which adjustment and correction factors, if any will apply. However, refer to 310.15(A)(2) and the Informational Note thereto. What it means is that the circuit ampacity is limited to the lesser of your adjusted and corrected conductor ampacity or the terminal temperature rating equivalent for the size used under Table 310.15(B)(16).

For example, say your 500kcmil conductors are rated 90?C and copper... and let's say there is no adjustment and correction, plus they run entirely in cable tray between termination enclosures. Ampacity under T310.15(B)(17) is 620A. The terminations are rated 75?C. This will limit your circuit ampacity to the 75?C column of Table 310.15(B)(16) for 500kcmil copper conductors, which is 380A. There are ways to bet around this limitation...

 

[turtlepokerman]

You can use Table 310.15(B)(7) allowable ampacity as the value which adjustment and correction factors, if any will apply. However, refer to 310.15(A)(2) and the Informational Note thereto. What it means is that the circuit ampacity is limited to the lesser of your adjusted and corrected conductor ampacity or the terminal temperature rating equivalent for the size used under Table 310.15(B)(16).

For example, say your 500kcmil conductors are rated 90?C and copper... and let's say there is no adjustment and correction, plus they run entirely in cable tray between termination enclosures. Ampacity under T310.15(B)(17) is 620A. The terminations are rated 75?C. This will limit your circuit ampacity to the 75?C column of Table 310.15(B)(16) for 500kcmil copper conductors, which is 380A. There are ways to bet around this limitation...

So in reality I would be better to place the cables in accordance with 392.80(A)(2)(b) (Do not have a one cable diameter spacing between cables) and take the 65% derating from the 310.15(B)(17) table. As long as that 65% derating of table 310.15(B)(17) is greater than the ampacity value of table 310.15(B)(16). The only time I would want to use table 310.15(B)(17) is if I had listed terminations that would allow me to use 310.15(B)(17).

Calculations:
620 * .65 = 403A Table 310.15(B)(17)
380A Table 310.15(B)(16) <--- Ampacity for circuit.

 

[Smart $]

So in reality I would be better to place the cables in accordance with 392.80(A)(2)(b) (Do not have a one cable diameter spacing between cables) and take the 65% derating from the 310.15(B)(17) table. As long as that 65% derating of table 310.15(B)(17) is greater than the ampacity value of table 310.15(B)(16).

That's one way of looking at it. You could use approximately half-as-wide tray.

FWIW, the derated value doesn't have to be greater than the terminal temperature limitation value under Table 310.15(B)(16)... it only has to equal or exceed the calculated load. When determining the terminal temperature limitation (aka minimum size conductor), continuous loads are factored 125%. When determining sufficient conductor ampacity, continuous loads are NOT factored 125%.

The only time I would want to use table 310.15(B)(17) is if I had listed terminations that would allow me to use 310.15(B)(17).

No. You're dealing with two completely separate parameters here: size and ampacity.

You must use the appropriate table for the conditions of use to determine conductor AMPACITY. In your case, that is the Table 310.15(B)(17) value as adjusted and corrected by other stipulations. That value must equal or exceed calculated load.

You must always use Table 310.15(B)(16) to determine the minimum conductor SIZE. The table value corresponding to terminal temperature and conductor size and element must equal or exceed calculated noncontinuous load plus 125% continuous load.

At best, you can use the 90?C corresponding value of Table 310.15(B)(16) for the main portion of the run if you make a transition using all 90?C-rated equipment amid the run at both ends... but there is no specified distance from the supply or load-end equipment. Some say at least 4' (the length of conductors used for testing terminal temperature limitations by UL), some say at least 10' [the maximum length given in 310.15(A)(2) Exception]. FWIW, 90?C transition splices (using approved means, of course) are permitted in cable tray without an enclosure or junction box.