Cable Tray Sizing

[rolltide315]

Article 392.22(B)(1)(b) states that the sum of the cross sectional areas of all single conductor cables betweem 250MCM and 900MCM should not be greater than the maximum cable fill shown in Column 1 of Table 392.22(B)(1). I have a situation requiring a separate equipment grounding conductor be carried back to a motor control center feeding 5 each 200HP motors. We are using single conductor cables to feed these motors. My question is this: Should the equipment grounding conductor for each motor be considered in the cable tray fill since it is not a current carrying conductor?

 

[GoldDigger]

Article 392.22(B)(1)(b) states that the sum of the cross sectional areas of all single conductor cables betweem 250MCM and 900MCM should not be greater than the maximum cable fill shown in Column 1 of Table 392.22(B)(1). I have a situation requiring a separate equipment grounding conductor be carried back to a motor control center feeding 5 each 200HP motors. We are using single conductor cables to feed these motors. My question is this: Should the equipment grounding conductor for each motor be considered in the cable tray fill since it is not a current carrying conductor?

This section does not mention "current carrying conductors" since it is not dealing with derating for heating. It is looking at the mechanical fill of the tray. Look at (a) - (d) in that section for specific guidelines based on the size of the EGCs and all other cables.

 

[Smart $]

Article 392.22(B)(1)(b) states that the sum of the cross sectional areas of all single conductor cables betweem 250MCM and 900MCM should not be greater than the maximum cable fill shown in Column 1 of Table 392.22(B)(1). I have a situation requiring a separate equipment grounding conductor be carried back to a motor control center feeding 5 each 200HP motors. We are using single conductor cables to feed these motors. My question is this: Should the equipment grounding conductor for each motor be considered in the cable tray fill since it is not a current carrying conductor?

The answer to your question as posed is yes. As noted by GoldDigger, fill includes all conductors.

That said, I have to wonder what the situation is that requires separate EGC's for each motor??? Engineering or manufacturer specification? ..'cause it certainly isn't required per NEC. Under NEC it is even permitted to use metal tray as the EGC if it meets the area requirement.

 

[rolltide315]

This section does not mention "current carrying conductors" since it is not dealing with derating for heating. It is looking at the mechanical fill of the tray. Look at (a) - (d) in that section for specific guidelines based on the size of the EGCs and all other cables.

I don't totally disagree with your theory but would your supposition not mean that you could bundle the 3-500MCM conductors in a triangle and just figure 10 each 500's since 5 of them are now on top of the other conductors?

 

[david luchini]

I don't totally disagree with your theory but would your supposition not mean that you could bundle the 3-500MCM conductors in a triangle and just figure 10 each 500's since 5 of them are now on top of the other conductors?

The area of the 5 conductors "on top" are still part of the fill of the cable tray.

 

[Julius Right]

I am afraid ,the filling percent mentioned in NEC 392.22(A) and (B) is not connected to mechanical resistance of the tray but to the cable ampacity. In art.392.17 it is written:
(3) Where a cable tray contains a combination of multiconductor and single-conductor cables, the allowable ampacities shall be as given in 392.17(A)(1) for multiconductor cables and 392.17(A)(2) for single-conductor cables, provided that the following conditions apply:
(1) The sum of the multiconductor cable fill area as a percentage of the allowable fill area for the tray calculated in accordance with 392.22(A), and the single-conductor cable fill area as a percentage of the allowable fill area for the tray calculated in accordance with 392.22(B), totals not more than 100 percent.
This it seems to be the J.Stolpe "spirit":
If we take ICEA P-54-440/NEMA WC 51-1986 in Part 1 GENERAL 1.1 General Information it is stated:
"Ampacity have been calculated based on 100 percent load with ALL CABLES FULLY LOADED and not spaced." That means : no unloaded cable is taken into consideration.
In "Ampacity for Cables in Randomly Filled Trays" by J.Stolpe-1970 it is mentioned: "no loaded cable will be neglected"
So, what the p.o. asked it is legitimate, I think.
:angel:

 

[rolltide315]

The area of the 5 conductors "on top" are still part of the fill of the cable tray.

I agree with what you say but it is in conflict with your theory about tray fill being strictly based on cable size. By stacking the cable you could get 1/3 more cable in the tray and not affect anything but the weight the tray carries.
I really don't think that the Code Panel put any thought into seperate EGC's as the most common practice would be to use the tray and its ground to connect the EGC to.

 

[david luchini]

I agree with what you say but it is in conflict with your theory about tray fill being strictly based on cable size. By stacking the cable you could get 1/3 more cable in the tray and not affect anything but the weight the tray carries.

I'm afraid I don't follow you. 392.22(B)(1)(b) tells you the maximum area of single conductor cables that can be installed in a tray of any given width. It say NOTHING about how the cables are installed, ie, in a single layer or stacked or triplexed. How you install the single conductor cables does not change the AREA of the single conductor cables.

For example, if you had a 12" cable tray, the maximum allowable fill would be 13 sq. in.

500mcm THHN has an area of 0.7073 sq. in., so you could put 18 #500mcm in the 12" tray (18 * 0.7073 = 12.73.)

The diameter of 500mcm is 0.949 in, so 18 500mcm in a single row would be 17.08 in.

The allowable fill from 392.22(B)(1)(b) allows for more 500mcm than would fit in a single layer.

 

[GoldDigger]

I'm afraid I don't follow you. 392.22(B)(1)(b) tells you the maximum area of single conductor cables that can be installed in a tray of any given width. It say NOTHING about how the cables are installed, ie, in a single layer or stacked or triplexed. How you install the single conductor cables does not change the AREA of the single conductor cables.

For some edge cases of mixed size cables, the section relates the sum of the cable diameters to the tray width, but does not make that relation dependent on how you then actually install the cables. A subtle distinction, as are many in the NEC.