Ampacity: MC in tray

[charlie b]

I don?t understand 392.11(A). It seems backwards.

I am contemplating using MC in tray as feeders to branch circuit panels. 392.11(A) says the ampacity is as shown in Table 310.16, as modified by (1) and (2) and (3).
? 392.11(A)(1) does not apply, as each feeder will have only 3 current carrying conductors.
? 392.11(A)(2) does not apply, as the tray will not be covered.
? 392.11(A)(3) tells me I have to use ?engineering supervision? ? that is, 310.15(C) ? to calculate the ampacity, but only if I keep ?one cable?s diameter? of separation between the cables. From my experience performing such ?under supervision? calculations, the answers are always, and I mean always, lower than the values in Table 310.16.

What if I do not keep that amount of separation between cables? What if I laid identical MC cables side-by-side in a single layer that goes the entire width of the tray? As I read it, that would mean that 392.11(A)(3) does not apply. That puts me back to 392.11(A), and I can then get ampacities directly from Table 310.16. Therefore, when I call for a configuration that clearly has more cables in closer proximity creating more heat, I get to use a higher ampacity that I would, if I spread them out.

What am I missing here?

 

[RUWired]

Therefore, when I call for a configuration that clearly has more cables in closer proximity creating more heat, I get to use a higher ampacity that I would, if I spread them out.

What am I missing here?

Charlie, Are your ambient temperature corrected ampacities lower than the fine print note refers us to in table B310.3 (which are multiconductor cables in free air).Those ampacities are higher than 310.16.
392.11 A3 is suggesting that if spacing is maintained,higher amps can be expected.
Rick

 

[drbond24]

392.11 says that you have to meet the requirements of 392.9. This regulates the number of multiconductor cables you can put in one tray and prevents the paradox you described. You'd violate the fill limitations if you laid the cables side-by-side along the entire width of the tray.

 

[RUWired]

You'd violate the fill limitations if you laid the cables side-by-side along the entire width of the tray.

Thats not so, 392.9 allows you to fill the tray in a single layer.
392.9 Number of Multiconductor Cables,
(1) Where all of the cables are 4/0 AWG or larger, the sum of the diameters of all cables shall not exceed the cable tray width, and the cables shall be installed in a single layer.

 

[drbond24]

Thats not so, 392.9 allows you to fill the tray in a single layer.
392.9 Number of Multiconductor Cables,
(1) Where all of the cables are 4/0 AWG or larger, the sum of the diameters of all cables shall not exceed the cable tray width, and the cables shall be installed in a single layer.

Ok, I re-read and agree with you. I don't have any other genius ideas at the moment.

 

[charlie b]

Just in case things are not already confusing enough, let?s do some math.

I will be using #1/4 MC cable with a #6 EGC. Diameter is 1.351 inches (according to one vendor). That equates to a cross-sectional area of 1.434 square inches.

I will be installing this type of cable, and no other, in a 30 inch wide tray. Per Table 392.9, Column 1, I can fill the tray up to a total of 35 square inches of cable. Dividing 35 by 1.434, I see that I can fill the tray with up to 24 such cables.

But if I put 24 of these cables side by side, they will occupy a width of 24 diameters times 1.351 inches per diameter, or a total width of 32.42 inches. In other words, I can fill the tray to more than its width, so long as I have no more than a single layer of cable.

So what I do is to place 23 of these cables in the tray, nearly filling the entire width. I am now in compliance with 392.9. So I go to 392.11, and see that Table 310.16 is all I need to determine ampacity (the table gives me 130 amps for a #1). However, if instead I chose to leave one cable diameter?s width between cables (i.e., I only install 12 cables in the tray, not 23), then 392.11(A)(3) would tell me I need to calculate the ampacity under engineering supervision. If I leave extra room, so that heat can dissipate more easily, I have to use an engineering calculation to get the ampacity! That is the part that confuses me. It seems backwards.

 

[drbond24]

I spend some of my time at work performing calculations for variable frequency drive cable. We have a consultant that wants to double the insulation thickness for the single conductors that this cable is made from. I calculated that this would increase the inductance of the cable because it increases the space between each conductor, thus allowing for more flux linkages. In some cases, this can triple the impedance of the cable. Perhaps the same principal applies here between separate cables? You would think that further apart would keep the temperature down, but wouldn't it also give the magnetic fields around each cable more room to mess with each other?

 

[RB1]

Charlie,

The FPN below 392.11(A)(3) references Table B.310.3. In earlier editions of the Code this table was included in Chapter 3 and therefore capable of being utilized without engineering supervision. The table appears to permit slightly higher ampacity values than 310.16.

 

[RUWired]

I get what your saying Charlie, You think that there should be engineering when the cables are next to each other instead of when theres spacing.They must think you can tweak a few more amps when in free air.Just the opposite of a duct bank,you have to take away a few amps.

 

[arubenstein]

Rekindling an old thread...

Rekindling an old thread...

Charlie, Are your ambient temperature corrected ampacities lower than the fine print note refers us to in table B310.3 (which are multiconductor cables in free air).Those ampacities are higher than 310.16.
392.11 A3 is suggesting that if spacing is maintained,higher amps can be expected.
Rick

I am looking for further clarification on this...

Table B.310.3 reads, "Ampacities of MC Cables with not more than three insulated conductors, rated 0 to 2000 volts, in free air ..."

What if you have a neutral? or an insulated ground?

Is that a typo, should it say "three current carrying conductors" ?