Single Conductors in Cable Tray

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lauraj

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My question pertains to Article 392.10(A), 2005 NEC (I believe it is the same for 2002).

Why are there different cale tray width requirements for using 1/0-4/0 AWG conductors (392.10(A)(4)), compared to using 250-1000 kcmil conductors (392.10(A)(2))? If using the larger conductors, the cable tray is allowed to be smaller (wires can be stacked), whereas the general rule for using the smaller conductors is that they have to be single layer.

I don't understand why this would be. Is this a heat dissapation issue?

Thanks,

Laura Jenkins
 
Re: Single Conductors in Cable Tray

Is this a heat dissapation issue?
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Absolutely. It is also related to rules for derating conductor ampacity and wire fill. I think the smaller wires are limited to a percentage fill. But the larger wires have no such requirement. The fill for larger wires is basically limited by the single layer with a single conductor space between the wires. And I don't think there is any derating required for the larger conductors because they can't be stacked.

(I didn't look any of that up, so take it with a grain of salt).

Steve
 
Re: Single Conductors in Cable Tray

Thanks for the reply Steve, but it's left me a little confused.


The fill for larger wires is basically limited by the single layer with a single conductor space between the wires.
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I'm not sure what you mean by this.

And I don't think there is any derating required for the larger conductors because they can't be stacked.
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The larger conductors can be stacked - the smaller conductors cannot be stacked.

I would think that the larger conductors would give off more heat than the smaller conductors due to the higher ampacity. But are they able to dissapate that heat better due to increased surface area?

Laura
 
Re: Single Conductors in Cable Tray

That's what I get for shooting from the hip and not looking up the reference you mentioned.

I guess I was thinking of 392.9(A)
(1) ...cables 4/0 and larger...shall be installed in a single layer.
(2) ...cables smaller than 4/0 shall not exceed the max fill per column 1 of table 392.9.

The rules in 392.10(A) (for ventilated or ladder tray only) look more complicated.

The rule I was thinking of for a conductors space between cables is 392.11(B)(3).

Steve
 
Re: Single Conductors in Cable Tray

There seems to be some differences between multiconductor cables, and single conductor cables.

I also think you may gain some insight if you read the rules for ampacity in 392.11. Conductors installed in a single layer may be allowed a higher ampacity than those that are stacked or bundled.

Steve
 
Re: Single Conductors in Cable Tray

I'm currently just curious about the single-conductor cable requirements. Reading through 392.11(B) - ampacity of single-conductor cables, doesn't clear much up.

So now we have cable tray size based off of (392.10(A):
1. 1/0 - 4/0 AWG
2. 250 - 1000 kcmil
3. 1000 kcmil and larger

Any we have conductor ampacity based off of (392.11(B)):
1. 1/0 - 500 kcmil
2. 600 kcmil and larger
3. Other special configurations

There doesn't seem to be any correlation between how cable tray is sized and how conductor ampacity is calculated for certain size conductors.
 
Re: Single Conductors in Cable Tray

Laura: It would be much less confusing if we break down the installation into two areas. (1)The number of conductors allowed in a certain style and size cable tray, and what is the ampacity of the conductors installed in a specific configuration. 392.10 addresses the fill and 392.11 addresses the ampacity.

(1) Determine the load requirements.(2) Choose cable tray style. (3)Choose the conductor size and type (single or multi-conductor) based on load. (4) Determine cable layout configuration. (5) Calculate cable tray fill to determine tray size.
 
Re: Single Conductors in Cable Tray

I understand the hows - just follow the rules. It's the why's I'm curious about.

WHY are there different cable tray width requirements for different size conductors?
 
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