Cable Tray Rungs Spacing

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D!NNy

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Can someone explain why 9" rung spacing is required per 392.10 for single conductor cables for sizes between 1/0 through 4/0? if this to avoid smaller conductors drooping (saging) between in the rungs, then why the code is not restricting with 9" rung spacing for multi conductor cables of sizes #10, #12, #8 etc which has less overall diameter less than #1/0?
 
D!NNy said:
Can someone explain why 9" rung spacing is required per 392.10 for single conductor cables for sizes between 1/0 through 4/0? if this to avoid smaller conductors drooping (saging) between in the rungs, then why the code is not restricting with 9" rung spacing for multi conductor cables of sizes #10, #12, #8 etc which has less overall diameter less than #1/0?
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Cable tray design criteria is nuts. It is very difficult to understand any of the reasons, or find any consistency within the cable tray design flowchart.

There are also a lot of unanswered holes in cable tray design requirements. It may be due to the fact that it is a relatively new technology, and the code hasn't caught up with understanding all of what can happen with cable tray systems.
 
It could be that multi-conductor cables are considered more flexible. Or maybe its because they just don't weight as much.
 
steve66 said:
It could be that multi-conductor cables are considered more flexible. Or maybe its because they just don't weight as much.
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If multiconductor cables are more flexible (which they are), it would make me conclude that multiconductor cables require a closer rung spacing, for equivalent amount of total KCMIL within the conductor.

The way that weight affects the strength/stiffness of a conductor in a cable tray isn't as we are ordinarily used to. The stiffness is proportional to diameter^4, and the strength is proportional to diameter^3. Lineal weight is proportional to diameter^2. As the conductor grows, its "structural" ablity to span the gap improves.

Given a multiconductor cable with a total diameter that is less than an equivalent single conductor, it wouldn't make sense for it to span a larger gap. Unless we are talking about a metal clad cable, where the metal cladding helps it. Not so much for SER/SEU or other non-metal enclosed cable assemblies.
 
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