Ampacity derating in Panduit wire duct

[akruse]

I am looking at chapter 12 in NFPA79 with regard to sizing conductors. I understand that Table 12.5.5(b) must be used to adjust ampacity when there are many conductors in a raceway or cable. My confusion is whether this also applies to conductors contained in slotted wiring duct. I was under the impression that since the slotted ducts allow airflow that they would not be considered a raceway.

However 12.5.5 says "Sizing of conductors within control enclosures in wiring harnesses or wiring channels shall be based on the ampacity in cable or raceway."

The verbiage is a little unclear to me. Am I supposed to treat conductors in a Panduit style wire duct like a raceway or cable?

Thanks in advance.

 

[DRWeig]

akruse,

I always treat them like in raceway -- the heating effect is not amplified so much by how the air circulates as it is by the heating effect of all the conductors' individual contributions when laid in a bundle. They're touching each other, and if there are enough in your wire duct, some will be completely surrounded by other current-carrying conductors.

My opinion anyway -- let's see who else pitches in.

Good on ya,

Goober Dave

 

[dpc]

I've always figured it as a raceway. Not a good raceway, but a raceway. Since the covers seem to invariably be left off eventually, that is probably a conservative approach.

 

[waross]

Often Panduit is used for control wiring and the current is small commpared to the ampacity, so heating and derating is not an issue.
However, if the conductors are loaded I would consider Panduit to be a raceway and derate accordingly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I have always treated it as raceway, but then again, I would rarely run power conductors in wire duct either. I don't like mixing voltage levels in bundles even when it is not "illegal". It's just a personal preference based on having been the poor schmuck that has to troubleshoot control panel wiring.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[akruse]

Thanks for the input. It sounds like the best thing to do is avoid running power conductors in the duct if possible. Otherwise I will derate accordingly.

 

[jraef]

What I would do in crowded panels was to use standoffs for the wire duct, then run the smaller power conductors on the surface of the back pan, strapped down of course, under the duct.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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