Chapter 9, Table 1, Note 2 - Any purpose?

[Brian M]

I recently was involved in a thread on another forum where the OP wanted to know how many 12-2 NM-B cables were allowed in a 12 ft length of 1" Sched 40 PVC used only for protection for 15/20 amp 120v residential circuits. Using the 2002 code I figured it at 2 cables, using Note 9 and a manufacturer's (Essex) major diameter of .410" as an example of how to do the math. But the consensus was that up to 4 cables were allowed before derating applied, and that Note 2 caused conduit fill to be a non-factor. I then asked what the answer would be if the OP had been using 1/2" PVC instead, but nobody responded to it.

Can anyone here shed any light on why there is no fill limitation on the number of conductors allowed to be jammed into an undefined "short section" of conduit used only to protect exposed conductors? What does installer intent have to do with the ability of conductors to withstand heat? And is conduit ever used for any other purpose except to protect conductors???

 

[George Stolz]

Re: Chapter 9, Table 1, Note 2 - Any purpose?

Originally posted by Brian M:
Can anyone here shed any light on why there is no fill limitation on the number of conductors allowed to be jammed into an undefined "short section" of conduit used only to protect exposed conductors?

Your indignation is justifiable.

Note 2's intentions are questionable, hopefully there will be some discussion on that, because it confuses me a bit too. Based on the wording, it seems as though you can fill a conduit used for protection of cables up to 100%, which seems like a bad idea. It seems to me that the cables would suffer damage from the initial installation, IMO.

What does installer intent have to do with the ability of conductors to withstand heat?

Provided 310.15(B)(2)(a) is followed, there should be no ill effects from filling a pipe to the brim in terms of heat.

Larger cables would have to be installed, requiring a larger pipe, which would bring everything well within safe heat limits, due to derating that must be done.

And is conduit ever used for any other purpose except to protect conductors???

It comes back to 300.18(A), where we're told that conduit must be installed complete between boxes, with the exception letting us know we can protect cables with conduit in violation of this main rule.

In the context we're talking about, the conduit is protecting cables, not conductors.

Welcome to the forum, Brian.

 

[Brian M]

Re: Chapter 9, Table 1, Note 2 - Any purpose?

georgestolz wrote:

Provided 310.15(B)(2)(a) is followed, there should be no ill effects from filling a pipe to the brim in terms of heat.

I don't think I agree with this idea that derating is protection enough without fill limitations in place. If this were the case, then there would be no need for conduit fill tables at all - yet every conduit article limits number of conductors by referencing Table 1 (where Note 2 then sends you spinning off) rather than referencing 310.15.

On the subject of 310.15, if heat is a problem for bundled cable outside of conduit, why isn't it an even greater problem within conduit which would require fill percentages to be strictly enforced rather than exempted?

Off the top of my head, I can't think of any case where fill is allowed to exceed 70% (nipples) except for Note 2 eliminating fill limits for vague "short sections". If 71% fill is too many conductors (even after derating) for conduit lengths up to 48" which terminate in boxes on both ends, then why are longer "short" sections of protective conduit better dissipators of heat if they are not connected to boxes and are allowed to be packed full of conductors?

Larger cables would have to be installed, requiring a larger pipe, which would bring everything well within safe heat limits, due to derating that must be done.

Common sense says that larger pipe would then be needed, but where does code require it if there are no fill limitations in effect? How do you know when a conduit should be increased in size, and how is the larger size to be chosen if not using fill percentage as your reference point?

 

[George Stolz]

Re: Chapter 9, Table 1, Note 2 - Any purpose?

Originally posted by Brian M:

Provided 310.15(B)(2)(a) is followed, there should be no ill effects from filling a pipe to the brim in terms of heat.

I don't think I agree with this idea that derating is protection enough without fill limitations in place. If this were the case, then there would be no need for conduit fill tables at all - yet every conduit article limits number of conductors by referencing Table 1 (where Note 2 then sends you spinning off) rather than referencing 310.15.

Yes there would - a conduit filled beyond these limits is becomes physically difficult to pull, and with a high risk of damage to the conductors. 310.15(B) is concerned with heat - this section does all the derating - it does all the work related to conductors in close proximity heating each other up and causing damage to each other.

On the subject of 310.15, if heat is a problem for bundled cable outside of conduit, why isn't it an even greater problem within conduit which would require fill percentages to be strictly enforced rather than exempted?

What in these tables change the requirements of 310.15?

And why do you consider a conduit to be more insulative than 24" of cellulose insulation? It doesn't hold water in that perspective, does it?

...why are longer "short" sections of protective conduit better dissipators of heat if they are not connected to boxes and are allowed to be packed full of conductors?

Just being clear -
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• <font size="2" face="Verdana, Helvetica, sans-serif">310.15 deals with heat</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">The Tables relating to conduit fill relate to how many conductors you can physically jam into a pipe without damaging the conductors and making it physically impossible to pull.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">

How do you know when a conduit should be increased in size, and how is the larger size to be chosen if not using fill percentage as your reference point?

 

[electric_instructor]

Re: Chapter 9, Table 1, Note 2 - Any purpose?

Discussion never included the ambient temperature of the nmb, & 1" PVC. Or the number of "current carrying" conductors.

These considerations alone limit the number of conductors. Either in conduit or not.

Based upon NEC Article 310.15(B)(2). Cables stacked or bundled longer than 24" without maintaining spacing shall be reduced based on Table 310.15(B)(2)(a).

In order to maintain the 20 amp ampacity of the #12 AWG a maximum number (if thhn) would be 7-9 conductors. This is .70 factor.
#12 THHN is rated at 30 amps X .7 = 21 amps

If the ambient temperature does NOT exceed 86 deg. F
AND
If all are 12/2, this would be 4 runs of 12/2 nm cable. If each cable includes 2 "Current Carrying Conductors".

You can't "bundle" cables together for more than 24", unless you derate them.

This includes leaving the panel! How many panels have more than 24" from the top of the panel, thru the wall plate, and have the cables "bundled" together?

 

[charlie b]

Re: Chapter 9, Table 1, Note 2 - Any purpose?

First, please note that the word "short" does not appear in Note 2 to Chapter 9 Table 1. But the word "exposed" is in that note.

I envision this note as referring to a run of NM that goes all around a garage, and that can be seen throughout the entire run, except for the section behind the work bench. That section has a conduit around the NM to keep it from being hit by a power tool.

In other words, if the entire run of cable is within a conduit, you can't say the conduit is "just for protection." To me, the key word in that note is the word, "exposed." The cable has to be exposed, except for the area that is within conduit to protect it from a hazard that exists in that area, but that does not present a hazard to the rest of the run.

Secondly, conduit fill is unrelated to derating. You can fill a conduit to 40% with two current carrying conductors and a ground. No derating. You can fill a conduit to 40% with 20 current carrying conductors and 10 grounds. 50% derating.

Finally, if you have a situation in which you can take advantage of the note, and pack the conduit as tightly as you please, you still have to deal with the derating for having more than 3 current-carrying conductors. This "conduit for protection" is not a "nipple," so you can't take advantage of Exception 3 to 310.15(B)(2)(a).

 

[Brian M]

Re: Chapter 9, Table 1, Note 2 - Any purpose?

Thanks to those responding for your time and help on clarifying this to me. I guess I've been mixing apples & oranges as far as conduit fill & derating/bundling/etc are concerned. Also, the reference to "short" was taken from the 99 handbook commentary and I confused that with the actual note text.