Type AC cable in Thermal Insulation

[don_resqcapt19]

John,

Do you feel that 320.80A overrules the exception? I can understand that if it's your interpretation, but I'm not sure I agree with it.

Yes. The rule is 320 requires a specific ampacity if the cable is in thermal insulation. It leaves no room for the application of the exception in 310.15. This is just like the tap rules in the 2002 code. They were revised in 2005 because the intent of the rule was not being complied with as a result of the same type of reasoning that is being used in this case.
The rule in the 2002 code said:

(1) Taps Not Over 3 m (10 ft) Long. Where the length of the tap conductors does not exceed 3 m (10 ft) and the tap conductors comply with all of the following:
(1) The ampacity of the tap conductors is
a. Not less than the combined computed loads on the circuits supplied by the tap conductors, and
b. Not less than the rating of the device supplied by the tap conductors or not less than the rating of the overcurrent-protective device at the termination of the tap conductors.

Many code users looked at this section and then applied the "round-up" rule found in 240.4(B). The code writers never intended that you could use the provisions 240.4(B) for the tap conductor OCPD. The intent was that the ampacity of the conductor be equal or greater than that of the OCPD. In the 2005 code additional wording was added to make the intent of the code rule very clear.

(B) Feeder Taps Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.

I see the logic that was being used to apply 240.4(B) to tap conductor in the same light as the logic being used in this application.
Don

 

[bob]

Pierre

I think that 320.80 & 334.80 need to be massaged somewhat adding that the exception to 310.15(A)(2) does not apply.
_________________
Instructor, Industry Advocate

"I think "

I tell the guys in class if they say, I think, then they really do not know.

Does this mean you are not sure?

 

[bob]

Did this question get resolved? I don't think so.

 

[Pierre C Belarge]

That is very quick of you Bob.

I misused my language... I was trying to be as "political" as is possible on this site.

It would help greatly if the wording of 320.80 was changed via a proposal to help with the confusion it seems to be causing.

 

[JohnE]

Did this question get resolved? I don't think so.

Regrettably, I still do not agree with Don. He agreed that ampacity of just the portion of cable in contact with thermal insulation is based on 60 degrees. So to determine overall ampacity we still use 310.15 requirements. 310.15 exception applies when different ampacities are determined for different portions of a conductior. 320.80 just guides us on how to figure the ampacity when in contact with thermal insulation, since 310.15 doesn't address this issue. If 320.80 were more prescriptive, I'd feel differently. If entire run (or large portion) were in contact with thermal insulation I'd feel differently.

While I agree with his taps example, I don't think the logic being applied here is the same.

Thanks,

John

 

[iwire]

I was asked to throw my opinion out here and unfortunately the person asking will be disappointed.

After reading this thread a couple of times I have to agree with Don about what the rule requires.

However I do not see the logic in it for real world applications.

 

[iwire]

OK lets get deeper into this.

Lets say this AC cable in contact with thermal insulation has more than 3 current carrying conductors or is in a high ambient temp.

Must I start my adjustments with the 60 C column?

 

[JohnE]

OK lets get deeper into this.

Lets say this AC cable in contact with thermal insulation has more than 3 current carrying conductors or is in a high ambient temp.

Must I start my adjustments with the 60 C column?

In this case there would be not adjustments since you'd be following 320.80 in it's entirety. 310.15 would not apply at all.

Now I don't believe this to be true, I feel 320.80 is used in conjuction with 310.15 as I've posted earlier.

I think your example helps substantiate my position. If I'm way off, please tell me 'cause I'm just not getting it then.

Thanks,

John

 

[don_resqcapt19]

Lets jump to 334.80 for a minute.

334.80 ... Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing that is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).

Does the exception in 310.15(A)(2) apply here too? If so why does the rule in 334.80 even exist?
Don

 

[rbalex]

... If so why does the rule in 334.80 even exist?
Don

The NEC is riddled with unintended consequences. Who knows? But:

7-150a Log #CP700 NEC-P07
(334-80)
Final Action: Accept
Submitter: Code-Making Panel 7
Recommendation:
Add a new paragraph to 334.80 to read as follows:
"334.80 Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing which is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a)."
Substantiation:
Code-Making Panel 6 Rejected Proposal 6-31 to add the proposed text to 310.15(B)(2)(a) and provided the following Panel statement:
"The Panel agrees with the intent of the Proposal, however, this material is more appropriately addressed in 334.80 since the Proposal only applies to one type of cable, and Code-Making Panel 6 covers all wiring methods. Therefore, Code-Making Panel 6 has forwarded this Proposal to Code-Making Panel 7 for action."
The substantiation provided by the submitter, Travis Lindsey, of Proposal 6-31 was:
"Recent experimentation shows the possibility of dangerous conditions when loaded circuits are brought into close proximity to each other inside a fire- or draft-stop, where the ability to dissipate heat is extremely limited. Cable temperatures well in excess of their 90?C rating were encountered, with no overcurrent protection present for these conditions. Results indicate that immediate adjustments should be made to the NEC to apply at least to the specific case represented by the experiment. Such a proposal is being made, with a supplemental report offered as technical support."
Panel Meeting Action: Accept
Number Eligible to Vote: 15
Ballot Results: Affirmative: 15

[rbalex bold added]

Note that 334.80 is almost verbatim what 320.80 is - except 320.80 is broken into subdivisions. Since 320.80 was revised after a MOS that had the same structural instructions with regard to subdivisions as the current one and both Articles are controlled by the same CMP, I may also reasonably infer the CMP intended to subordinate 320.80(A) to the main rule in 320.80.

 

[JohnE]

In reviewing 380.20(A), I noticed a second sentence has been added for 2005.

It says "the 90 degree rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60 degree conductor."

That certainly means to me that article 310.15 applies in it's entirety except as modified by 320.80.

Then 320.80 limits ampacity to 60 degree.

I think the only question remaining in OP example is whether 60 degree applies to entire conductor or portion in contact with insulation. I don't think the article is clear on this, but I can see an argument for either case.