Type AC cable in Thermal Insulation

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infinity

infinity

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Recently had a discussion with an inspector about a type AC cable in thermal insulation. A 75' branch circuit for an A/C unit (MCA=31amps) run in 10-2 AC cable with 5' of the run going through a wall with fiberglass insulation. He claims that if any portion of the run is within the insulation than 320.80(A) applies and the entire circuit length can only be rated for 60 degrees. I argued that the 60 degree ampacity only applies to the portion of the cable within the insulation, in this case the 5' of cable. By then applying 310.15(A)(2) Exception, the 5' would not need to have the 60 degree rating applied to it. He doesn't agree. Any thoughts?
 
My NEC Handbook agrees with you. However, who ever wrote that exception should be beaten with a stick. It could have been written more clearly.
 
Trevor,
How does the exception in 310.15(A)(2) apply to the rule in.15 320.80(A)? 310.15 applies to determining the correct ampacity for conductors. That is not the case for the rule in 320.80. It tells us what the ampacity of the conductors shall be. I don't see how the exception applies.
Don
 
What are the temperature ratings of the terminations of this circuit? If you do not know that they are 75 degree or 90 degree, you will need to use the 60 degree rating anyway, assuming the conductors are smaller than 1/0.
See 110.14(C).
There are very few cases where the 90 degree ampacity may be used for other than derating.
 
How does the exception in 310.15(A)(2) apply to the rule in.15 320.80(A)?
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Don
My reasoning was that in the book it says 320.80 Ampacity The ampacity shall be determined by 310.15.
 
bob said:
How does the exception in 310.15(A)(2) apply to the rule in.15 320.80(A)?
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Don
My reasoning was that in the book it says 320.80 Ampacity The ampacity shall be determined by 310.15.
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This was my reasoning too. Since 310.15(A)(2) is part of 310.15 and since 320.80 directed me to 310.15, than IMO, 310.15(A)(2) should apply to the 5' of AC cable within the insulation.
 
infinity said:
bob said:
How does the exception in 310.15(A)(2) apply to the rule in.15 320.80(A)?
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Don
My reasoning was that in the book it says 320.80 Ampacity The ampacity shall be determined by 310.15.
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This was my reasoning too. Since 310.15(A)(2) is part of 310.15 and since 320.80 directed me to 310.15, than IMO, 310.15(A)(2) should apply to the 5' of AC cable within the insulation.
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That reasoning works for the general ampacity of AC cable, but 320.80(A) is a subsection and that part gets treated separately from the other.
 
Are we making an assumption here that ampacity is the reason for this restriction? After all, AC-90's 90 degree insulations are similar to MC's 90 degree insulation, but MC doesn't have the restriction.

What is there different about AC vs MC? The paper wraps maybe?
 
That reasoning works for the general ampacity of AC cable, but 320.80(A) is a subsection and that part gets treated separately from the other.
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If you had an installation where there was 12 inches of AC that passed thru an insulated wall, I would think that most people would agree that
it would not be necessary to require that the 60C rating be used to operate the cable safely.
I think 320.80A is referring to a cable totally installed in an insulated wall and it would be reasonable to require a 60C rating. 320.80 provides a means to use the in exception in 310.15 when just a small portion of the cable is installed in insulation.
 
"I think "

I tell the guys in class if they say, I think, then they really do not know. In an instance like this, a yes or no written type question to NFPA may be the cure.


That 1 foot in the insulation may be treated the same as the last paragraph in 334.80... we will have to find out.

The funny thing is, how many installations are already installed this way?? Are they finding problems with them?
 
I'd agree that 320.80 applies to only the portion of the cable installed in thermal insulation, and then that the portion is small enough to qualify the exception to 310.15 (A)2.

I feel that 320.80 directly refers you to 310.15, so the exception does apply.
 
There is no room to use the exception even if only 1" of cable is in the insulation. The code section is very clear.
(A) Thermal Insulation Armored cable installed in thermal insulation shall have conductors rated at 90?C (194?F). The ampacity of cable installed in these applications shall be that of 60?C (140?F) conductors.
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There is no room to apply the exception.
Don
 
Insulation vs "ambient", they are not the same.

I would have to agree that you cannot apply 310.15 exception in 320.80 here is an example of 310.15 from 2005 HB. No mention of "insulation".

(2) Selection of Ampacity Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.

Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
Example

Three 500-kcmil THW conductors in a rigid conduit are run from a motor control center for 12 ft past a heat-treating furnace to a pump motor located 150 ft from the motor control center. Where run in a 78?F to 86?F ambient temperature, the conductors have an ampacity of 380 amperes, per Table 310.16. The ambient temperature near the furnace, where the conduit is run, is found to be 113?F, and the length of this particular part of the run is greater than 10 ft and more than 10 percent of the total length of the run at the 78?F to 86?F ambient. Determine the ampacity of total run in accordance with 310.15(A)(2).

Solution

In accordance with the correction factors for temperature at the bottom of Table 310.16, the ampacity is 0.82 ? 380 amperes, or 311.6 amperes. This, therefore, is the ampacity of the total run, in accordance with 310.15(A)(2).

Had the run near the furnace at the 113?F ambient been 10 ft or less in length, the ampacity of the entire run would have been 380 amperes, in accordance with the exception to 310.15(A)(2). The heat-sinking effect of the run at the lower ambient temperature would have been sufficient to reduce the temperature of the conductor near the furnace.
 
don_resqcapt19 said:
There is no room to use the exception even if only 1" of cable is in the insulation. The code section is very clear.
(A) Thermal Insulation Armored cable installed in thermal insulation shall have conductors rated at 90?C (194?F). The ampacity of cable installed in these applications shall be that of 60?C (140?F) conductors.
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There is no room to apply the exception.
Don
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I don't think this is very clear. Does this mean that the ampacity of the entire cable is to be that of 60 degrees, or just the portion in contact with thermal insulation. I was reading it as that portion which is in contact with the insulation, for which the exception would apply. Why do you read this as the ampacity of the entire cable?

I don't think the above example really even fits the description of "installed in thermal insulation". More like "barely touching thermal insulation".
 
John,
I don't think this is very clear. Does this mean that the ampacity of the entire cable is to be that of 60 degrees, or just the portion in contact with thermal insulation. I was reading it as that portion which is in contact with the insulation, for which the exception would apply. Why do you read this as the ampacity of the entire cable?
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You are correct the only part that has to be used at the 60?C ampacity is the part in the insulation. You could install a J-box on each side of the insulation and a larger conductor within the insulation, or you can do it the easy way and protect the complete cable at or below its 60?C ampacity.
There is nothing that lets you use an exception from one code rule on a different code rule. If there was an exception to 320.80 it would have to be a part of 320.80. You can't take an exception from 310.15 and apply it to 320.80.
From the NEC Style Manual:
2.6 Exceptions.
2.6.1 Placement and Order. Exceptions shall immediately follow the main rule to which they apply. Where exceptions are made to items within a numbered list, the exception shall clearly indicate the items within the list to which it applies. Exceptions containing the mandatory terms shall or shall not are to be listed first in the sequence. Permissive exceptions containing shall be permitted are to follow any mandatory exceptions and be listed in their order of importance as determined by the Code-Making Panel.
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Don
 
I?m responding to this one from a PM I received.

There will always be some level of ambiguity in any document like the NEC. That is the REAL purpose of the ?? has the responsibility for making interpretations ?? clause in 90.4.

Don properly cited the NEC Manual of Style (MOS) as a basis of proper interpretation; I?d like to add one more to muddy the issue for a while:

2.1.5 Subdividing Sections. Sections shall be permitted to be subdivided for clarity, with each subdivision representing either a rule or a part of a rule. Up to three levels of subdivisions shall be permitted, and any level shall be permitted to contain a list.
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The question then is, ?Is 320.80(A) a ?rule? or ?part of a rule??? If it is only ?part of a rule,? I would argue 310.15 and its exceptions apply because it is subordinated to the main rule(320.80). Especially in light of the remaining text of the Subsection. If it a "whole" rule in itself, then the 310.15 exception does not apply as Don pointed out.

Unfortunately the MOS doesn?t clarify how to determine the status of a Subdivision.

When I first joined the Forums, I asked if anyone felt there should be any standard ?Rules of Interpretation.? I was surprised that it caused some discord. I have my opinions (of course) but I?ll let this one ride a while.
 
Don.

320.80 tells us to determine ampacity by 310.15. So why doesn't all of 310.15 including the exception apply? My thought is that the only reason for 320.80A is that 310.15 does not make any adjustment in ampacity for installations in thermal insulation, hence the need for it in 320. 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.

I ask this with respect, as I've read and benefitted from many of your posts.

John
 
don_resqcapt19 said:
Bob,
I don't see how a rule in Article 320 can be a subdivision of a rule in Article 310.
Don
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[Edit Add: When Rule "A" says "Do it per Rule "B," the exceptions to Rule "B" must logically apply also. The question is as I stated earlier. Is 320.80(A) a 'rule' or 'part of a rule?']

I'd start with making the observation that other Articles that have analogous (xxx.80) Sections either have no subdivisions (e.g. 330, 334,336, & 340) or they are fairly obvious "parts of a rule" that give additional information about a specific application (e.g., 332). I'd also observe that 320.80(B) is only "part of a rule," why would 320.80(A) not be?

I would also ask myself if there is something unique about Type AC that makes it unusually dangerous in the envisioned specific application, such that the 310.15(B)(2) Exception should be avoided.

In all honesty I don't have any experience with Type AC. It is obvious that someone who does feels a need for additional caution. Whether that experience would say 10' or less of the cable in thermal insulation would warrant derating the entire circuit I don't know.
 
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