AC wire size

Status
Not open for further replies.
Dennis Alwon said:
I think they are saying the conduit used cannot be used in an area more than 60C. If the temp was more than 60C then you would have to use another raceway... I think
Click to expand...
Sounds like both:

356.12 Uses Not Permitted. LFNC shall not be used as follows:
(1) Where subject to physical damage
(2) Where any combination of ambient and conductor temperatures is in excess of that for which it is listed
(3) In lengths longer than 1.8 m (6 ft), except as permitted by 356.10(5) or where a longer length is approved as essential for a required degree of flexibility
(4) In any hazardous (classified) location, except as permitted by other articles in this Code
Click to expand...
 
david luchini said:
The mca of the conductor has nothing to do with the operating temperature of the conduit. The actual load current and the ambient conditions affect the operating temperature of the conduit.
Click to expand...
110.14(C) starts off "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."

Are you saying that LFNC is not a "termination, conductor, or device," and that therefore it is OK to "select" the temperature of 75C (when applicable) for ampacity for conductors in 60C rated LFNC, as long as some other assurance is made that the LFNC will not exceed 60C?

Cheers, Wayne
 
wwhitney said:
110.14(C) starts off "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."

Are you saying that LFNC is not a "termination, conductor, or device," and that therefore it is OK to "select" the temperature of 75C (when applicable) for ampacity for conductors in 60C rated LFNC, as long as some other assurance is made that the LFNC will not exceed 60C?

Cheers, Wayne
Click to expand...
Yes, LFNC is neither a termination, conductor nor device. So yes, 1104.14(C) says you can use the 75deg rating associated with the ampacity of the #14 THHN. And yes, you have to make sure that the conditions of use don't exceed the temp rating of the LFNC.
 
david luchini said:
Yes, LFNC is neither a termination, conductor nor device. So yes, 1104.14(C) says you can use the 75deg rating associated with the ampacity of the #14 THHN. And yes, you have to make sure that the conditions of use don't exceed the temp rating of the LFNC.
Click to expand...
OK, I think that 110.14(C) should probably include a term that covers LFNC in that list (without being so broad as to include all equipment), but it doesn't, so I have to agree.

Now, how about an analysis of 356.12 (Uses Not Permitted) "(2) Where any combination of ambient and conductor temperatures is in excess of that for which it is listed". How would you rebut the claim that that means the conductor temperature rating selected may not exceed 60C for 60C rated LFNC?

BTW, I checked the UL 1660 standard, and all LFNC is 60C wet rated. Only the dry and oil temperature ratings vary depending on testing results.

Cheers, Wayne
 
electrofelon said:
So how are we to determine the operating temperature of a conduit?
Click to expand...
My thoughts exactly. I see Davids point, the MCA already has 25% added to the largest load meaning the compressor so the actual running current is below 15 amps. One would have to assume that the #14 conductors will not heat up above 60° C.
 
infinity said:
My thoughts exactly. I see Davids point, the MCA already has 25% added to the largest load meaning the compressor so the actual running current is below 15 amps. One would have to assume that the #14 conductors will not heat up above 60° C.
Click to expand...
I see his point too, but I'm not sure what else we have to go off. Not sure that telling the inspector "there's no way it will get that hot" or showing him a bunch of heat calculations on the back of a napkin will cut it 😉
 
electrofelon said:
I see his point too, but I'm not sure what else we have to go off. Not sure that telling the inspector "there's no way it will get that hot" or showing him a bunch of heat calculations on the back of a napkin will cut it 😉
Click to expand...
I'm with you, I would need something more definitive.
 
electrofelon said:
I see his point too, but I'm not sure what else we have to go off. Not sure that telling the inspector "there's no way it will get that hot" or showing him a bunch of heat calculations on the back of a napkin will cut it 😉
Click to expand...
Well you can always write the calculations on the back of a big enough check...
 
wwhitney said:
Now, how about an analysis of 356.12 (Uses Not Permitted) "(2) Where any combination of ambient and conductor temperatures is in excess of that for which it is listed". How would you rebut the claim that that means the conductor temperature rating selected may not exceed 60C for 60C rated LFNC?
Click to expand...

I think 110.14(C) already rebutted the claim that you have to use a conductor temperature rating based on 60C.

But...
Suppose you had 3 #14 THWN with a load of 15A in a LFNC in an ambient of 30C. You would expect the load current to add 30C to the 30C ambient temperature so that the operating temperature is 60C.

Suppose you had 3#14 THWN with a load of 16.2A in a LFNC in an ambient of 25C. You would expect the load current to add 35C to the 25C ambient temperature so that the operating temperature is 60C.

Now suppose you had 3#14 THWN with a load of 15.9A in a LFNC in an ambient of 26C. You would expect the load current to add 34C to the the 26C ambient temperature so that the operating temperature is 60C.

In the 26C ambient example the load current is 15.9A, which exceeds the Allowable Ampacity of the 60C rating of #14 conductors in a 26C ambient, but the conditions of use from 356.12 are being met. In other words, the Code doesn't require you to use the 60C ampacity.

And in the single phase case, there are only two current carrying conductors, so the heat generated by the load current would be less.
 
david luchini said:
And in the single phase case, there are only two current carrying conductors, so the heat generated by the load current would be less.
Click to expand...
And how would you quantify that?

I think I can get behind the type of analysis you give an example of. And it has the advantage of making some sense of the non-multiple of 15C temperature ratings that LFNC may have, such at 80C dry or 70C oily.

But it does imply that for an A/C unit with 17A MCA, #14 Cu in LFNC in a wet location with 3 CCCs and an ambient of 30C would not suffice. Which (other than the 3 CCCs question which is outstanding) was my original point.

Cheers, Wayne
 
All well and good but when you take into account that many inspectors still have trouble with the concept of allowing a 25 amp overcurrent device on that conductor to begin with, good luck in convincing him/her the ampacity can be less than the MCA (other than using the method in post #29)
 
wwhitney said:
But it does imply that for an A/C unit with 17A MCA, #14 Cu in LFNC in a wet location with 3 CCCs and an ambient of 30C would not suffice. Which (other than the 3 CCCs question which is outstanding) was my original point.
Click to expand...
It doesn't imply that at all...you are inferring that. The MCA is not the operating current. If the operating current of the 17MCA unit is 15A or less, then#14Cu in LFNC in a wet location with 3ccc's and an ambient of 30C would be fine.

The MCA doesn't tell you what the actual current is, which was Don's point back in post #10.
 
david luchini said:
It doesn't imply that at all...you are inferring that. The MCA is not the operating current. If the operating current of the 17MCA unit is 15A or less, then#14Cu in LFNC in a wet location with 3ccc's and an ambient of 30C would be fine.
Click to expand...
I'm going to say that the phrase "conditions of use" in the definition of ampacity includes "not overheating the conduit the conductor is in." So if 15A is the most current that the #14s can continuously carry without exceeding the 60C LFNC rating, then the ampacity is no more than 15A. Which is less than the 17A MCA.

Edit: the fact that the equipment isn't going to be loading the conductor to 17A continuously is immaterial. The equipment label says it must be supplied with conductors that can carry 17A continuously.

Cheers, Wayne
 
wwhitney said:
I'm going to say that the phrase "conditions of use" in the definition of ampacity includes "not overheating the conduit the conductor is in." So if 15A is the most current that the #14s can continuously carry without exceeding the 60C LFNC rating, then the ampacity is no more than 15A. Which is less than the 17A MCA.
Click to expand...
Of course that's not what the definition actually says. It says the maximum current that the CONDUCTOR can carry continuously under the conditions of use without exceeding ITS TEMPERATURE RATING.

For #14 THWN in raceway in 30C ambient, that's 20A...which is greater than the 17A MCA.
 
Sorry, I'm still inclined to disagree. The full definition of ampacity is "The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."

I don't find a definition of "condition of use" anywhere. If you can demonstrate that "conditions of use" means only "ambient temperature and number of current carrying conductors in a cable/conduit" then I will agree with you. Otherwise, I find it more rational to say that "conditions of use" includes temperature limitations of the conduit carrying the conductors. As well as possibly other limitations.

Which leads me to another ambiguity in the use of the word "ampacity": when the allowable current on a conductor is controlled by the temperature rating of one of its connections, terminologically, does that reduce the conductor's ampacity? Or is the ampacity still the same, and the termination limitation is an additional limitation? The distinction might matter for applying something like 240.4, "shall be protected
against overcurrent in accordance with their ampacities."

The wording of 240.4(D) "small conductors" is an example where the OCPD size is limited, but the language involved doesn't change the conductor ampacity. But 110.14(C) says "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated . . ." Which suggests to me that the ampacity itself is changed by termination limitation.

Cheers, Wayne
 
wwhitney said:
Sorry, I'm still inclined to disagree. The full definition of ampacity is "The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."

I don't find a definition of "condition of use" anywhere. If you can demonstrate that "conditions of use" means only "ambient temperature and number of current carrying conductors in a cable/conduit" then I will agree with you. Otherwise, I find it more rational to say that "conditions of use" includes temperature limitations of the conduit carrying the conductors. As well as possibly other limitations.

Which leads me to another ambiguity in the use of the word "ampacity": when the allowable current on a conductor is controlled by the temperature rating of one of its connections, terminologically, does that reduce the conductor's ampacity? Or is the ampacity still the same, and the termination limitation is an additional limitation? The distinction might matter for applying something like 240.4, "shall be protected
against overcurrent in accordance with their ampacities."

The wording of 240.4(D) "small conductors" is an example where the OCPD size is limited, but the language involved doesn't change the conductor ampacity. But 110.14(C) says "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated . . ." Which suggests to me that the ampacity itself is changed by termination limitation.

Cheers, Wayne
Click to expand...
You did touch on it some, but there is two ampacities for most every conductor - one based on the termination temperature and one based on insulation temperature.

A bare conductor would only have the termination temp for the most part, next would be how much can any supporting items can withstand, then ultimately they all have a max before the conductor itself starts to melt, stretch, etc.
 
kwired said:
You did touch on it some, but there is two ampacities for most every conductor
Click to expand...
I think parts of the NEC might be clearer if they were written in terms of two or more different ampacities, e.g. insulation ampacity, termination ampacity, maybe baseline ampacity for the number directly from the table, etc. But the way it's currently written and the definition above doesn't admit that, there is only one ampacity for a conductor. And so some of the rules aren't clear about whether they are changing that ampacity or not. As well as some sections require constructions such as "ampacity before the application of correction or adjustment factors".

Cheers, Wayne
 
wwhitney said:
I think parts of the NEC might be clearer if they were written in terms of two or more different ampacities, e.g. insulation ampacity, termination ampacity, maybe baseline ampacity for the number directly from the table, etc. But the way it's currently written and the definition above doesn't admit that, there is only one ampacity for a conductor. And so some of the rules aren't clear about whether they are changing that ampacity or not. As well as some sections require constructions such as "ampacity before the application of correction or adjustment factors".

Cheers, Wayne
Click to expand...
might be a little confusing, but at same time there is two different aspects that need to apply and mostly not dependent on one another either.

A good designer, designer/installer needs to know the reasons for both aspects or not be involved in that part of the design process.

And NEC does simplify things to some extent, we could probably use smaller conductors in many places if we wanted to make it more complex.
 
Status
Not open for further replies.
Top