Table 310.15(B)(16)

[MBRUESCHKE]

What is the determining factor on which column to use when sizing conductors, 60-Degree-C, 75-Degree-C, or 90-Degree-C. My journeyman told me to always default to the 60-Degree-C column. But there has to be more to it then that.

 

[iwire]

What is the determining factor on which column to use when sizing conductors, 60-Degree-C, 75-Degree-C, or 90-Degree-C. My journeyman told me to always default to the 60-Degree-C column. But there has to be more to it then that.

Well there is more to it than that, a lot more.

For starters the terminal rating or if it is marked at all, if you are derating or not, what kind of insulation the wire has and if the code specifies a temp for a specific wiring method like it does with NM.

What kind of work do you do and we can try to relate it to that.

 

[infinity]

What is the determining factor on which column to use when sizing conductors, 60-Degree-C, 75-Degree-C, or 90-Degree-C. My journeyman told me to always default to the 60-Degree-C column. But there has to be more to it then that.

Actually it can be pretty complicated as Bob indicated. Defaulting to the 60? C column is not the answer. :roll:

 

[jumper]

Actually it can be pretty complicated as Bob indicated. Defaulting to the 60? C column is not the answer. :roll:

Oh yeah, I agree.

 

[James S.]

I will pick it up since OP has disappeared. But, my question is more about what row. What is the determining factor in what row to use. I am in Phoenix, AZ. our record temp was 123 a few years ago. (It actually shut down the airport because the planes couldn't get enough lift) Is that what should be used or is there something else? Obviously it gets even hotter in attics but I have never seen any derating for NM.

 

[Smart $]

I will pick it up since OP has disappeared. But, my question is more about what row. What is the determining factor in what row to use. I am in Phoenix, AZ. our record temp was 123 a few years ago. (It actually shut down the airport because the planes couldn't get enough lift) Is that what should be used or is there something else? Obviously it gets even hotter in attics but I have never seen any derating for NM.

I a bit perplexed regarding "what row to use"...

Your mention of elevated temperature is more regarding ampacity correction for ambient temperature. In one aspect of sizing, correction for ambient is considered, in the other, not...

In 215.2(A)(1)...

The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.

In 210.19(A)(1)...

Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable ampacity
not less than the noncontinuous load plus 125 percent
of the continuous load.

Motor applications aside, there are at least four elements to sizing a conductor:

1) 110.14(C)
2) First sentence of 210.19(A)(1) or 215.2(A)(1)
3) Second sentence of 210.19(A)(1) or 215.2(A)(1)
4) 240.4

Each element determines a minimum size. The largest of the four is the required minimum size.

 

[Dennis Alwon]

James the attic temp. has nothing to do with what col. to use. The wire insulation is what is used and the ampacities are based on 86?F. From there you would derate for temp. BTW I have never seen anyone use derating in attic either

 

[James S.]

I a bit perplexed regarding "what row to use"...

Your mention of elevated temperature is more regarding ampacity correction for ambient temperature. In one aspect of sizing, correction for ambient is considered, in the other, not...

In 215.2(A)(1)...


In 210.19(A)(1)...


Motor applications aside, there are at least four elements to sizing a conductor:

1) 110.14(C)
2) First sentence of 210.19(A)(1) or 215.2(A)(1)
3) Second sentence of 210.19(A)(1) or 215.2(A)(1)
4) 240.4

Each element determines a minimum size. The largest of the four is the required minimum size.

Sorry, I had a question and searched for ambient temp then misread the OP of this thread. I probably should have just started a new one. Anyway, what I was talking about was the correction factors for ambient temp. 310.15(B)(2)(a) [Formerly Table 310(16)]. What determines what row you use?

 

[James S.]

James the attic temp. has nothing to do with what col. to use. The wire insulation is what is used and the ampacities are based on 86?F. From there you would derate for temp. BTW I have never seen anyone use derating in attic either

yes, the derating is what I was talking about. How do you determine the "ambient temperature"?

 

[infinity]

Sorry, I had a question and searched for ambient temp then misread the OP of this thread. I probably should have just started a new one. Anyway, what I was talking about was the correction factors for ambient temp. 310.15(B)(2)(a) [Formerly Table 310(16)]. What determines what row you use?

You use the row that is based on the type of insulation that you have on the conductor.

 

[xformer]

I a bit perplexed regarding "what row to use"...

Your mention of elevated temperature is more regarding ampacity correction for ambient temperature. In one aspect of sizing, correction for ambient is considered, in the other, not...

In 215.2(A)(1)...


In 210.19(A)(1)...


Motor applications aside, there are at least four elements to sizing a conductor:

1) 110.14(C)
2) First sentence of 210.19(A)(1) or 215.2(A)(1)
3) Second sentence of 210.19(A)(1) or 215.2(A)(1)
4) 240.4

Each element determines a minimum size. The largest of the four is the required minimum size.

This is probably the best way I have ever sen this phrased..

 

[xformer]

yes, the derating is what I was talking about. How do you determine the "ambient temperature"?

You can get ambient temps from a ASHRAE handbook - fundamentals.

 

[Dennis Alwon]

There is a chart that supposedly tells us but I cannot for the life me understand how one can determine the ambient temp. The ambient temp is the temp at the time of the install. It does not mean average. No one seems to know if we are supposed to use the highest temp ever or an avg over the hottest days, etc.

Here is a read on ambient temps but it has more to do with rooftop

http://www.copper.org/publications/pub_list/pdf/Ambient_REPRINT.pdf

Here is another one temps by cities

http://www.copper.org/applications/electrical/building/pdf/rooftop.pdf

 

[Smart $]

There is a chart that supposedly tells us but I cannot for the life me understand how one can determine the ambient temp. The ambient temp is the temp at the time of the install. It does not mean average. No one seems to know if we are supposed to use the highest temp ever or an avg over the hottest days, etc.

Here is a read on ambient temps but it has more to do with rooftop

http://www.copper.org/publications/pub_list/pdf/Ambient_REPRINT.pdf

Here is another one temps by cities

http://www.copper.org/applications/electrical/building/pdf/rooftop.pdf

The note seems to indicate the average is to be used for rooftops...

Informational Note to Table 310.15(B)(3)(c): The temperature
adders in Table 310.15(B)(3)(c) are based on the results of
averaging the ambient temperatures

If you take that on face value, the temperature adder (a) is added to the average ambient temperature (b). Yet the ambient temperature one is supposed to use for basic correction is the highest ever (c). So you add the adder (a) to the average (b), then use the higher of (a+b) or (c)???

 

[GoldDigger]

The note seems to indicate the average is to be used for rooftops...
If you take that on face value, the temperature adder (a) is added to the average ambient temperature (b). Yet the ambient temperature one is supposed to use for basic correction is the highest ever (c). So you add the adder (a) to the average (b), then use the higher of (a+b) or (c)???

They might be trying (and failing) to say that they have taken the observed adders for different ambient temperatures and averaged them.
The experimental adder for a winter day with 0 degree ambient may be lower than the experimental adder for mid-summer when the the ambient is 100 AND the increase in heat from the sun on the roof is higher.
Treating the adder itself as an average would result in taking the maximum ambient and adding the average adder, which makes more sense than summing the average ambient and the average adder and then throwing away that result.
What will be affecting the wires, of course, will be the maximum ambient plus the maximum adder, but you do not have the latter number to work with.

 

[Smart $]

They might be trying (and failing) to say that they have taken the observed adders for different ambient temperatures and averaged them.
The experimental adder for a winter day with 0 degree ambient may be lower than the experimental adder for mid-summer when the the ambient is 100 AND the increase in heat from the sun on the roof is higher.
Treating the adder itself as an average would result in taking the maximum ambient and adding the average adder, which makes more sense than summing the average ambient and the average adder and then throwing away that result.
What will be affecting the wires, of course, will be the maximum ambient plus the maximum adder, but you do not have the latter number to work with.

You got my head spinning :blink:

The note says the adders are based on averaging the ambient temperatures. I don't believe there is that much to read into it.

When it comes right down to it, 310.15(B)(2) does not say how we are to apply ambient temperature correction factors. Unless the conductors are in a controlled-temperature environment, the ambient temperature is going to vary. So what if I use the ambient temperature correction factor on the day I do the installation... and itjust happens to be the coldest day of winter. Is my installation compliant?

 

[GoldDigger]

You got my head spinning :blink:
The note says the adders are based on averaging the ambient temperatures. I don't believe there is that much to read into it.

I think the problem is that, as written, there is nothing to read into the statement at all, because it does not say anything.
Adders have to be based on an increase of the raceway temperature (and therefore the even higher conductor temperature) above what they would otherwise be.
If all you do is measure overall ambient temperatures and average them, you have learned nothing except what the average ambient temperature is.
If the ambient temperatures they are averaging are the actual raceway temperatures, then you have some data.
And the natural way to get the adders is to take 10 different ambient temperatures and the corresponding circular raceway temperatures, and average both sets of numbers and then take the difference or else take the 10 differences and then average them (which will always give you exactly the same number!).
That could be described as averaging the adders corresponding to each ambient temperature.
There is nothing there that tells me that the adders should only be used to correct the average ambient temperature. (Although in all fairness, it does not tell me to do anything else either.) It is implicit in "adders" that I have to add them to something, and in the absence of any direction to the contrary I would add them to whatever temperature I would have used had I not been told to apply the correction.

Returning to the question of what should be used for the ambient temperature, the first rule is:

No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved.

Note that it does not say "at the time of installation". There is no magic about doing an install in the winter that allows you to use smaller wires.
It is not crystal clear whether this should be taken as year round average, but given the following mention that:

Ambient temperature — ambient temperature may vary along the conductor length as well as from time to time.

I have to infer that I am supposed to use the highest temperature point along the raceway in question and the time at which the temperature is the highest.

 

[infinity]

So what if I use the ambient temperature correction factor on the day I do the installation... and it just happens to be the coldest day of winter. Is my installation compliant?

Given the fact we discussing this with this much detail means that the code section needs to be rewritten. :roll:

 

[Dennis Alwon]

Given the fact we discussing this with this much detail means that the code section needs to be rewritten. :roll:

That was my point. There is no info on how to determine ambient temp. when the temp changes. Obviously if you are in an area that is a constant 120? we know what to do but when temps can swing some 90? or so we have no answers.

The fact of the ambient temp of an attic for a dwelling has never seemed to be an issue. I have never seen insulation break down or trouble with NM cable that has been in an attic. Now AZ may be different however it sounds like they don't worry either as most cables are barely loaded.

 

[Smart $]

I think the problem is that, as written, there is nothing to read into the statement at all, because it does not say anything.
Adders have to be based on an increase of the raceway temperature (and therefore the even higher conductor temperature) above what they would otherwise be.
If all you do is measure overall ambient temperatures and average them, you have learned nothing except what the average ambient temperature is.
If the ambient temperatures they are averaging are the actual raceway temperatures, then you have some data.
And the natural way to get the adders is to take 10 different ambient temperatures and the corresponding circular raceway temperatures, and average both sets of numbers and then take the difference or else take the 10 differences and then average them (which will always give you exactly the same number!).
That could be described as averaging the adders corresponding to each ambient temperature.
There is nothing there that tells me that the adders should only be used to correct the average ambient temperature. (Although in all fairness, it does not tell me to do anything else either.) It is implicit in "adders" that I have to add them to something, and in the absence of any direction to the contrary I would add them to whatever temperature I would have used had I not been told to apply the correction.

What we have are requirement statements which are either implicit, inferred, or explicit. I do not see any of the last, perhaps at best bordering the middle, and a lot of the first. Your last sentence alone proves you have to make a leap of faith to implement.

Returning to the question of what should be used for the ambient temperature, the first rule is:

Note that it does not say "at the time of installation". There is no magic about doing an install in the winter that allows you to use smaller wires.
It is not crystal clear whether this should be taken as year round average, but given the following mention that:

I have to infer that I am supposed to use the highest temperature point along the raceway in question and the time at which the temperature is the highest.

For the most part, I agree with the notion we should use the highest temperature... but you can't average the high temperature for any one year period. It is either the high or it is not. Anything lower is whatever you want to call it, whether accurate, useful, or not.

All I'm saying is if the adders are based on "averaging the ambient temperatures", we have no true grounds to add them to the highest anticipated ambient temperature. To do so is an assumption not based on fact.