When utilizing wire with a higher insulation temp rating than that of the connected terminations, and where both the wire and the terminations are in an ambient greater than 30 deg. C, it appears that the code allows you to apply the ambient temp correction factor to the wire insulation temp rating ampacity column instead of the connected termination temp rating ampacity column (Art. 310 tables). Is this an accurate interpretation? It would seem that, although this method certainly protects the conductor insulation, it provides no ambient temp correction to protect the connected terminations.
conductor ampacity ambient temp derating
- Thread starter bozo
- Start date
- Status
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: conductor ampacity ambient temp derating
yes that is correct. For example the 90 C rating of 12 THHN is 30 amperes, for derating 30 amps is used, in most cases you can not operate it above the 20 ampere rating per 240.4(D). You are starting out at 30 amps not 20. The ampacity of 12 THHN is 30, not 20 amperes. Some code articles (motors for example) allow you to use other than the 240.4 (D) ampacity.
yes that is correct. For example the 90 C rating of 12 THHN is 30 amperes, for derating 30 amps is used, in most cases you can not operate it above the 20 ampere rating per 240.4(D). You are starting out at 30 amps not 20. The ampacity of 12 THHN is 30, not 20 amperes. Some code articles (motors for example) allow you to use other than the 240.4 (D) ampacity.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conductor ampacity ambient temp derating
It does protect the terminations, but in a way that is not obvious. Take another example, but with a larger conductor, so 240.4(D) does not come into play. Consider a #8 THHN in an ambient of 35C. To protect the terminations, you have to stay below the 75C column (i.e., 50 amps). But you have to derate the conductor for the higher ambient. The derating factor for a 35C ambient and for a THHN conductor (90C column of the Correction Factors) is 0.96. You multiply that 0.96 times the 90C ampacity of a #8 THHN (i.e., 55 amps), and you get the result of 52.8 amps. You conclude that the conductor can safely handle 52.8 amps, ?safely? meaning that this current would not cause damage to its insulation system. However, you must still stay below the 50 amp limit, in order to protect the terminations.
It does protect the terminations, but in a way that is not obvious. Take another example, but with a larger conductor, so 240.4(D) does not come into play. Consider a #8 THHN in an ambient of 35C. To protect the terminations, you have to stay below the 75C column (i.e., 50 amps). But you have to derate the conductor for the higher ambient. The derating factor for a 35C ambient and for a THHN conductor (90C column of the Correction Factors) is 0.96. You multiply that 0.96 times the 90C ampacity of a #8 THHN (i.e., 55 amps), and you get the result of 52.8 amps. You conclude that the conductor can safely handle 52.8 amps, ?safely? meaning that this current would not cause damage to its insulation system. However, you must still stay below the 50 amp limit, in order to protect the terminations.
Re: conductor ampacity ambient temp derating
I don't see the answer to his question. Let's get away from the smaller conductors with the rating exceptions and deal with a more defined scenario.
Let's take a 68 amp continuous load, THHN 90 deg C wire. Per 110.14.C.1.a.1 or 2, since this is below 100 amps (and we don't know what the termination temp rating is) we are required to use the 60 deg c column from table 310.16 for sizing the wire to protect the termination device. We have a conductor requirement of 1.25 x 68 = 85 amps. This requires a #3 wire size at 30 deg C ambient temperature from the 60 Deg C column of table 310.16.
Now if we have an elevated ambient of 45 deg C, then according to some sources such as http://www.ecmweb.com/ar/electric_sizing_continuously_loaded/ we should only use the actual THHN wire insulation (90 deg C column) to apply the temperature derate (.87 x 110 instead of .71 x 85). This is all good and well for testing the protection of the insulation, but won't that allow the conductor to break the 60 deg C termination barrier. #3 THW wire has the same amount of copper as #3 THHN, no more no less. The termination's ambient temperature as well as the wire is now 15 degrees C higher than it was for the original calculation, and I would think that would cause a wire operating temperature greater than 60 degrees C. I agree that this test will make sure that the wire insulation is protected (THHN) but won't the termination temperature be breached by a wire that was once sized at 60 deg C with a 30 deg C ambient now running 15 degrees C higher? How can the temperature derating only be applied to the insulation, when the equipment will see overheating due to the conductor? (I realize that the 1.25 multiplier was put in the code because equipment was not designed for 100% and is a buffer, but the buffer is eaten away quickly by the elevated ambient)
I don't see the answer to his question. Let's get away from the smaller conductors with the rating exceptions and deal with a more defined scenario.
Let's take a 68 amp continuous load, THHN 90 deg C wire. Per 110.14.C.1.a.1 or 2, since this is below 100 amps (and we don't know what the termination temp rating is) we are required to use the 60 deg c column from table 310.16 for sizing the wire to protect the termination device. We have a conductor requirement of 1.25 x 68 = 85 amps. This requires a #3 wire size at 30 deg C ambient temperature from the 60 Deg C column of table 310.16.
Now if we have an elevated ambient of 45 deg C, then according to some sources such as http://www.ecmweb.com/ar/electric_sizing_continuously_loaded/ we should only use the actual THHN wire insulation (90 deg C column) to apply the temperature derate (.87 x 110 instead of .71 x 85). This is all good and well for testing the protection of the insulation, but won't that allow the conductor to break the 60 deg C termination barrier. #3 THW wire has the same amount of copper as #3 THHN, no more no less. The termination's ambient temperature as well as the wire is now 15 degrees C higher than it was for the original calculation, and I would think that would cause a wire operating temperature greater than 60 degrees C. I agree that this test will make sure that the wire insulation is protected (THHN) but won't the termination temperature be breached by a wire that was once sized at 60 deg C with a 30 deg C ambient now running 15 degrees C higher? How can the temperature derating only be applied to the insulation, when the equipment will see overheating due to the conductor? (I realize that the 1.25 multiplier was put in the code because equipment was not designed for 100% and is a buffer, but the buffer is eaten away quickly by the elevated ambient)
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: conductor ampacity ambient temp derating
My two cents;
Here is an example of derating due to the ambient temperature surrounding the conductors.
Load at 125% = 85A
Conductor size required for 60c rated terminations = 3AWG copper (any insulation type from table).
This will be the minimum size of copper conductor required for this load under most circumstances.
Now we add an adjustment because the conductors will be installed in an ambient temperature which is 15c higher than the table ratings of 30c.
Then we need to see if the same 3AWG conductor will be able to safely carry the maximum load of 85A. Because we know that type THHN insulation is one of the higher temperature rated types, we look at THHN?s rating for amperes which is 110A in an ambient of 30c. We then have to reduce it?s rating because it will be installed in an ambient 15c higher than the table.
We look at the table column for THHN and find at 45c the current carrying ability of the THHN insulated conductor is reduced by a factor of 0.87. We then take the 30c rating and perform the calculation: 110A x 0.87 = 95.7A. Since the original load of 85A is well below the THHN?s current carrying ability of 95.7A in an ambient of 45c, we may still install this same conductor.
Now to consider the terminations which are listed to handle temperatures up to 60c, we find that the 45c ambient is still below the terminations upper limit.
The only time the termination?s temperature limit will be a consideration is when the termination is in an ambient temperature higher than it?s rating.
The ambient temperature of the conductors does not change or detract from the termination?s rating, it simply lowers the conductors current carrying ability.
[ March 22, 2004, 06:57 PM: Message edited by: websparky ]
My two cents;
Here is an example of derating due to the ambient temperature surrounding the conductors.
Load at 125% = 85A
Conductor size required for 60c rated terminations = 3AWG copper (any insulation type from table).
This will be the minimum size of copper conductor required for this load under most circumstances.
Now we add an adjustment because the conductors will be installed in an ambient temperature which is 15c higher than the table ratings of 30c.
Then we need to see if the same 3AWG conductor will be able to safely carry the maximum load of 85A. Because we know that type THHN insulation is one of the higher temperature rated types, we look at THHN?s rating for amperes which is 110A in an ambient of 30c. We then have to reduce it?s rating because it will be installed in an ambient 15c higher than the table.
We look at the table column for THHN and find at 45c the current carrying ability of the THHN insulated conductor is reduced by a factor of 0.87. We then take the 30c rating and perform the calculation: 110A x 0.87 = 95.7A. Since the original load of 85A is well below the THHN?s current carrying ability of 95.7A in an ambient of 45c, we may still install this same conductor.
Now to consider the terminations which are listed to handle temperatures up to 60c, we find that the 45c ambient is still below the terminations upper limit.
The only time the termination?s temperature limit will be a consideration is when the termination is in an ambient temperature higher than it?s rating.
The ambient temperature of the conductors does not change or detract from the termination?s rating, it simply lowers the conductors current carrying ability.
[ March 22, 2004, 06:57 PM: Message edited by: websparky ]
Re: conductor ampacity ambient temp derating
We are still ignoring the conductor operating temperature.
Riddle me this: What is the operating temperature (of the copper) of an insulated #3 conductor with a 125% load of 85 amperes in a 30 deg C ambient? Isn't it approaching 60 degrees C?
If the conductor is approaching 60 deg C due to the internal resistance of the wire and the load, and then you raise the ambient 15 deg C, now isn't the conductor's operating temperature going to be higher than 60 deg C? And won't that conductor, being attached to a 60 deg C device, be overheated?
I suppose the "legal" allowance using a conductor that may actually be operating at a higher temperature than the termination rating is allowed in 110.14.C "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."
But I don't have to like it! I guess I could check the equation in 310.15.C.....if I could get source data for all items....but... by solving for TC you get
TC=I*I[RDC(1+YC)RCA]+TA+deltaTD.
I (current) stays constant,
RDC (dc resistance) will go up slightly with temperature, maybe 4 to 5%,
RCA (thermal resistance to ambient) will, at best, stay constant,
YC (skin effect) will basically stay constant,
We can assume Delta TD is nearly constant (capacitance)
Assuming all is constant except TA (ambient) then
TC = K + TA, where K is a constant
and we have a conductor temperature that changes linearly with ambient temperature changes degree for degree. So if we were anywhere near the 60 deg C wire operating temperature at 30 deg C ambient, we are now running near 75 deg C with the 45 deg C ambient. Not good since this will be heating the termination above 60 degrees C - REGARDLESS OF THE INSULATION TYPE.
I guess I need convincing that the wire operating temperature is not really approaching the insulation temperature under full load conditions.....
We are still ignoring the conductor operating temperature.
Riddle me this: What is the operating temperature (of the copper) of an insulated #3 conductor with a 125% load of 85 amperes in a 30 deg C ambient? Isn't it approaching 60 degrees C?
If the conductor is approaching 60 deg C due to the internal resistance of the wire and the load, and then you raise the ambient 15 deg C, now isn't the conductor's operating temperature going to be higher than 60 deg C? And won't that conductor, being attached to a 60 deg C device, be overheated?
I suppose the "legal" allowance using a conductor that may actually be operating at a higher temperature than the termination rating is allowed in 110.14.C "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."
But I don't have to like it! I guess I could check the equation in 310.15.C.....if I could get source data for all items....but... by solving for TC you get
TC=I*I[RDC(1+YC)RCA]+TA+deltaTD.
I (current) stays constant,
RDC (dc resistance) will go up slightly with temperature, maybe 4 to 5%,
RCA (thermal resistance to ambient) will, at best, stay constant,
YC (skin effect) will basically stay constant,
We can assume Delta TD is nearly constant (capacitance)
Assuming all is constant except TA (ambient) then
TC = K + TA, where K is a constant
and we have a conductor temperature that changes linearly with ambient temperature changes degree for degree. So if we were anywhere near the 60 deg C wire operating temperature at 30 deg C ambient, we are now running near 75 deg C with the 45 deg C ambient. Not good since this will be heating the termination above 60 degrees C - REGARDLESS OF THE INSULATION TYPE.
I guess I need convincing that the wire operating temperature is not really approaching the insulation temperature under full load conditions.....
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conductor ampacity ambient temp derating
Sorry kralcp, but I do not follow your point. Let me first assert that the temperature of the conductor (i.e., the copper) means absolutely nothing in its own right. The only reason we care about the copper getting hotter is that it will, in turn, cause the insulation to get hotter. If you performed a laboratory experiment in which you continued to raise current (i.e., well beyond the NEC limits) until ?something happens,? that something is going to be an insulation something long before the copper undergoes the slightest change.
If you are running the NEC limited current through a 60C rated conductor in a 30C ambient, what you will not get is an insulation temperature above 60C. That means that there will be a temperature rise within the insulation system of 30C, and that it will have been caused by a temperature rise of more than 30C within the copper. Heat flows from the hotter area (i.e., the copper) to the cooler area (i.e., the insulation system). So yes, the copper is above 60C. But we do not care.
If you were then to raise the ambient by, let us say, 15 degrees, then the copper temperature will go above 75C, and the insulation system should approach, but not exceed, 75C. That would be a bad thing, since the insulation system is only rated for 75C. Therefore, we are required to reduce the current in the conductor, using the temperature correction factors associated with Table 310.16. This causes the temperature rise within the copper to be less, and results in a temperature of the insulation system that does not exceed its 60C rating.
Does this perspective help in any way, or do you still have a need for convincing?
Sorry kralcp, but I do not follow your point. Let me first assert that the temperature of the conductor (i.e., the copper) means absolutely nothing in its own right. The only reason we care about the copper getting hotter is that it will, in turn, cause the insulation to get hotter. If you performed a laboratory experiment in which you continued to raise current (i.e., well beyond the NEC limits) until ?something happens,? that something is going to be an insulation something long before the copper undergoes the slightest change.
If you are running the NEC limited current through a 60C rated conductor in a 30C ambient, what you will not get is an insulation temperature above 60C. That means that there will be a temperature rise within the insulation system of 30C, and that it will have been caused by a temperature rise of more than 30C within the copper. Heat flows from the hotter area (i.e., the copper) to the cooler area (i.e., the insulation system). So yes, the copper is above 60C. But we do not care.
If you were then to raise the ambient by, let us say, 15 degrees, then the copper temperature will go above 75C, and the insulation system should approach, but not exceed, 75C. That would be a bad thing, since the insulation system is only rated for 75C. Therefore, we are required to reduce the current in the conductor, using the temperature correction factors associated with Table 310.16. This causes the temperature rise within the copper to be less, and results in a temperature of the insulation system that does not exceed its 60C rating.
Does this perspective help in any way, or do you still have a need for convincing?
Re: conductor ampacity ambient temp derating
I appreciate the responses. It appears that my interpretation of the code concerning ambient temp rating was correct. However, just to make sure, let me rephrase my interpretation and see if anyone agrees. (1)It appears that the code requires ambient temp derating of the conductor to protect the conductor insulation. (2)The only requirement for the protection of the terminations is to limit conductor ampacity to the termination temp rating ampacity regardless of the ambient temp.
I appreciate the responses. It appears that my interpretation of the code concerning ambient temp rating was correct. However, just to make sure, let me rephrase my interpretation and see if anyone agrees. (1)It appears that the code requires ambient temp derating of the conductor to protect the conductor insulation. (2)The only requirement for the protection of the terminations is to limit conductor ampacity to the termination temp rating ampacity regardless of the ambient temp.
Re: conductor ampacity ambient temp derating
I have no problems with the insulation system or current rating of the conductors. My concern lies solely with protection of the terminating device from being overheated beyond its rating.
110.14(C)Temperature Limitations.
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....
It goes on to say that for circuits 100 amps or less you must assume a 60 deg C device termination rating if you don't know what it really is. 110.14(C)(1)(a)(1)
So now we have a hot conductor running near 60 deg C, ambient at 30 deg C. I'm happy because the termination is rated 60 for deg C operation.
But now we elevate the ambient to 45 degrees C. Even though my wire with 75deg C or 90 Degree C insulation is ok, my termination is now being overheated above its rating, because the conductor temp has gone over 60 deg C.
Its like having an iron rod in a fire, and the end you grab is at 115 degrees F, its on a nice day, say 75 deg F. (I don't care what the insulation is because my hand, the terminating device, is not touching the insulation, just the rod). Now all of the sudden it gets hot outside, say 100 Degrees F. Now the rod cannot dissipate the heat to the ambient as fast, and the temperature of the rod at the connection rises above 120 degrees (let's say that is where blistering starts), and the hand gets burned. Failure of the terminating device. The hand was rated for 120 degrees, but it failed due to an increased ambient. Sure the insulation that was on the rod is still ok because it had a higher rating, the the hand still failed.
My concern is how can we ignore the termination temperature over-heating....and just be concerned if the insulation is overheated.
It appears to me, even though allowed by the code, that we should be using the 60 degree C column of 310.16 to take the temp derate from, and not the actual conductor we use because of the termination heating effect.
I guess I may be able to answer my own question. Is that why the overcurrent device must be rated for the 125% of the continuous load
plus the non-continuous, to be a buffer for devices not being able to run at more than 80% continuously, which is a purely heat transfer requirement of the equipment? If so this is really confusing using one column for sizing overcurrent device settings and another for wire sizing based on temperature and fill derating.
I have no problems with the insulation system or current rating of the conductors. My concern lies solely with protection of the terminating device from being overheated beyond its rating.
110.14(C)Temperature Limitations.
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....
It goes on to say that for circuits 100 amps or less you must assume a 60 deg C device termination rating if you don't know what it really is. 110.14(C)(1)(a)(1)
So now we have a hot conductor running near 60 deg C, ambient at 30 deg C. I'm happy because the termination is rated 60 for deg C operation.
But now we elevate the ambient to 45 degrees C. Even though my wire with 75deg C or 90 Degree C insulation is ok, my termination is now being overheated above its rating, because the conductor temp has gone over 60 deg C.
Its like having an iron rod in a fire, and the end you grab is at 115 degrees F, its on a nice day, say 75 deg F. (I don't care what the insulation is because my hand, the terminating device, is not touching the insulation, just the rod). Now all of the sudden it gets hot outside, say 100 Degrees F. Now the rod cannot dissipate the heat to the ambient as fast, and the temperature of the rod at the connection rises above 120 degrees (let's say that is where blistering starts), and the hand gets burned. Failure of the terminating device. The hand was rated for 120 degrees, but it failed due to an increased ambient. Sure the insulation that was on the rod is still ok because it had a higher rating, the the hand still failed.
My concern is how can we ignore the termination temperature over-heating....and just be concerned if the insulation is overheated.
It appears to me, even though allowed by the code, that we should be using the 60 degree C column of 310.16 to take the temp derate from, and not the actual conductor we use because of the termination heating effect.
I guess I may be able to answer my own question. Is that why the overcurrent device must be rated for the 125% of the continuous load
plus the non-continuous, to be a buffer for devices not being able to run at more than 80% continuously, which is a purely heat transfer requirement of the equipment? If so this is really confusing using one column for sizing overcurrent device settings and another for wire sizing based on temperature and fill derating.
Re: conductor ampacity ambient temp derating
The NEC tables and NEMA standards are conservative and have taken all your worries into consideration.
How would you use the Neher McGrath formulas except to use the ambient averages?
There is no way the conductors or terminations could be calculated to give us an exact control point due to the ambient extremes in 90 percent of the worlds attics for example.
Roger
[ March 23, 2004, 03:50 PM: Message edited by: roger ]
The NEC tables and NEMA standards are conservative and have taken all your worries into consideration.
How would you use the Neher McGrath formulas except to use the ambient averages?
There is no way the conductors or terminations could be calculated to give us an exact control point due to the ambient extremes in 90 percent of the worlds attics for example.
Roger
[ March 23, 2004, 03:50 PM: Message edited by: roger ]
- Status