I post this because it was a subject of great debate and confusion in a NABCEP class I took recently.
Below is my understanding of how the ampacity tables should really be used.
We all know that THHN and THWN-2, commonly used for PV wiring, can be allowed to run up to 90degC. Their insulation is chosen so that it doesn't melt or degrade at that temperature. If we want to know how much current it takes for the wire to reach its temperature limit, this is what table 310.15(B)(16) tells us. For example, look up 10AWG, and you will see that it takes 40 amps to raise it to 90C, starting at 30C ambient , which is what the whole table is based on.
Point 1: Starting at 30C ambient, Any 10AWG wire will get up to 90C with this same 40A current;
but you can't let that happen to THW for example, or it will melt.
THW has to be kept below 75degC. The current required to raise 10AWG to 75degC is, from the table, 35 amps, so that is the limit for THW
Point 2: Any 10AWG wire will rise from a 30degC ambient to 75degC with 35 amps going through it. THHN too, which is very happy at 75C
where it well within its design limit.
Is there any time we would not want to let THHN rise as far as its insulation limit of 90C?
Yes - when it is attached to a terminal that can only stand 75degC.
Article 110.14(C) says that the wire may not exceed the terminal temperature. So even if it's THHN, if it is attached to a 75degC terminal,
you cannot let it rise above 75C. To find the current that would raise the temperature to 75degC, you look in the 75degC column of
table 310.15(B)(16); this says 35 amps for 10AWG , if we are starting with a 30degC ambient.
Now, if the ambient starts out higher than 30C, it will take less current to raise the temperature to 75C.
In fact, if we start with an ambient of 74degC, we are not going to be able to put squat through the wire without raising the temperature above the terminal temperature and violating 110.14(C).
So we know that the derating for higher ambients, which is the purpose of table 310.15(B)(2)(A) must take the allowed current down to zero as the ambient approaches the allowed temperature. The allowed temperature is the LOWER of the wire insulation temperature rating and the terminal temperature rating. If the wire alone could be allowed to reach 90 degrees, but the allowed terminal temperature is 75 degrees, then the allowed temperature is 75 degrees ( 110.14(C) )
Looking in the 75degC column of table 310.15(B)(2)(A) we see that the current is indeed derated to zero when ambient is in the range 71-75degC. Therefore this is the column we have to use to keep the wire temeprature below 75.
There have been suggestions that you can take the ampacity from the 75 degC column of table 310.15(B)(16) and then take the derating for ambient from the 90degC column of table 310.15(B(2)(A), but this is quite wrong. You can see this if you look at the derating factor in the 90 degree column for an ambient of 75. The derating factor is 0.5 there, which means that if you used this column, it is saying you can STILL put 50% of the 310.15(B)(16) current through the wire, even when the ambient is already 75! Clearly this is wrong, because the wire would rise way over 75degC and thus violate 110.14(C).
So, it is important to use the same temperature column in table 310.15(B)(2)(A) as used in 310.15(B)(16) during the same ampacity
calculation.
I can see where the confusion arises, as the columns are headed "Tempeature rating of the conductor" which is 90C for THHN, right?
Well, only if not limited by something else, i.e. the terminals.
It may help to the think of "temperature rating of the conductor" as meaning "How hot can we let it get?" , not "what is the wire's design limit"
So the temperature rating of THHN, at the point it connects to a 75 degC terminal, is only 75 degC, not 90 degC.
Of course away from the terminal, a foot or so down the conduit, 90 degrees is fine for THHN. So when doing the ampacity calculations for wire remote from the terminals by even just a few inches, it is OK to use the wire temperature rating un-limited by the terminal temperature.
Below is my understanding of how the ampacity tables should really be used.
We all know that THHN and THWN-2, commonly used for PV wiring, can be allowed to run up to 90degC. Their insulation is chosen so that it doesn't melt or degrade at that temperature. If we want to know how much current it takes for the wire to reach its temperature limit, this is what table 310.15(B)(16) tells us. For example, look up 10AWG, and you will see that it takes 40 amps to raise it to 90C, starting at 30C ambient , which is what the whole table is based on.
Point 1: Starting at 30C ambient, Any 10AWG wire will get up to 90C with this same 40A current;
but you can't let that happen to THW for example, or it will melt.
THW has to be kept below 75degC. The current required to raise 10AWG to 75degC is, from the table, 35 amps, so that is the limit for THW
Point 2: Any 10AWG wire will rise from a 30degC ambient to 75degC with 35 amps going through it. THHN too, which is very happy at 75C
where it well within its design limit.
Is there any time we would not want to let THHN rise as far as its insulation limit of 90C?
Yes - when it is attached to a terminal that can only stand 75degC.
Article 110.14(C) says that the wire may not exceed the terminal temperature. So even if it's THHN, if it is attached to a 75degC terminal,
you cannot let it rise above 75C. To find the current that would raise the temperature to 75degC, you look in the 75degC column of
table 310.15(B)(16); this says 35 amps for 10AWG , if we are starting with a 30degC ambient.
Now, if the ambient starts out higher than 30C, it will take less current to raise the temperature to 75C.
In fact, if we start with an ambient of 74degC, we are not going to be able to put squat through the wire without raising the temperature above the terminal temperature and violating 110.14(C).
So we know that the derating for higher ambients, which is the purpose of table 310.15(B)(2)(A) must take the allowed current down to zero as the ambient approaches the allowed temperature. The allowed temperature is the LOWER of the wire insulation temperature rating and the terminal temperature rating. If the wire alone could be allowed to reach 90 degrees, but the allowed terminal temperature is 75 degrees, then the allowed temperature is 75 degrees ( 110.14(C) )
Looking in the 75degC column of table 310.15(B)(2)(A) we see that the current is indeed derated to zero when ambient is in the range 71-75degC. Therefore this is the column we have to use to keep the wire temeprature below 75.
There have been suggestions that you can take the ampacity from the 75 degC column of table 310.15(B)(16) and then take the derating for ambient from the 90degC column of table 310.15(B(2)(A), but this is quite wrong. You can see this if you look at the derating factor in the 90 degree column for an ambient of 75. The derating factor is 0.5 there, which means that if you used this column, it is saying you can STILL put 50% of the 310.15(B)(16) current through the wire, even when the ambient is already 75! Clearly this is wrong, because the wire would rise way over 75degC and thus violate 110.14(C).
So, it is important to use the same temperature column in table 310.15(B)(2)(A) as used in 310.15(B)(16) during the same ampacity
calculation.
I can see where the confusion arises, as the columns are headed "Tempeature rating of the conductor" which is 90C for THHN, right?
Well, only if not limited by something else, i.e. the terminals.
It may help to the think of "temperature rating of the conductor" as meaning "How hot can we let it get?" , not "what is the wire's design limit"
So the temperature rating of THHN, at the point it connects to a 75 degC terminal, is only 75 degC, not 90 degC.
Of course away from the terminal, a foot or so down the conduit, 90 degrees is fine for THHN. So when doing the ampacity calculations for wire remote from the terminals by even just a few inches, it is OK to use the wire temperature rating un-limited by the terminal temperature.
