Table 310.15 B11 VS 310.15B12 ambient temp correction

[Jpflex]

As currently worded in NEC, various ambient temperatures may quailfy to be used in either table because ambient temperatures may be simultaniously other than 30 degrees celcius table 310.15B11 and or other than 40 degrees celcius table 310.15 B12

Therefore, the NEC is not clear on which to use.

For example:

A mid point ambient temperature between 30 and 40 degrees celcius could be 35 degrees celcius

An ambient temperature of 35 degrees celcius is both "other than 30 degrees celcius" table 310.15B11 and is also "other than 40 degrees celcius"

Therefore, which table does NEC expect the electrician to use for ambient temperature correction factors and wire ampacity determination based on their wording?

I do understand that the second table includes higher ambient temperatures but this still does answer which table to use according to the info within table titles



following is an example by the 2023 NEC handbook. Notice that the answer is based on a 90 degree THHN copper conductor insulation which we know cannot terminate on most utilization equipment typically rated for 75 degrees maximum


2) NEC hand book example
Determine the ampacity of 2 AWG THHN copper conductors to be installed in a raceway in an ambient temperature of 50 degrees celcius

NEC Answer

Table 310.16

Start at 130 ampere THHN at 90 degree insulation

Next NEC chose table 310.15B111, instead of table 310.15B12?

130i x 0.82 (derating factor based on 90 degree insulation) = 106.6 amperes

However if not simply terminating to a connector, bus or power block rated for 90 degrees celcius, should the ampacity of the wire be reduced to its maximum current value of 115 amperes (table 310.16) for a 75 degree utilization device termination point?

Also if the second table 310.15B12 were used instead we would have had an original wire ampacity of 130i x 0.89 = 115.7 amperes but NOT the 106.6 amperes shown by the NEC hand book as an answer.

Thanks

 

[david luchini]

As currently worded in NEC, various ambient temperatures may quailfy to be used in either table because ambient temperatures may be simultaniously other than 30 degrees celcius table 310.15B11 and or other than 40 degrees celcius table 310.15 B12

Therefore, the NEC is not clear on which to use.

The ampacity tables tell you which to use. If you have a conductor from an ampacity table that tells you the ampacity is for conductors installed in a 30C ambient, you would "other than 30C correction factor. Similar for conductor ampacities from ampacity table for conductors installed in a 40C ambient.

For example...Table 310.16 Note 1 mentions "where the ambient temperature is other than 30C".

Table 310.18 Note 1 mentions "where the ambient temperature is other than 40C."

 

[Jpflex]

The ampacity tables tell you which to use.

For example...Table 310.16 Note 1 mentions "where the ambient temperature is other than 30C".

Table 310.18 Note 1 mentions "where the ambient temperature is other than 40C."

Like i was explaining in this post, the ampacity tables do not tell you which table to use because an ambient temperature can simultaneously be both "other than 30 decrees celcius and 40 degrees celcius"

An ambient temperature range plus or minus 30 and 40 degrees fits this example

 

[wwhitney]

Therefore, which table does NEC expect the electrician to use for ambient temperature correction factors and wire ampacity determination based on their wording?

The table to use for temperature correction factors needs to match the ambient temperature assumed in the ampacity table from which the base ampacity was chosen.

If you used an ampacity table that specifies it is based on 30C ambient, use the correction table for 30C ambient. If used an ampacity table that specifies it is based on 40C ambient, use the correction table for 40C ambient.

Or just use the formula:

temperature correction factor
= sqrt (actual allowable temperature rise / temperature rise the table was based on)
= sqrt ((insulation temperature - actual ambient) / (insulation temperature - assumed ambient listed in the ampacity table))

Cheers, Wayne

 

[david luchini]

Like i was explaining in this post, the ampacity tables do not tell you which table to use because an ambient temperature can simultaneously be both "other than 30 decrees celcius and 40 degrees celcius"

An ambient temperature range plus or minus 30 and 40 degrees fits this example

As I explained in my post, the ampacity tables tells you which correction factor to use. An ampacity from Table 310.16 would use the "other than 30C" correction factor, because that ampacity table tells you to use the "other than 30C" correction factor.

 

[Jpflex]

The table to use for temperature correction factors needs to match the ambient temperature assumed in the ampacity table from which the base ampacity was chosen.

If you used an ampacity table that specifies it is based on 30C ambient, use the correction table for 30C ambient. If used an ampacity table that specifies it is based on 40C ambient, use the correction table for 40C ambient.

for example is the "yearly highest average?" ambient temperature for a region as explained by mike holt using copper.net to find a regions ambient temperature, and the regions ambient temperature is found at 35 degrees celcius, this may fall into either table because it is not 30 degrees celcius nor 40 degrees celcius

 

[david luchini]

for example is the "yearly highest average?" ambient temperature for a region as explained by mike holt using copper.net to find a regions ambient temperature, and the regions ambient temperature is found at 35 degrees celcius, this may fall into either table because it is not 30 degrees celcius nor 40 degrees celcius

The yearly highest average ambient temperature has nothing to do with the tables.

The ambient that the conductors will be installed in is what matters to the table.

 

[wwhitney]

for example is the "yearly highest average?" ambient temperature for a region as explained by mike holt using copper.net to find a regions ambient temperature, and the regions ambient temperature is found at 35 degrees celcius, this may fall into either table because it is not 30 degrees celcius nor 40 degrees celcius

No.

Do you have "Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried)"? Then your ampacity value is taken from Table 310.16, and that table says it is based on 30C ambient. So to that ampacity value you would only apply correction factors taken from Table 310.15(B)(1)(1) which is for deviation from assumed 30C ambient. If you only ever look use Table 310.16 for ampacities, you can ignore Table 310.15(B)(1)(2) and stick with Table 310.15(B)(1)(1).

Or do you have "Conductors Supported on a Messenger"? Then your ampacity value is taken from Table 310.20, and that table says it is based on 40C ambient. So for that ampacity value you would only apply correction factors taken from Table 310.15(B)(1)(2), which is for deviation from assumed 40C ambient.

Cheers, Wayne

 

[Jpflex]

An ambient temperature of 35 degrees celciu

As I explained in my post, the ampacity tables tells you which correction factor to use. An ampacity from Table 310.16 would use the "other than 30C" correction factor, because that ampacity table tells you to use the "other than 30C" correction

As I explained in my post, the ampacity tables tells you which correction factor to use. An ampacity from Table 310.16 would use the "other than 30C" correction factor, because that ampacity table tells you to use the "other than 30C" correction factor.

The information within the two ambient temperature correction tables are contradicting for ambient temperatures other than 30 degrees celcius and 40 degrees celcius because any ambient temperature other than 30 or 40 devrees celcius may qualify and be subject to either table because such an ambient temperature is simultaneously "other than" 30 and 40 degreec celcius.

 

[wwhitney]

The information within the two ambient temperature correction tables are contradicting for ambient temperatures other than 30 degrees celcius

No. The title of Table 310.15(B)(1)(1) is "Ambient Temperature Correction Factors Based on 30°C". If you are using an ampacity based on 30C ambient, you can use that table. If you are using an ampacity based on some other ambient temperature, you can not.

Cheers, Wayne

 

[david luchini]

The information within the two ambient temperature correction tables are contradicting for ambient temperatures other than 30 degrees celcius and 40 degrees celcius because any ambient temperature other than 30 or 40 devrees celcius may qualify and be subject to either table because such an ambient temperature is simultaneously "other than" 30 and 40 degreec celcius.

The information is not contradictory. In your original post you selected an ampacity from table 310.16 for #2 THHN

If you install that #2 THHN in an ambient temperature of other than 30C, 310.16 tells you to use the correction factors for "Other than 30C".

The correction factor table gives you correction factors for a whole range of temperatures that are "Other than 30C".

For example, the correction factor for 35C ambient for your ampacity selected from Table 310.16 is 0.96.

The correction factor for 40C ambient for your ampacity selected from Table 310.16 is 0.91.

The correction factor for 75C ambient for your ampacity selected from Table 310.16 is 0.50, etc.

 

[Jpflex]

No.

Do you have "Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried)"? Then your ampacity value is taken from Table 310.16, and that table says it is based on 30C ambient. So to that ampacity value you would only apply correction factors taken from Table 310.15(B)(1)(1) which is for deviation from assumed 30C ambient. If you only ever look use Table 310.16 for ampacities, you can ignore Table 310.15(B)(1)(2) and stick with Table 310.15(B)(1)(1).

Or do you have "Conductors Supported on a Messenger"? Then your ampacity value is taken from Table 310.20, and that table says it is based on 40C ambient. So for that ampacity value you would only apply correction factors taken from Table 310.15(B)(1)(2), which is for deviation from assumed 40C ambient.

Cheers, Wayne

The foot note 1 for table 310.16 referrs you to 310.15B.

310.15B says

Ampacities for ambient temperatures OTHER than those shown in the ampacity tables (30 deg celcius in our case) shall be corrected innaccordance with table 310.15B11 OR table 310.15B12...


Therefore we are told fo use either table but doind so yields different answers

 

[wwhitney]

The foot note 1 for table 310.16 referrs you to 310.15B.

310.15B says

Ampacities for ambient temperatures OTHER than those shown in the ampacity tables (30 deg celcius in our case) shall be corrected innaccordance with table 310.15B11 OR table 310.15B12...


Therefore we are told fo use either table

Sure. So we look at the table titles, and use the table whose title matches the 30C ambient that Table 310.16 is based on. We must use table 310.15(B)(1)(1) and may not use 310.15(B)(1)(2).

Cheers, Wayne

 

[Jpflex]

If you install that #2 THHN in an ambient temperature of other than 30C, 310.16 tells you to use the correction factors for "Other than 30C".

Here is where the problem is:

Footnote 1 from ampacity table 310.16 says to referr to 310.15B for ambient temperature other than 30 degrees celcius


310.15B says

Ampacities for ambient temperature other than those shown in the ampacity tables (30 degrees celcous in our case table 310.16) SHALL BE corrected in accordance with table 310.15 B111 OR!!! 310.15B12....


Now with this shown, a regions ambient temperature can be simultaneously other than 30 and 40 degrees celcius and therefore subject a wire to both tables. The resulting ampacity calculation will vary accordingly

 

[david luchini]

The foot note 1 for table 310.16 referrs you to 310.15B.

The foot note 1 for Table 310.16 says...Section 310.15(B) shall be referenced for ampacity correction factors WHERE THE AMBIENT TEMPERATURE IS OTHER THAN 30C.

Table 310.15(B)(1) says "For ambient temperatures other than 30C..."

Table 310.15(B)(2) does NOT say "For ambient temperatures other than 30C..."

Only one of the Tables 310.15(B)(1) and 310.15(B)(2) applies "For ambient temperatures other than 30C." (Hint: it's (B)(1), not (B)(2)).

 

[Jpflex]

Table 310.15(B)(2) does NOT say "For ambient temperatures other than 30C..."

Only one of the Tables 310.15(B)(1) and 310.15(B)(2) applies "For ambient temperatures other than 30C." (Hint: it's (B)(1), not (B)(2))

Table 310.15B2 does not say for ambient temperatures other than 30 degrees celcius. CORRECT

However, it says for ambient temperatures other than 40 degrees celcius

Therefore is an ambient temperature of 35 degrees cecius not simultaneously other than both 30 degrees celcius and other than 40 degrees celcius?

Therefore table 310.15B11 can accept ambient temperatures less than or greater than 30 degrees celcius

Table 310.15B12 can accept ambient temperatures less than or greater than 40 degrees celcius

There is overlapping of ammbient temperature ranges that will fit into the criteria of either table

 

[david luchini]

Table 310.15B2 does not say for ambient temperatures other than 30 degrees celcius. CORRECT

However, it says for ambient temperatures other than 40 degrees celcius

Therefore is an ambient temperature of 35 degrees cecius not simultaneously other than both 30 degrees celcius and other than 40 degrees celcius?

Therefore table 310.15B11 can accept ambient temperatures less than or greater than 30 degrees celcius

Table 310.15B12 can accept ambient temperatures less than or greater than 40 degrees celcius

There is overlapping of ammbient temperature ranges that will fit into the criteria of either table

40C does not equal 30C. That seems pretty obvious.

I'm wondering at this point if you are trolling. That's considered impolite.

 

[wwhitney]

Table 310.15B2 does not say for ambient temperatures other than 30 degrees celcius. CORRECT

However, it says for ambient temperatures other than 40 degrees celcius

The 2023 NEC eliminated the small ambiguity that you have identified. Table 310.15(B)(1)(1) is followed by a Note that says "Note: Table 310.15(B)(1)(1) shall be used with Table 310.16 and Table 310.17 as required." Likewise Table 310.15(B)(1)(2) says "Note: Table 310.15(B)(1)(2) shall be used with Table 310.18, Table 310.19, Table 310.20, and Table 310.21."

Still, even without the notes, with some thought it's clear enough that you use the "other than 30C" table with ampacity tables based on 30C ambient, and the "other than 40C" table with ampacity tables based on 40C ambient.

So if your original complaint is an honest "I don't know which table to use," hopefully this threaded has settled that. If it's "the NEC should be more explicit in instructing the user which table to use," well the 2023 NEC has taken care of it.

Either way, your objections have been more persistent than is reasonable.

Cheers, Wayne

 

[Jpflex]

Table 310.15B2 does not say for ambient temperatures other than 30 degrees celcius. CORRECT

However, it says for ambient temperatures other than 40 degrees celcius

Therefore is an ambient temperature of 35 degrees cecius not simultaneously other than both 30 degrees celcius and other than 40 degrees celcius?

Therefore table 310.15B11 can accept ambient temperatures less than or greater than 30 degrees celcius

Table 310.15B12 can accept ambient temperatures less than or greater than 40 degrees celcius

There is overlapping of ammbient temperature ranges that will fit into the criteria of either table

No i am not trying to nit pick, but this is trully not making sense to me because

Table 310.15B11 can accept ambient temperatures for example from 28, 29, 31, 32...

Table 310.15B12 can accept ambient temperatures for example 28, 29, 30, 31, 32,33,34,35,36,37,38,39,41,42....

As can be seen there are ambient temperatures which may correspond to both tables.


In other words ANY ambient temperature is acceptable other than 30 degrees celcius for table 310.15B11

And ANY ambient temperature is acceptable other than 40 degrees celcius for table 310 15B12

I Read some commentary that table 310.15B12 is only for nessenger wires but i cant find any such distiction between the two tables in NEC

 

[Jpflex]

The post above was responding to your last post