Table 310.15 B11 VS 310.15B12 ambient temp correction

Jpflex said:
Wwhitney the last post was respobse to you. My phone is acting up
Click to expand...
Hopefully the 2023 NEC settles your concerns. The notes to each of the two tables correction factor tables specify which ampacity tables they may be used with. And we've explained multiple times the logic involved.

Cheers, Wayne
 
Jpflex said:
As can be seen there are ambient temperatures which may correspond to both tables.
Click to expand...
Of course there are ambient temperatures that correspond to both tables. 50 deg C is both Other than 30degC and other than 40deg C.

Let's try this. According to 310.16, what ambient temperature must three #1/0 THWN copper conductors in a raceway be installed in, in order for the ampacity of those conductors to be 150?
 
wwhitney said:
Hopefully the 2023 NEC settles your concerns. The notes to each of the two tables correction factor tables specify which ampacity tables they may be used with. And we've explained multiple times the logic involved.

Cheers, Wayne
Click to expand...
Yes, you and David certainly have and if the OP can not understand it's pretty much futile. IMO this appears to be trolling.
 
david luchini said:
Of course there are ambient temperatures that correspond to both tables. 50 deg C is both Other than 30degC and other than 40deg C.

Let's try this. According to 310.16, what ambient temperature must three #1/0 THWN copper conductors in a raceway be installed in, in order for the ampacity of those conductors to be 150?
Click to expand...
I Will answer your question.

Based on this table 310.16 and what im sure you meant was the individual ampacity of 1/0 THWN with 75 deg c insulation but not the total ampacity of three paralleled 1/0 conductors in a raceway.

I understabd the ambient temp would need to be 86 degrees c. However that is not my point. What im saying is that the same ambient temperature other than 30 and 40 deg c can apply to both tables with varied results

And for those who think im trolling i just wont use table 310.15B12
 
roger said:
Yes, you and David certainly have and if the OP can not understand it's pretty much futile. IMO this appears to be trolling.
Click to expand...
No, I don't believe he is.
Just frustration on this topic.

We have all been there on something over the years.

Jpflex
I dug the following out of IEEE Buff Book...9.5.1.3 TDF (Temperature Correction Factor.
Tn - Normal Loading Temp
Ta - Base Ambient Temp
Ta1 - New Ambient Temp
In - Normal loading current at base ambient
Ix - Current value at new ambient temp

TDF (multiplying factor) = SQRT { (Tn - Ta) / (Tn - Ta1) }

Ix = In * TDF

Play with some different cable sizes and temps and you should be able to solve for ANY ambient you encounter.
 
MyCleveland said:
No, I don't believe he is.
Just frustration on this topic.

We have all been there on something over the years.

Jpflex
I dug the following out of IEEE Buff Book...9.5.1.3 TDF (Temperature Correction Factor.
Tn - Normal Loading Temp
Ta - Base Ambient Temp
Ta1 - New Ambient Temp
In - Normal loading current at base ambient
Ix - Current value at new ambient temp

TDF (multiplying factor) = SQRT { (Tn - Ta) / (Tn - Ta1) }

Ix = In * TDF

Play with some different cable sizes and temps and you should be able to solve for ANY ambient you encounter.
Click to expand...
That is brilliant

Can we solve for final temperature, with a given load?
 
ramsy said:
How would you solve that for final temperature, with a given load?
Click to expand...
You're never going to get actual temperatures starting with NEC ampacity tables--the ampacity values are chosen to ensure that if all the unspecified factors are the worst plausible for heat retention, the temperature rise will not exceed the allowable rise (difference between insulation temperature limit and ambient temperature) at a current equal to the ampacity.

But the general idea is that that temperature rise is proportional to the square of the current. So if you measured some resistive component that was carrying, say, 15A, and it had a temperature rise of 9 degrees F, you can expect that if you push 30A through it, it will have a temperature rise of about 36F. [About because the resistance may rise with the higher temperature, so you might end up a bit higher than 36F temperature rise.] I.e. twice the current, four times the temperature rise; 3 times the current, 9 times the temperature rise; etc.

Cheers, Wayne
 
wwhitney said:
You're never going to get actual temperatures
Click to expand...
We solve Ohm’s law for each variable, why not solve equation 310.15(B) for a different variable?

Lets review 310.15(B):

I'-I*Sqrt(Tc-T'a/Tc-Ta)

I′ = ampacity corrected for ambient temperature
I = ampacity shown in the tables
Tc = temperature rating of conductor (°C)
Ta′ = new ambient temperature (°C)
Ta = ambient temperature used in the table (°C)

Proposed solving for T2 (Final Conductor Temperature) at given load

T2 = (Ambient + Temp.Rise)
Load = Given Circuit Amps

T2 = T1 + (TR-T1) * Load^2 / Imax^2

T1 = Ambient Temperature
TR = Conductor Max Temperature Rating
Imax = Max Amps at TR, derated for ccc's
 
ramsy said:
We solve Ohm’s law for each variable, why not solve equation 310.15(B) for a different variable?
Click to expand...
Sure you can do that, but the temperatures you solve for won't match what you'd get if you set up an experiment and measured the actual conductor temperature. The calculated temperature should be noticeably higher.

If you use a conductor at its 60C ampacity (based on 30C ambient) in 30C ambient and otherwise normal conditions, the temperature had best not hit 60C. It's supposed to be guaranteed to be not be more than 60C regardless of what other unregulated conditions are present (e.g. embedded in 6" of polyiso), so in otherwise normal conditions, the temperature better be quite a bit lower.

Cheers, Wayne
 
ramsy said:
That is brilliant

Can we solve for final temperature, with a given load?
Click to expand...
Thank IEEE…not me.

Yes, you can solve for conductor temp. The next section in the book goes over this.
I use it in a VD app where if your load amps are less than 80% of your conductor amps it returns VD based on recalculated R value, as compared to NEC chap 9 values, and tells you the new conductor operating temp.
Not that anyone cares.
 
ramsy said:
Was hoping you might say that. Was I getting close in post #31 above?
Click to expand...
Cannot say one way or the other.
No history of working with these formulas and I literally go cross eyed looking over others math.

Why does the adjusted temp interest you?
Only any Amp or R value has any value, at least in my mind.
 
MyCleveland said:
Why does the adjusted temp interest you?
Click to expand...
IMHO, less thermal imaging reports would follow overheated wire if more electricians paid attention to 220.18 for inductive ballast & motor loads, where current rises with voltage drop.

The equation wants Amps, reminding electricians Watts is useless without knowing Power Factor.
 
wwhitney said:
Doing that requires at least one real world temperature measurement to use as a starting point.
Click to expand...
Only needs to avoid thermal imaging errors in violation of 110.14(C).

In post #33 above, MyCleveland may select variables for worst case ambient, loaded to 80% ampacity.
wwhitney said:
You can not do that by calculation only just using the NEC.
Click to expand...
Professional engineers may not want the NEC to publish such an equation, if design-build shops can use it without engineering supervision.
 
Jpflex said:
I Will answer your question.

Based on this table 310.16 and what im sure you meant was the individual ampacity of 1/0 THWN with 75 deg c insulation but not the total ampacity of three paralleled 1/0 conductors in a raceway.

I understabd the ambient temp would need to be 86 degrees c. However that is not my point. What im saying is that the same ambient temperature other than 30 and 40 deg c can apply to both tables with varied results

And for those who think im trolling i just wont use table 310.15B12
Click to expand...
30degC.

The answer is 30degC.

310.16 literally tells you that the conductors are installed in a 30degC ambient temperature.
 
david luchini said:
30degC.

The answer is 30degC.

310.16 literally tells you that the conductors are installed in a 30degC ambient temperature.
Click to expand...
That is what i said 86 deg ferenheight* or 30 deg c. However thus was not my point. My point was for ambient temperatures other than 30 and 40 which may fall into both tables. No one is getting this?
 
You must log in or register to reply here.
Top