Derating confusion

[Nathanael47]

I keep on running into teachers and contractors who do correction and adjustment differently. I've bumped into people who say you make temperature corrections from the 90 degree column but adjustments from the 60 degree column (that seems pretty clearly wrong). Others say that you ALWAYS derate from the 90 degree column if your conductor insulation allows it. And finally, I've bumped into people who say that you derate based on the temperature rating of your terminals at either end if they are lower than the conductor insulation rating.

Who is right here?

The sentence that explains this in the code is incredibly confusing and I think it's the thing that has led to all of this disagreement. It's the second paragraph of 310.15(A)

"The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with 110.14(C)."

I've read that sentence probably 40 times and changed my mind probably 10 times about what it means. But the last 20 or so times I've read it I've become more and more convinced about its meaning: 110.14(C) does NOT change the temperature rating of the conductor, nor does it change the correction and adjustment factors. What does 110.14(C) do? Provide the upper bound for the corrected and adjusted ampacity.

But what happens if the corrected and adjusted ampacity DOES exceed the ampacity for the temperature rating of the termination? What do you apply correction and adjustment factors to then?

Is the implication of 310.15(A) that then instead of applying correction and adjustment factors to the temperature rating of the conductor must be applied to the temperature ratings of the TERMINALS? If that's true then there's an implied sentence -- it would be something like:

"The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with 110.14(C)." If the corrected and adjusted ampacity does exceed the ampacity for the temperature rating of the termination, then you select ampacity based on the temp rating of the terminal by using 310.16 and that's it - you don't do further temperature correction.

Maaaybe. But I don't see where in the code it tells you to pretend like 310.16 turns into "temperature rating of conductor... or termination"

How does this actually work?

 

[tom baker]

Mike Holt has a number of videos on this subject. Search his site or You Tube

 

[don_resqcapt19]

....
But what happens if the corrected and adjusted ampacity DOES exceed the ampacity for the temperature rating of the termination? What do you apply correction and adjustment factors to then?
...

Then you use a conduct sized based on the temperature rating of the termination. If the termination is rated for 60°C, you select the conductor size based on that column in the table.

 

[Nathanael47]

Then you use a conduct sized based on the temperature rating of the termination. If the termination is rated for 60°C, you select the conductor size based on that column in the table.

That makes sense to me!

 

[MiElectrician]

Derating seems complicated but it's not. You're derating for number of conductors in a pipe, since you won't be splicing in the pipe you can disregard the splice temperatures. After you get done derating make sure you don't go above the 75 degree column unless you have ninety degree terminations. I'll give you an example at lunch. This is something you have to know, so read the code, read these forums, ask at work, but a lot of guys either don't know or know incorrectly and will feed you garbage.

 

[Dennis Alwon]

Derating seems complicated but it's not. You're derating for number of conductors in a pipe, since you won't be splicing in the pipe you can disregard the splice temperatures. After you get done derating make sure you don't go above the 75 degree column unless you have ninety degree terminations. I'll give you an example at lunch. This is something you have to know, so read the code, read these forums, ask at work, but a lot of guys either don't know or know incorrectly and will feed you garbage.

Some panels have 90C lugs but the panel itself is still listed as 75C, therefore you need to use the 75C rating anyway.

 

[infinity]

Some panels have 90C lugs but the panel itself is still listed as 75C, therefore you need to use the 75C rating anyway.

I agree, almost all equipment is rated at 75°C so that is the maximum usable conductor ampacity.

 

[Krusscher]

I agree, almost all equipment is rated at 75°C so that is the maximum usable conductor ampacity.

if it is unknown then you would use 110.14(C)(1) correct?

 

[Dennis Alwon]

if it is unknown then you would use 110.14(C)(1) correct?

Yes based on the conductor sizes will determine 75C or 60c when the temp is not known

 

[augie47]

For your 15-30 amp lighting and receptacle circuits, the 210.4 (D) restrictions takes care of most devices that are rated 60°

 

[dbpeleed]

I agree, almost all equipment is rated at 75°C so that is the maximum usable conductor ampacity.

It's a UL thing. The UL White Book places the limit on the NEC 310.15(B)(16) ampacity column you use. In a nutshell, if you terminate at a circuit breaker (DHJR in the White Book) or safety switch, you can only assign the conductor ampacity with the 75°C column even if it's 90°C wire, like THHN (unless it's 60°C conductor then you use the 60°C column).

 

[Nathanael47]

For your 15-30 amp lighting and receptacle circuits, the 210.4 (D) restrictions takes care of most devices that are rated 60°

You mean 240.4 (D) right - small conductor rules?
A somewhat related question: What happens when 240.4 (D) and 110.14 (C) fight? I have always assumed that small conductor rules trump everything else and but I've started seeing 75 degree rated switches and receptacles. If every link in the chain is rated for 75 degrees would 110.14 (C)(3) overrule 240.4 (D)?

 

[infinity]

If every link in the chain is rated for 75 degrees would 110.14 (C)(3) overrule 240.4 (D)?

The small conductor rules in Article 240 are still the ones you follow. Same would apply to something like 334.80 which limits the ampacity of NM cable to the 60° C ampacity even when everything else in the circuit may be rated for 75° C.

 

[Bill Snyder]

You're right that the language in the NEC regarding temperature correction and adjustment factors can be confusing. Let me try to clarify how it works.

First of all, it's important to understand that the ampacity of a conductor depends on its temperature rating. The higher the temperature rating of the conductor, the higher its ampacity. However, in practice, conductors are often operated at temperatures below their maximum ratings, which means that their ampacity can be increased by applying correction and adjustment factors to account for the lower operating temperature.

The general rule for applying temperature correction and adjustment factors is as follows:

1. Determine the temperature rating of the conductor based on its insulation type.
2. Find the ampacity of the conductor in the appropriate column of Table 310.15(B)(16) based on its temperature rating and the number of current-carrying conductors in the raceway or cable.
3. Apply correction and adjustment factors to the ampacity based on the actual operating temperature of the conductor, as determined by the ambient temperature, the presence of other heat sources, and any other relevant factors.
4. If the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination (i.e., the device or equipment to which the conductor is connected), then the conductor is appropriately sized.
5. If the corrected and adjusted ampacity does exceed the ampacity for the temperature rating of the termination, then you must select a conductor with a higher temperature rating or a termination with a higher ampacity rating.

So, in answer to your question, if the corrected and adjusted ampacity exceeds the ampacity for the temperature rating of the termination, then you must either choose a conductor with a higher temperature rating or a termination with a higher ampacity rating. You do not apply further temperature correction or adjustment factors.

However, I want to emphasize that it's important to carefully follow the requirements of the NEC and any applicable local codes and standards when sizing conductors and selecting terminations. If you're uncertain about how to properly apply temperature correction and adjustment factors, it's always a good idea to consult with a qualified electrical engineer or other expert. -Bill Snyder- (maybe)

 

[don_resqcapt19]

...
So, in answer to your question, if the corrected and adjusted ampacity exceeds the ampacity for the temperature rating of the termination, then you must either choose a conductor with a higher temperature rating or a termination with a higher ampacity rating. You do not apply further temperature correction or adjustment factors.
....

So that sounds like you are telling me that I can't use a 2 AWG THHN with an 80% adjustment factor on an old 90 amp circuit breaker with a 60°C rating. I think this is just a language issue. I can use the 2 AWG on the 60°C rated 90 amp device as long as the load does not exceed 90 amps.