Temperature adjustments

[electrofelon]

I had a more specific question about this in the PV forum, but I have been thinking about it and wanted to discuss this more generally and see what everyone's thoughts and experience has been on this. How often do you do temp corrections for conductors? Ill be honest and say I dont recall ever doing a temp adjustment for an actual installation, and have never really seen or heard of anyone else doing them either. What about attics? Shouldnt we do them there? I am in the northeast, what about down south where it is hotter?

IT can go the other way too. What about correcting an underground installation to get more ampacity? Anyone do that?

 

[infinity]

Personally I can't think of any time when I considered temperature adjustments outside of a licensing exam. In the real world it's just not really an issue.

 

[electrofelon]

Personally I can't think of any time when I considered temperature adjustments outside of a licensing exam. In the real world it's just not really an issue.

Ok good to know its not just me....

 

[infinity]

Ok good to know its not just me....

Yes for a time we had those dumb adjustments for roofs. Then we had an exception for XHHW. Now the entire mess is for all intents and purposes has been proven to be nonsense.

 

[tom baker]

Yes for a time we had those dumb adjustments for roofs. Then we had an exception for XHHW. Now the entire mess is for all intents and purposes has been proven to be nonsense.

Correct nonsense, I was at the IAEI meeting , 2007 Boise where the initial change was discussed. The original rule was for conduit and EMT is not conduit. The change was based on a study by the IAEI and maybe submitted by the copper organization. And next to me was the chief electrical inspector for Las Vegas, he said in 30 years he never saw any overheated wires on rooftops.

 

[electrofelon]

But we have the requirement in the code to adjust for the temp do we not? Even a flat TPO or PVC roof with direct sun is hotter than 86 degrees is it not? Then of course get into something dark like EPDM or shingles it's going to be even hotter.

 

[LarryFine]

I haven't, because none of the environments were abnormal, or it was designed.

I also haven't taken advantage abnormally low temps either, so I figure I'm even.

 

[jaggedben]

As a PV installer I have usually done it on the roof but not anywhere else. Some AHJs pretty much enforced it that way. My area is quite temperate.

 

[ptonsparky]

I adjusted some for direct buried with IDR how many feet of conductor up on each end. Purely academic because no one asked or probably cared. Waste of my time other than I knew it was correct. I think.

 

[wwhitney]

A few comments:

Where your wire size is effectively based on the 60C ampacity, e.g. most #14-#10 circuits because of 240.4(D), but you are using 90C rated wire with no ampacity adjustment for more than 3 CCCs, you are effectively using only half the available heat rise at 30C ambient. That mean you could have a 60C ambient, or 140F, and still not have an issue with ampacity correction due to ambient temperature. Which is almost never.

On the other hand when you are taking advantage of the 75C ampacity, then you're only good for an ambient up to 45C, or 113F. So outdoors in very hot climates, or many unconditioned attics, that is something that really should be considered.

Of course, when you have more than 3 CCCs, then the impact of temperature correction on top of the ampacity adjustment will be much more significant, as the ampacity adjustment already uses much or all of the headroom in the computation.

Cheers, Wayne

 

[electrofelon]

A few comments:

Where your wire size is effectively based on the 60C ampacity, e.g. most #14-#10 circuits because of 240.4(D), but you are using 90C rated wire with no ampacity adjustment for more than 3 CCCs, you are effectively using only half the available heat rise at 30C ambient. That mean you could have a 60C ambient, or 140F, and still not have an issue with ampacity correction due to ambient temperature. Which is almost never.

On the other hand when you are taking advantage of the 75C ampacity, then you're only good for an ambient up to 45C, or 113F. So outdoors in very hot climates, or many unconditioned attics, that is something that really should be considered.

Of course, when you have more than 3 CCCs, then the impact of temperature correction on top of the ampacity adjustment will be much more significant, as the ampacity adjustment already uses much or all of the headroom in the computation.

Cheers, Wayne

Yeah I thought about that. So single or non-bundled NM cables in attics, not an issue. Perhaps something running through the Attic like a 200 amp feeder using 4/0 SER would be an issue where you are just barely above a standard size ocpd as is. As I already mentioned, another common one would probably be on flat roofs feeding HVAC units. Frequently I am feeding multiple units in one pipe so we already have a CCC derating.

 

[LarryFine]

So single or non-bundled NM cables in attics, not an issue.

In my opinion, the low rating of NM takes this into account. It comes "pre-derated" from the factory.

 

[ramsy]

Please delete duplicate

 

[infinity]

In a laboratory we can test these things and find a particular scenario when there is a problem like stuffing a hole full of NM cables, cranking up the load to the maximum, and then filling it with spray foam. But in the real world these things are typically not an issue and end up in the code without any real world problems or substantiation.

For example back in the 80's there were real world problems with furniture partitions, harmonics, and shared neutrals because of the power supplies in PC's. These partitions were burning up due to overloaded neutrals. That was a real world scenario that the code addressed becuse it had an actual real world substantiation. This other nonsense about roofs, temperature adjustments, ampacity adjustment, etc., are for most part non-issue.

 

[ptonsparky]

...
For example back in the 80's there were real world problems with furniture partitions, harmonics, and shared neutrals because of the power supplies in PC's. These partitions were burning up due to overloaded neutrals. That was a real world scenario that the code addressed becuse it had an actual real world substantiation. This other nonsense about roofs, temperature adjustments, ampacity adjustment, etc., are for most part non-issue.

Is this an issue yet for new installations?

 

[infinity]

Is this an issue yet for new installations?

Not really. For one partition manufactures changed the wiring scheme to where there was a single neutral for two circuits instead of three. They also employed over-sized neutrals. And lastly computer power supplies have changed so the harmonic issues from a few decades ago are pretty much gone.

 

[ptonsparky]

The harmonic issue was what I was wondering about.