Temperature Derating - When?

[Robert G]

I have a question about derating conductors for ampacity versus temperature, art. 310.16.

My scenario takes place in a manufacturing facility. We have a 24 foot ceiling with 4 conduits running the length of the plant, 4 foot down from the ceiling, (paralleled feeders). Only 3 current carrying THHN conductors per conduit, 480v 3 phase Delta.

Although it?s warm inside the plant in the summer time, the other 3 out of 4 seasons inside the plant here in NJ are cool.

At what point does a person have to use temperature correction factors for conductors concerning ambient temperature?

In other words, how long does the temperature have to stay at a certain range and for how long or short a time before correction factors need to be applied?

Thanks.

 

[charlie b]

Ah, one of my favorite topics. It has been discussed (debated?? argued??) on this forum before. It might be worth looking up some of the previous discussions, in order to quickly get you to the point of confusion that the rest of us enjoy. :grin:

My opinion, one that by the way is shared by very few members of this forum, is that it is technically acceptable to use the "year round average temperature" for this purpose. Unfortunately, nothing in the NEC wording explicitly supports that point of view. On the other hand, nothing in the NEC explicitly states what others believe: that the highest temperature seen during the year is the temperature you should use for derating purposes.

I think the only practical answer is that the rule means what your AHJ says the rule means. So my suggestion is to talk with the local authority.

 

[haskindm]

This is subject to interpretation as Charlie says, as the term is not defined by the NEC. If your interpretation (or mine, or Charlie's, or anyone else's) does not agree with the AHJ in your area, you will have a problem. Find out how they interpret it. Charlie's method makes sense to me. Other people use the different formulas. See what method the AHJ in your area recommends.

 

[beanland]

Thermal Design

Thermal Design

I perform steady state and dynamic cable loading calculations for clients regularly. The problem is that you need to be able to model system to get its thermal time constant.

If the conductors and conduit are just hung in the air, they have little thermal mass and so they heat up rapidly (1 hour?). So, if you are going to fully load the cables during the hottest time of the year, you need to use the hottest hourly temperature for your adjustment. If not, you will violate the rating for the conductors by allowing their operating temperature to exceed their rating, an NEC violation.

If the conductors are buried or in conduit in the ground, the thermal time constant is much longer (2-3 days) and you can then use a different temperature (hottest daily average temperature?) for derating.

However, the modeling is complex and the AHJ still needs to approve of the installation.

 

[nakulak]

I know that you guys have worn this subject out, but imsho if the NEC is going to say that continuous use is 3 hrs, then why wouldn't temperature derating be about the same, since imsho the point is that 3 hrs is how long it takes wiring to cook up a fire ? (unless otherwise de-energized) (or at least this is how I interpret the intent)