WHEN TO USE TEMPERATURE DERATING FACTORS FOR CONDUCTORS

[PE (always learning)]

Hey everyone,

This may be an obvious question to others, but I haven't had an opportunity to use temperature derating factors much outside of running conduit outdoors. I have a project where we are providing a brand new stand alone shop class for a school and the shop class only has heating for the winter and no A/C for the summer. There is a makeup air unit with louvers and exhaust, but that's about it. So my question is do I need to provide temperature derating for the conductors in the shop class. I'm assuming the ambient temperature could be over 86 degrees fahrenheit in the summer time, but we are feeding equipment that won't be running for long continuous amounts of time. For example, there are arc welders and table saws. Just curious if anyone has come across a similar project.

Best Regards

 

[Dennis Alwon]

If the ambient is higher than 86° then you probably should de-rate. With thwn-2 conductors it may not hurt you because of the high temp of the insulation

 

[infinity]

Take a look at Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C (86°F). Although if the temperature exceeds 86° F you need to make an adjustment but with the 90° C conductor ampacity that adjustment will likely not change anything.

 

[charlie b]

Since you are an EIT, I will do what I can to confuse you. No, scratch that. I mean I will give you something to think about. :happyyes:

There has been some debate, without resolution of which I am aware, concerning how to determine what temperature correction to apply. For example, if your shop gets up to 89 degrees for two days during the entire summer, is that enough to bring the temperature correction into play? What if you recorded the maximum temperature observed each day for a year, and calculated the average, would that be the correct basis for determining temperature correction? All I can say for certain is that the NEC does not answer such questions.

You should be aware that the issue with temperature is all about degradation of the conductor's insulation system. You can essentially assume that if you keep a conductor at 30C for its entire life, and if you keep the current levels no higher than the posted ampacity values, the conductor's insulation system will not degrade to the point of failure for its entire design useful life (which I believe to be 40 years). The impact of higher temperatures on degradation follows an exponential function (Side Note: Look up the "Arrhenius Equation"). A rule of thumb is that adding 10 degrees C to the ambient temperature (keeping this value constant until the conductor's insulation fails) will reduce the useful life by 50%. However, reducing the ambient temperature will significantly increase the useful life.

Think of it like starting with a full tank of gas. If you travel at the car's "optimal" speed, you will burn up the gas at one rate. If you travel faster for some amount of time, you will use up gas at a faster rate. And once the gas has been burned, it is gone forever. So travelling fast will reduce the "useful life" of the gas tank. On the other hand, travelling slower will (generally speaking) burn up gas at a lower rate, preserving more of the tank for future use, and extending its "useful life."

Getting back to conductors and ambient temperature, the purpose of derating for temperature is that if the ambient is high but you reduce the ampacity (i.e., lower I2R heating), the combined effect will be to keep the original useful life at its original expectations. You can see that having a higher temperature in the summer will reduce the conductor's life, and having a lower temperature the rest of the year will extend the conductor's life. But how do we calculate the plusses and minuses? How do we assign an ampacity value, when we know the temperature will sometimes be high and the rest of the time will be low? Sadly, I have no answer to give you. In my defense, I don't think anyone has that answer.

 

[Carultch]

Since you are an EIT, I will do what I can to confuse you. No, scratch that. I mean I will give you something to think about. :happyyes:

There has been some debate, without resolution of which I am aware, concerning how to determine what temperature correction to apply. For example, if your shop gets up to 89 degrees for two days during the entire summer, is that enough to bring the temperature correction into play? What if you recorded the maximum temperature observed each day for a year, and calculated the average, would that be the correct basis for determining temperature correction? All I can say for certain is that the NEC does not answer such questions.

This explains why it is common practice per the NEC's informational note to calculate according to ASHRAE values of temperature for derating purposes. Just about anyone who's lived for several years in any of the cities of reference, can testify that they've experienced weather hotter than that. Or colder than that, for the design low temperatures. It isn't an issue of absolute failure if the temperature occasionally gets above the design temperature, it is an issue of accumulated damage to the wire for operating at that temperature for extended time.

 

[PE (always learning)]

Thanks everyone for your answers, they are much appreciated. I ended up using the correction factor in the table for the 96-104 degrees far. category. This correction factor ended up mostly affecting the larger conductors. I figured why not upsize a little bit to provide for a worst case scenario.