When did rooftop table 310.15(B)(3)(c) appear in the code?

[ggunn]

I agree...and all they are doing is proposing the removal the table 310.15(B)(3)(c). The Code will still have tables 310.15(B)(2)(a) & (B)(2)(b) and they must be applied as applicable.

So, we will have to figure out what temperature the conductors will get to and derate accordingly? How is that different from using 310.15(B)(3)(c)?

 

[don_resqcapt19]

So, we will have to figure out what temperature the conductors will get to and derate accordingly? How is that different from using 310.15(B)(3)(c)?

No...without the adder in the table, you just use the ambient for the area. You do not take into account that the interior of the raceway or cable will be hotter than the ambient.

 

[ggunn]

No...without the adder in the table, you just use the ambient for the area. You do not take into account that the interior of the raceway or cable will be hotter than the ambient.

I get it, but it makes no sense to me. When the ambient temperature is 100 degrees the temperature on a rooftop in areas exposed to sunlight can easily be 140 degrees. We derate for the 20 degree difference between 80 degrees and 100 degrees but not for the 40 degree difference between 100 degrees and 140 degrees? Why derate at all for temperature?

 

[shortcircuit2]

I get it, but it makes no sense to me. When the ambient temperature is 100 degrees the temperature on a rooftop in areas exposed to sunlight can easily be 140 degrees. We derate for the 20 degree difference between 80 degrees and 100 degrees but not for the 40 degree difference between 100 degrees and 140 degrees? Why derate at all for temperature?

If it is 140 degrees on the rooftop, apply that temperature to the conductors.

If it is 140 degrees in a boiler room, apply that temperature to the conductors.

If it is 140 degrees on the rooftop, PVC can't be used up there.

 

[ggunn]

If it is 140 degrees on the rooftop, apply that temperature to the conductors.

That's what 310.15(B)(3)(c) does as an adjustment to the ambient temperature. If we still must make the same calculation, why not leave the table in the code? Why fix it if it ain't broke?

 

[wwhitney]

That's what 310.15(B)(3)(c) does as an adjustment to the ambient temperature. If we still must make the same calculation, why not leave the table in the code? Why fix it if it ain't broke?

The ambient temperature is the air temperature in the vicinity of the wires in question. So if the air temperature is 100 degrees outside the building at ground level, and 130 degrees while standing on the roof, the ambient temperature for wires in conduit on the roof would be 130 degrees. The Table 310.15(B)(3)(c) adders were on top of this raised ambient, based on the idea that the solar heating phenomenon that raises the air temperature on the roof would be concentrated and occur to a greater extent for conduit close to the rooftop. It is this latter premise that is being discarded, so the Table 310.15(B)(3)(c) adders will be gone.

At least, that is my understanding.

Cheers, Wayne

 

[don_resqcapt19]

The ambient temperature is the air temperature in the vicinity of the wires in question. So if the air temperature is 100 degrees outside the building at ground level, and 130 degrees while standing on the roof, the ambient temperature for wires in conduit on the roof would be 130 degrees. The Table 310.15(B)(3)(c) adders were on top of this raised ambient, based on the idea that the solar heating phenomenon that raises the air temperature on the roof would be concentrated and occur to a greater extent for conduit close to the rooftop. It is this latter premise that is being discarded, so the Table 310.15(B)(3)(c) adders will be gone.

At least, that is my understanding.

Cheers, Wayne

I disagree. The ambient is based on ambient design temperatures used by the heating and air conditioning designers. See the informational note that follows 310.15(B)(3)(c).

Informational Note: One source for the ambient temperatures in various locations is the ASHRAE Handbook — Fundamentals.

 

[wwhitney]

I disagree. The ambient is based on ambient design temperatures used by the heating and air conditioning designers. See the informational note that follows 310.15(B)(3)(c).

Informational Note: One source for the ambient temperatures in various locations is the ASHRAE Handbook — Fundamentals.

Hi Don,

That informational note is just telling you "one source" for temperature data--which implies it is not the definitive source or will cover all conditions. I don't see any NEC definition of ambient temperature. So here's the definition of ambient Google selects:

am·bi·ent--of or relating to the immediate surroundings of something.

Based on that definition, it is clear to me that ambient temperature at ground level and ambient temperature on a rooftop are two different things. So if the ASHRAE Handbook includes statistical temperature data collected at, say, 5 feet above a flat rooftop, that would be appropriate to use for installing conductors 5 feet above a flat rooftop. If it only has data collected at ground level, then that source would only be appropriate for use at ground level.

Cheers, Wayne

 

[don_resqcapt19]

Hi Don,

That informational note is just telling you "one source" for temperature data--which implies it is not the definitive source or will cover all conditions. I don't see any NEC definition of ambient temperature. So here's the definition of ambient Google selects:



Based on that definition, it is clear to me that ambient temperature at ground level and ambient temperature on a rooftop are two different things. So if the ASHRAE Handbook includes statistical temperature data collected at, say, 5 feet above a flat rooftop, that would be appropriate to use for installing conductors 5 feet above a flat rooftop. If it only has data collected at ground level, then that source would only be appropriate for use at ground level.

Cheers, Wayne

The whole point of the temperature adder table was to make an adjustment to the ambient, but you are correct it is an undefined term.
While there was lots of evidence that the temperatures above the roof are higher than the weather based ambient as published in various sources, there was never any evidence presented to the CMP that higher temperature has caused problems or failures of conductors within roof top conduits.

To me it is clear that the intended ambient is based on the weather records and the published design ambient temperatures. However three is clearly room for other views. There is also the question of how long a temperature must exist before it is an "ambient". The maximum temperature for a minute, hour, day? The average of the maximum ambient over a number of years? The whole concept of derating based on ambient, not just the roof top derating, really needs a lot of work.

 

[wwhitney]

The whole concept of derating based on ambient, not just the roof top derating, really needs a lot of work.

Agreed.

Cheers, Wayne

 

[jaggedben]

The nice thing about the table is that it provided a NEC approved method for guessing how much hotter it is above a roof surface compared to the general outside ambient temperature. Whether I use ASHRAE or anything else, those temperatures are for the general outside ambient temperature, which isn't the same as the the close-to-the-roof temperature. I am not going to sit around taking rooftop temperature measurements all year on a project site to determine how different they are from published temperatures. So the table is nice to have if an AHJ questions me on whether I'm using an appropriate 'ambient' temperature for my rooftop conduits.

As for whether any of this temperature derating is truly justified based on real world results, well, I'm certainly open to learning more about that.

 

[ggunn]

The whole concept of derating based on ambient, not just the roof top derating, really needs a lot of work.

That's the one thing I guess we can all agree on. IMO simply deleting 301.15(B)(3)(c) makes things less clear, not more.

 

[don_resqcapt19]

That's the one thing I guess we can all agree on. IMO simply deleting 301.15(B)(3)(c) makes things less clear, not more.

In my opinion, the deletion just puts us back to where we were before the table was in the code. That is we just use the general ambient based on the weather records.

None of the substantiations for the temperature adder showed any real world problems that resulted from the higher temperatures that exist close to the roof. Without any evidence of cable failures or other problems, there was nothing that needed fixing. So there was never a real world need for the temperature adder table. (as I recall it was the Copper Development Association, that funded the tests that were used to show the higher temperatures)

 

[ggunn]

In my opinion, the deletion just puts us back to where we were before the table was in the code. That is we just use the general ambient based on the weather records.

None of the substantiations for the temperature adder showed any real world problems that resulted from the higher temperatures that exist close to the roof. Without any evidence of cable failures or other problems, there was nothing that needed fixing. So there was never a real world need for the temperature adder table. (as I recall it was the Copper Development Association, that funded the tests that were used to show the higher temperatures)

I get it. I really do.

But it still makes no sense to me to have to derate conductor ampacity for an ambient of 100 degrees and then totally ignore the temperature of 140 degrees (or whatever) on a rooftop. Maybe I should give up on the NEC being consistent...

 

[don_resqcapt19]

I get it. I really do.

But it still makes no sense to me to have to derate conductor ampacity for an ambient of 100 degrees and then totally ignore the temperature of 140 degrees (or whatever) on a rooftop. Maybe I should give up on the NEC being consistent...

You are correct....I am a bit surprised that after they added the exception to the roof top temperature derating for XHHW that there was not a move to show an ampacity higher that 90°C for derating purposes for XHHW.

 

[jaggedben]

...None of the substantiations for the temperature adder showed any real world problems that resulted from the higher temperatures that exist close to the roof. Without any evidence of cable failures or other problems, there was nothing that needed fixing. ...

If that's the case, then the logical conclusion, to me, is to remove ambient temperature derations from the code entirely.

Here's my logic:
- Roof top surfaces can be tens of degrees hotter than the general ambient temperature.
- tests show no 'real world problems' from conductors at those temperatures, according to your post.
- If differences of tens of degrees don't cause 'real world problems' then we don't need to be need to be adjusting anything for those temperatures. Anywhere.

I think I'm saying the same thing as ggunn in a different way.

 

[Strathead]

If that's the case, then the logical conclusion, to me, is to remove ambient temperature derations from the code entirely.

Here's my logic:
- Roof top surfaces can be tens of degrees hotter than the general ambient temperature.
- tests show no 'real world problems' from conductors at those temperatures, according to your post.
- If differences of tens of degrees don't cause 'real world problems' then we don't need to be need to be adjusting anything for those temperatures. Anywhere.

I think I'm saying the same thing as ggunn in a different way.

After reading through the entire thread again. Which certainly swayed from my original question in a good way, as my question was answered very promptly...

I was coming to the same conclusions as you do. The number of current carrying conductors should also go away. When was the last time you found a wire that was too hot to touch? And that is around 42º centigrade. A conductor reaching 75 plus is not likely then when installed to all of the other code requirements. It would seem that conduits near a roof surface in a hot climate would be far more likely to get hot than an extra couple wires in a 3/4" conduit.

 

[ggunn]

If that's the case, then the logical conclusion, to me, is to remove ambient temperature derations from the code entirely.

Here's my logic:
- Roof top surfaces can be tens of degrees hotter than the general ambient temperature.
- tests show no 'real world problems' from conductors at those temperatures, according to your post.
- If differences of tens of degrees don't cause 'real world problems' then we don't need to be need to be adjusting anything for those temperatures. Anywhere.

I think I'm saying the same thing as ggunn in a different way.

Yes, you are. Not that much different, actually.

 

[MasterTheNEC]

Just for clarity, yes the table is going away but the amended remnants of the rule is still in play.

 

[Strathead]

Just for clarity, yes the table is going away but the amended remnants of the rule is still in play.

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Talk about arbitrary! Why should only XHHW-2 not be subject to this adjustment. What about glass insulated wire or other really high insulation temp wires? How about they just make it simple and eliminate the allowance for low temp insulation altogether for all building wiring going forward? It wouldn't add nearly as much to the cost of construction as other less proven benefits like arc fault breakers.