Do I need to derate?

[kwired]

I see what you are saying but under that line of reasoning shouldn't we do away with 310.15 (B)(6).

310.15(B)(6) is a different ballgame, the conductor must be supplying entire dwelling before it can be used. The load diversity is greater when supplying entire dwelling than when just supplying a portion of it.

You said in OP that this feeder was for HVAC equipment. HVAC equipment does not have much of a load diversity at all at times. If this feeder is sized close to actual HVAC load then the load plus the ambient temp will be more of a problem. I understand you are feeding this from subfeed lugs from the main panel and it likely is not going to be loaded to its capacity, and may never give any trouble, but that does not change the fact that you have a conductor that has an derated ampacity of maybe 125 amps being protected by a 200 amp overcurrent device. I don't know what justification an inspector would have to say this is acceptable. Run same conductor with same calculated load and same deration factors through a boiler room and I bet he does not accept it. Why is it any different?

 

[jumper]

I heard a rumor once that someone derated for an attic, but I never believed it.

 

[Finite10]

The ambient would be based on the year round average, not the record. I'm not sure how you'd figure that. I'd recommend running on the roof, at least you'll know how much to derate.

That's exactly right.
It's; how many inches ABOVE a roof, not under.

Okay, how about under the insul and WITH an attic fan...?

 

[Volta]

The ambient would be based on the year round average, not the record. I'm not sure how you'd figure that....

That sounds like trouble to me.

I'm sure that the thought would be different for some process room that reached 150 deg F for just one shift a day. I doubt we'd take an annual average on that.

What's the annual average in FC, 60 deg or so?

 

[Dennis Alwon]

The ambient would be based on the year round average, not the record.

Although you are probably correct, that is not what ambient means. If that were the case then they should say average.

 

[George Stolz]

Although you are probably correct, that is not what ambient means. If that were the case then they should say average.

Compare the language of 310.15(B)(3)(c) to it's informational note.

 

[Dennis Alwon]

Compare the language of 310.15(B)(3)(c) to it's informational note.

I hadn't noticed the FPN but the temp. in an attic in all the different scenarios would be harder to determine. I always assumed they meant average but I don't know why they just don't state it in the article.

 

[Volta]

Do you guys not feel that ambient simply means the surrounding temperature?

And then, dropping all semantics, does it not stand to reason that the conductors subjected to that surrounding temperature (when it is higher) will experience a rise in temperature?

If we figure the ampacity from the temperature on a 70 deg day, or if that is the ASHRAE temperature, but then the next day is hotter, won't the installation be in violation of 310.15(B), as were would not be in compliance with the tables?

 

[Dennis Alwon]

I don't believe the conductors will have trouble where the temp. is higher for portions of the day and few days of the year. I suspect the conductor would need to be exposed to the high loads and high temp. for extended periods of time to cause damage to the conductor.

 

[kwired]

I don't believe the conductors will have trouble where the temp. is higher for portions of the day and few days of the year. I suspect the conductor would need to be exposed to the high loads and high temp. for extended periods of time to cause damage to the conductor.

And if it is supplying air conditioning loads it will be loaded the hardest when the ambient temperature is highest.

 

[iMuse97]

And if it is supplying air conditioning loads it will be loaded the hardest when the ambient temperature is highest.

And for another monkey wrench in the fan: AC loads under these circumstances (the hottest days and highest loads) may sometimes run for more than 3 hours, thus constituting a continuous load. What about that, guys? :happyyes:

 

[George Stolz]

Do you guys not feel that ambient simply means the surrounding temperature?

And then, dropping all semantics, does it not stand to reason that the conductors subjected to that surrounding temperature (when it is higher) will experience a rise in temperature?

If we figure the ampacity from the temperature on a 70 deg day, or if that is the ASHRAE temperature, but then the next day is hotter, won't the installation be in violation of 310.15(B), as were would not be in compliance with the tables?

No more than a ground fault would constitute a violation - for a moment that 12 AWG can see hundreds of amps.

Think of the typical transformer serving a rural house. I have a 15 kVA transformer supplying a 200A service. Will it supply 200A continuously? Yes. Will it last forever under those conditions? No. Will it serve a long time, given periods to cool down? Sure - and that's why the utility is okay with it.

You see a six foot run of LFNC with 12 AWG conductors sitting in the sun on a 100? day and panic, but it will withstand that for an extended period of time as long as it doesn't stay 100? from now until forever.

The "average ambient" concept recognizes that it's good enough to split the difference.

And if it is supplying air conditioning loads it will be loaded the hardest when the ambient temperature is highest.

And for another monkey wrench in the fan: AC loads under these circumstances (the hottest days and highest loads) may sometimes run for more than 3 hours, thus constituting a continuous load. What about that, guys? :happyyes:

440.32 already covers that.

 

[blueheels2]

I must not have been clear in my original statement. The feeders going through the attic will feed a 200 amp sub panel which is feeding the entire house. That's why I referenced 310.15 (B)(6).

 

[kwired]

No more than a ground fault would constitute a violation - for a moment that 12 AWG can see hundreds of amps.

Think of the typical transformer serving a rural house. I have a 15 kVA transformer supplying a 200A service. Will it supply 200A continuously? Yes. Will it last forever under those conditions? No. Will it serve a long time, given periods to cool down? Sure - and that's why the utility is okay with it.

You see a six foot run of LFNC with 12 AWG conductors sitting in the sun on a 100? day and panic, but it will withstand that for an extended period of time as long as it doesn't stay 100? from now until forever.

The "average ambient" concept recognizes that it's good enough to split the difference.





440.32 already covers that.

440.32 basically recognizes that AC equipment is considered a continuous load, what does that have to do with ambient temperature? Continuous loading plus high temperature is two separate derations for the same conductor and the effect is additive.

 

[George Stolz]

440.32 basically recognizes that AC equipment is considered a continuous load, what does that have to do with ambient temperature?

I never said it did - I was pointing out that the continuous aspect was already covered. After that, we would need to consider ambient and any other considerations.

Continuous loading plus high temperature is two separate derations for the same conductor and the effect is additive.

Agreed.

 

[kwired]

I never said it did - I was pointing out that the continuous aspect was already covered. After that, we would need to consider ambient and any other considerations.


Agreed.

I must have misunderstood, but thanks for clarification.

 

[Dennis Alwon]

And if it is supplying air conditioning loads it will be loaded the hardest when the ambient temperature is highest.

Most residential a/c units are not on a roof but on the ground. We were talking NM in general- I thought

And for another monkey wrench in the fan: AC loads under these circumstances (the hottest days and highest loads) may sometimes run for more than 3 hours, thus constituting a continuous load. What about that, guys? :happyyes:

A/c loads and heating loads already have 125% built in. Are you suggesting doing that twice?

 

[kwired]

Most residential a/c units are not on a roof but on the ground. We were talking NM in general- I thought



A/c loads and heating loads already have 125% built in. Are you suggesting doing that twice?

We are talking about derating NM cable for ambient temperature in an attic. AC units typically are not on a roof but one may run a cable across an attic if that is the easiest way to get to where you need to go, especially existing construction.