310.15(B)(3)(c) & Copper.Org

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George Stolz

George Stolz

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I know this is not an extremely recent change, and not even all that new of a topic, but I hadn't looked into it much until today. I was looking around, and found this link pertaining to how to find the temperature of a rooftop in the US and Canada. The numbers they provide are based on continuous readings taken from June through August for 10 years.

Does anyone else take issue with the fact that only the hottest months factor into their tables? I get no credit for the other 75% of the year when the conductors are not as abused, and whose useful life therefore is extended?

Has another source of data from some party other than the copper vendors that is less strict come along yet?
 
George Stolz said:
I know this is not an extremely recent change, and not even all that new of a topic, but I hadn't looked into it much until today. I was looking around, and found this link pertaining to how to find the temperature of a rooftop in the US and Canada. The numbers they provide are based on continuous readings taken from June through August for 10 years.

Does anyone else take issue with the fact that only the hottest months factor into their tables? I get no credit for the other 75% of the year when the conductors are not as abused, and whose useful life therefore is extended?

Has another source of data from some party other than the copper vendors that is less strict come along yet?
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"310.15 Ampacities for Conductors Rated 0-2000 Volts.
(A) General.
(3) Temperature Limitation of Conductors. ....

Informational Note No. 1: The temperature rating of a
conductor [see Table 310.104(A) and Table 310.104(C)] is
the maximum temperature, at any location along its length,
that the conductor can withstand over a prolonged time
period without serious degradation...."

I have thought that the "prolonged time period" would be measured in hours rather than months.
 
I have always felt that the nec has never given us guidelines as to how long a wire must be exposed to high ambient temp. to be considered an issue. Ambient temp means nothing- it means the temp at any given time. If they want average, hotest 10 days in a row or whatever then we should be given that info.

IMO, this rule is generally not even followed in attics.

Has anybody done studies that show at what percentage of the load and for how long will a wire be affected? I can't imagine that a week of high temperature & full ampacity would damage a wire but I really have no idea.
 
George Stolz said:
I know this is not an extremely recent change, and not even all that new of a topic, but I hadn't looked into it much until today. I was looking around, and found this link pertaining to how to find the temperature of a rooftop in the US and Canada. The numbers they provide are based on continuous readings taken from June through August for 10 years.

Does anyone else take issue with the fact that only the hottest months factor into their tables? I get no credit for the other 75% of the year when the conductors are not as abused, and whose useful life therefore is extended?

Has another source of data from some party other than the copper vendors that is less strict come along yet?
Click to expand...

I know Charlie wants us to look at it like you suggest but IMO it has to be about the max temp not the average temp.

To look at it as the average temp we might as well look at the average amperage on the circuit because ultimately the current rating is the temp limitations of the conductor.

In other words if we can stress the insulation due to high ambient why not let us stress it with high current if it is only once in a while.
 
iwire said:
I know Charlie wants us to look at it like you suggest but IMO it has to be about the max temp not the average temp.
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So one day out of the year it is the hottest and we should use that? That makes no sense to me. Certainly a conductor can handle that. Maybe if they take the average temp. for 2 weeks during the hottest time of year but I still would like to know if studies have been done to show that a conductor is affected or more precisely how long it takes a conductor to begin to break down or have adverse affects.
 
Dennis Alwon said:
So one day out of the year it is the hottest and we should use that? That makes no sense to me. Certainly a conductor can handle that.
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Sure it can handle that.

By the same token a 12 AWG can handle 45 amps for repeated periods of time.

In both cases it is the insulation that is being compromised.
 
I am working on a Changes class, and am dipping back a cycle or two to cover things that still haven't really been pounded too hard in the field. This topic fit the bill.

Here's a portion of what I've written so far:

Using these numbers, let's imagine a 200A feeder with a 180A non-continuous load running 100' on a rooftop exposed to sunlight.

If the ambient temperature is 123?F inside the raceway, then I can divide my 180A load by .76 to find the ampacity I am looking for in Table 310.15(B)(16), an adjusted ampacity of 237A. This would require a 4/0 CU THWN-2.

Prior to this change, I could have used the ambient temperature given, 93?F, and done the math thusly:

180 ? .96 = 187.5A ---> 2/0 CU THWN-2

Let's try another example:

500A feeder, 500A non-continuous load installed 2" above roof surface

500 ? .71 = 704A ---> (2) sets of 400 kcmil CU THWN-2 or (3) sets of 4/0 CU THWN-2

Same conditions under the 2005 NEC:

500 ? .96 = 521A ---> (2) sets of 4/0 CU THWN-2

Essentially, this change nets the wire and raceway manufacturers 50% more material if you don't get your conduits away from the roof!
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One of the comments that repeatedly came up during the ultimate acceptance of this proposal was, where are the damaged conduits in sunlight that were installed prior to the 2008 taking effect?

Edit to add: I would propose the idea that perhaps the conductor insulation is the proof - if there is no damage, then perhaps the balance of time in freezing weather and during periods of inactivity make up for brief spells of overages.
 
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George Stolz said:
Like we do with 250.122? :)
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Or with motor loads ........


But you are trying to compare a short duration high load that has no time to really heat the conductors with a long term condition.

Can you give a me a good reason why overheating the insulation via current alone over a weeks period would be worse than overheating the insulation vial current and ambient combined for a weeks time?

IMO we tend to look a ampacity ratings like they are firm numbers. 20 amps is OK, 21 amps is bad. But the truth is the ampacity rating is directly and inescapably related to the ability of the insulation to tolerate temperature.


As far as where is the trail of damaged roof top conductors I would ask where the trail of damaged 14 AWG is when protected by 30 amp Edison base fuses which is (or was) very common in older homes. My point it is simply that the NEC has always been conservative. :)

Just rambling a bit...... :)
 
iwire said:
....... have you wrapped your head around 690.8 and its ramifications? :cool:
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Beyond the distinct feeling I do not want anything to do with solar... no. :D

As far as where is the trail of damaged roof top conductors I would ask where the trail of damaged 14 AWG is when protected by 30 amp Edison base fuses which is (or was) very common in older homes. My point it is simply that the NEC has always been conservative. :)

Just rambling a bit...... :)
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As am I, really. There's little to knock this down, and people tried. Best I can try for is finding the most minimal bar to base an ambient on.

Here's another nugget from a fortune cookie: If the NEC has always been conservative, then doesn't a 50% additional level of conservatism raise it from a wink and a nod to an outcry?
 
Forgot half your quote. :ashamed:

iwire said:
But you are trying to compare a short duration high load that has no time to really heat the conductors with a long term condition.
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I agree, there is a difference. But, then again, the small utility conductors and transformers testify to long term ability to withstand as well.

Can you give a me a good reason why overheating the insulation via current alone over a weeks period would be worse than overheating the insulation vial current and ambient combined for a weeks time?

IMO we tend to look a ampacity ratings like they are firm numbers. 20 amps is OK, 21 amps is bad. But the truth is the ampacity rating is directly and inescapably related to the ability of the insulation to tolerate temperature.
Click to expand...

How can we see things in this matter so similarly, yet sit on opposite sides of the debate? :cool:
 
iwire said:
Sure it can handle that.

By the same token a 12 AWG can handle 45 amps for repeated periods of time.

In both cases it is the insulation that is being compromised.
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Of course it is the insulation but do we have evidence that a few days in the high temperature will affect it. That's what I was getting at. The copper is fine it is almost always the insulation we worry about with ambient temp.
 
In the case of Table 250.122 (or 250.66 as well) we are expecting the current levels durations to be the clearing time of an OCPD, surely on the order of a minute or less. In addition in 250 the insulation (excepting isolated grounds) is merely decorative.

I've certainly seen PVC conduit installed near roofs to be brittle and broken. I don't know if this is solar heating, ultraviolet, and heating from conductors carrying current or just solar and ultraviolet. Preserving the mechanical characteristics of the conduit is important and if having reduced conductor heating helps this then I'm all for it.
 
Off topic

Off topic

What does T250.66 have to do with OCPDs?

"250.68 (C) Metallic Water Pipe and Structural Metal. Grounding
electrode conductors and bonding jumpers shall be permitted
to be connected at the following locations and used
to extend the connection to an electrode(s):
....
(2) The structural frame of a building that is directly connected
to a grounding electrode as specified in
250.52(A)(2) or 250.68(C)(2)(a), (b), or (c) shall be
permitted as a bonding conductor to interconnect electrodes
that are part of the grounding electrode system,
or as a grounding electrode conductor."

I presume the conductors for this purpose are based on T220.66.

I further presume that a fault consisting of a phase conductor shorting to structural steel would be cleared by an OCPD.
 
fmtjfw said:
What does T250.66 have to do with OCPDs?

"
I presume the conductors for this purpose are based on T220.66.

I further presume that a fault consisting of a phase conductor shorting to structural steel would be cleared by an OCPD.
Click to expand...


That's not the primary purpose of Part III. The conductor sizing is for the purposes outlined in 250.4(A)(1), grounding.

Bonding is per 250.104.
 
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