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Old 07-04-2012, 11:51 PM   #1
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Default Ambient Temperatures/Humidity

We are doing a very complex job in the near future. The ambient temps are going to come into play in a major way. I won't go into great detail as the complexity and secrecy of this project will be involved (government project). I can't even say where. There will be some very technical thermodynamic equations involved as well. But, i will say that we need to find out what humidity (actually the dewpoint) levels were used for the ambients. I can't seem to locate that. I've already tried to find the answer over at that mike holt site but no one knows there either? For example, 100F at 50F dewpoint is much different than 100F with a 78F dewpoint. Just like 100F in vegas is much different than 100F in Louisianna. Any idea what dewpoint levels were used for the ambients in the 2011 nfpa nec 310?

Thanks in advance

Bob (the one legged vet DAMMIT !)
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Old 07-04-2012, 11:52 PM   #2
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We are doing a very complex job in the near future. The ambient temps are going to come into play in a major way. I won't go into great detail as the complexity and secrecy of this project will be involved (government project). I can't even say where.
I know what it is, they are going to give you a new leg from a donkey.
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Old 07-04-2012, 11:54 PM   #3
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Bob (the one legged vet DAMMIT !)
Sure you are, Cletis.
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Old 07-05-2012, 12:41 AM   #4
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Humidex or windchill only has effect on bare skin. The "feels like" temperature. It has no effect on inanimate objects. Is this what you're getting at? It doesn't matter what the humidity is in the electrical code.
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Old 07-05-2012, 12:53 AM   #5
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Are you sure about that?
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Old 07-05-2012, 12:54 AM   #6
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Are you sure about that?
Yes, I'm sure you are Cletis.
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Old 07-05-2012, 01:03 AM   #7
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C&P from Wikipedia:

The heat index (HI) or humiture is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature

Yes I'm sure. Wikipedia says so...lol. Or take a thermodynamics course
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Old 07-05-2012, 01:05 AM   #8
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Ampacity is a portmanteau for ampere capacity defined by National Electrical Safety Codes, in some North American countries. Ampacity is defined as the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration. Also described as current rating or current-carrying capacity, ampacity is the RMS electric current which a device or conductor can continuously carry while remaining within its temperature rating.
The ampacity of a conductor depends on:
its insulation temperature rating;
the electrical resistance of the conductor material;
frequency of the current, in the case of alternating current;
ability to dissipate heat, which depends on conductor geometry and its surroundings;
ambient temperature.

2nd law of T
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Old 07-05-2012, 01:09 AM   #9
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Good, you can copy and paste too. Humidity has no bearing on whether a conductor can dissapate heat. Wind will help cool it convectively, but it will never get below ambient temperature.
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Old 07-05-2012, 01:27 AM   #10
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I was thinking more along the lines of Convective Heat Loss (Qc) which is the heat loss due to fluid (in this case air). I first was going to use the Nusselt number Nu which will obviously be based off the reynolds number Re.

Re=vDy/n

v = component of wind speed

D = diameter of cable

y = specific mass of air

n = dynamic viscosity of air

delta = thermal conductivity of air

Please correct me if I am wrong though
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Old 07-05-2012, 01:34 AM   #11
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You can try to dazzle me all you want with formulas and numbers. Good for you. If you find that humidity affects conductors so much that we would care about it, why is it not in the code book? Why do we care? We don't.
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Old 07-05-2012, 01:37 AM   #12
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You can try to dazzle me all you want with formulas and numbers. Good for you. If you find that humidity affects conductors so much that we would care about it, why is it not in the code book? Why do we care? We don't.
Bionic Sparky is just an expert troll. The scenario in his OP is fictional and should be ignored completely.
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Old 07-05-2012, 01:54 AM   #13
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I'm not trying to "dazzle" anyone. I'm just an average citizen trying to get to the bottom of this. The NEC doesn't really care I guess because wire manufactorers know all about this. Don't take my work for it. Go look at 3.1 under the ampacity chapter in the Mining Cable Engineering Handbook.

Cliff Notes

Ampacity calculation should take into account natural variables such as solar warming, wind, air density, viscosity, and thermal conductivity.

https://docs.google.com/viewer?a=v&q...-hUR5rccCH9u4Q
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Old 07-05-2012, 02:02 AM   #14
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I'm not trying to "dazzle" anyone......Don't take my word for it. Go look at 3.1 under the ampacity chapter in the Mining Cable Engineering Handbook.
If your read the introduction of NFPA-72, it does not cover mining applications, so it's apples and oranges.
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Last edited by CADPoint; 07-05-2012 at 02:04 AM.
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Old 07-05-2012, 09:00 AM   #15
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Originally Posted by CheapCharlie View Post
You can try to dazzle me all you want with formulas and numbers. Good for you. If you find that humidity affects conductors so much that we would care about it, why is it not in the code book? Why do we care? We don't.

Actually humidity can affect the insulation resistance of insulating materials, windings in transformers and motors.
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Old 07-05-2012, 09:04 AM   #16
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I can't even say where.
My guess is Utah.
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