# Ambient temperature



## B-Nabs (Jun 4, 2014)

Peter Goldwing said:


> Help me understand this ( after more than 30 years being in business lol)
> 
> How is ambient temperature computed.
> If I put wires in a conduit in a FL attic it should be computed at a lesser ampacity then the same situation in Alaska?
> Again how you get the ambient temperature. Thanks


It is the temperature that an area is likely to reach. So yes, a FL attic gets hotter than an AK one.

Sent from my SM-G920W8 using Tapatalk


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## RePhase277 (Feb 5, 2008)

The ampacity table in the NEC, Table 310.15 (B)(16), formerly 310.16, has the temperature correction multiplier on the bottom half.

You find the highest ambient temperature available for the conditions in your area, then find the corresponding temperature in the table. There is a decimal multiplier for that temperature that you multiply the conductor ampacity by.


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## ralpha494 (Oct 29, 2008)

Try here:

https://www.copper.org/applications/electrical/building/pdf/rooftop.pdf


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## ralpha494 (Oct 29, 2008)

I write the cities and max temperatures of cities I would work in right in the code book.


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## telsa (May 22, 2015)

One engineers for the WORST case.

That's why Romex is so de-rated. 

In practical terms, the tables have forced the de-rating upon you.

That's why you don't see fellas up-sizing to #12, instead they just stay with #14 for 15A Residential circuits.

One SHOULD de-rate for hot climates like Las Vegas -- particularly for feeders. 

This reaches extremes in Araby. It's nothing for the ambient temperature to hit 50C in ceilings.


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## Peter Goldwing (Sep 23, 2011)

If you have 4 current carrying conductors then the ampacity is 80% of the 310.15(B)16
The heat correction comes AFTER already derated conductors?


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## Dennis Alwon (May 9, 2009)

310.15(B)(2)(a) would be used to do the temp. If you have thhn then use the 90C column


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## MechanicalDVR (Dec 29, 2007)

telsa said:


> One engineers for the WORST case.
> 
> That's why Romex is so de-rated.
> 
> ...


50 Celsius (122F) really isn't uncommon in attics in many locations.


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## Peter Goldwing (Sep 23, 2011)

Dennis Alwon said:


> 310.15(B)(2)(a) would be used to do the temp. If you have thhn then use the 90C column


Even that has to be derated for higher temperatures. 
What is the current ampacity for(4) #6 in a conduit in Arizona attic 123 F.
for the tables it is good for 75A. 
Because its 4 conductors has to be derated to 60A
From the temp tables has to be derated 76%

76% of which ampacity ,the original one75A, or the 60A one?
Id really like to understand this


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## telsa (May 22, 2015)

The heating derating is not a compounded one.

You're looking to find the materials limit of the conductor.

This is calculated twice. 

Crowding is 'crowding' because it's a form of heat concentration.

BTW, you start with 90C if you're dealing with THHN or THWN-2.


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## Peter Goldwing (Sep 23, 2011)

telsa said:


> The heating derating is not a compounded one.
> 
> You're looking to find the materials limit of the conductor.
> 
> ...


Im a bit slow Tesla I need more help. I DID start with 90C. But under the tables it says to look for temp correction. 
What is the magic number these (4)#6 are good for in that Arizona attic 123 F


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## RePhase277 (Feb 5, 2008)

Peter Goldwing said:


> Im a bit slow Tesla I need more help. I DID start with 90C. But under the tables it says to look for temp correction.
> What is the magic number these (4)#6 are good for in that Arizona attic 123 F


At 123 degree F, the derating factor for THHN is 0.76 and the factor for 4 current carrying conductors is 0.8. #6 copper THHN is 75 amps in the 90 degree column. Applying the corrections, you get 75 x 0.76 x 0.8 = 45.6 amps


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## Peter Goldwing (Sep 23, 2011)

Now thats a straight answer.TY


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