Ambient temperture Calculation

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Ive been brushing up on my calculations of ampacity. But i forgot how to do the 'ambient' temperture adjustments.

Im looking at table 310.16 of the 2008 code . when it says ".....Based on Ambient temperture of 30 degrees C (86 degrees F)"

And then there are the correction Factors at the bottom for ambient temperture and values you used to multiply with.

Do you multiply the ampacity when the temperture out side is at least 21-25 (the first ambient temp in Celsius) degress of fluctuation?? or is it something different.
 
You find the ambient temp in the table below 310.16 and that will give you your adjustment multiplier. Then multiply the ampacity of the wire with the multiplier and that will give you the ampacity of the wire.
 
GilbeSpark said:
The temp in the table below if for Ambient Temperatures, not temperature fluctuations.
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I suppose thats where im a little off its been a while. . when is the ambient temperture applied.

For example. Lets say i just have a regular house and im runing a 20 amp circuit . I know i can use thhn #12 no problem

But lets say im in a industrial enviroment and it gets REALLY HOT lets say about 300 degrees F. How does the ambient temp come into play to help determine the ampacity to use?? I see the table at the bottom just needs some examples to help understand.
 
brother said:
But lets say im in a industrial enviroment and it gets REALLY HOT lets say about 300 degrees F. How does the ambient temp come into play to help determine the ampacity to use?? I see the table at the bottom just needs some examples to help understand.
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I am not sure what you do at 300 degrees-- You would need a different type of insulation than those listed in T. 310.16. However if the ambient temp were 150 F and you were using #6 thhn wire we would normally use 65amps for this wire size. Now the correction factor.

Use the 90C column (Thhn) and then go to the 150 C row on the right side of the correction col. Follow it to the 90C and we get a correction factor of .58.

Now we must take the 65amps and multiply it by .58. That gives us 37.7 amps. Now the #6 wire is only good for 38 amps. Hope this helps
 
Dennis Alwon said:
I am not sure what you do at 300 degrees-- You would need a different type of insulation than those listed in T. 310.16.
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I don't know what type insulation it is, but the thermocouple in my oven (it's a "K" type) has two wires to it that can handle -450?F (-270?C) to 2500?F (1370?C).

(I don't think I can get my oven that cold... not that it can get any hotter than about +600?F...)

t_27843.jpg


The old control went out so I replaced it with an old laptop, an acquisition system, and a K thermocouple. And don't tell me about all the NFPA79 rules I probably broke in the process...

EDIT: Found the package for the thermocouple. It says the wire type is CHROMEL-P and ALUMEL. Whatever that means. I don't know what its limits are, but the voltage/amperage/gauge are all very small. Edit to correct min and max temp of wire.
 
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Dennis Alwon said:
I am not sure what you do at 300 degrees-- You would need a different type of insulation than those listed in T. 310.16. However if the ambient temp were 150 F and you were using #6 thhn wire we would normally use 65amps for this wire size. Now the correction factor.

Use the 90C column (Thhn) and then go to the 150 C row on the right side of the correction col. Follow it to the 90C and we get a correction factor of .58.

Now we must take the 65amps and multiply it by .58. That gives us 37.7 amps. Now the #6 wire is only good for 38 amps. Hope this helps
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Ok that helps some. the reason i chose 300 degrees was just a number I pulled out the air. I wanted to be sure it was high enuff passed the insulation TEMP rating.. Oh by the way, I noticed that #6 THHN is actually good for 75ampp. ;) But your example sent the point home.

I know some electricians don't even concern themselves with Ambient temperture correction just like some don't derate.

Next example. Lets say im running conduit/circuit on the outside of building located in the Arizona DESERT for a unit that pulls (has a load of) 28 amps. the various temperture in this DESERT is between 110-120 degrees F is the HOTTEST(summertime) it will get.

I want to run #10 copper with type ThWN insulation. I see #10 ThWn is good for 35 amps. I look at the Ambient Temp Correction Factor on the Right hand side for 114-122 F (since the temp is 110-120 the Hottest it will get) and the multiplier is 0.75 Under the column of ThWN copper. I take 0.75 * 35 amps and i get 26.25 amps. So this #10 ThWn is NOT going to work for the location because the load amps i need is 28 amps.

So I check with #8 copper ThWN. It has a of rating 50 amps. 0.75 * 50 amps and i get 37.5 amps. I can use #8.

Is this correct? Let me know if i left something out. And remember i just Chose the Arizona desert to use as an example. I really have NO idea how hot it gets there!!
 
Oh one more question for anyone to answer. I see in the 2008 code Adjustment Factors 310.15(2)(c) Coduits Exposed to Sunlight on ROOFTOPS.

They added a chart to ADD more temperture to the correction factors of the ambient temperture in below the ampacity tables.

Was this really an ISSUE for them to add this in there?? Were there wires MELTING in conduits that ran on the roof tops??
 
brother said:
Were there wires MELTING in conduits that ran on the roof tops??
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I think that is a fair question.

As I understand it the answer to your question is no.

From what I understand some temperature tests of those areas were done and it proved that the temps got higher in exposed roof top conduits.

I don't recall seeing anything mentioned about real life failures.
 
iwire said:
I think that is a fair question.

As I understand it the answer to your question is no.

From what I understand some temperature tests of those areas were done and it proved that the temps got higher in exposed roof top conduits.

I don't recall seeing anything mentioned about real life failures.
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I may agree to disagree with that one Bob .,, but not to flame in here for a min but i did see a near failure on one conduct on the roof and i did have a service call and the wires were pretty much very stiff and kinda near bittle and it was due the sun was beating on black roof and that really build up pretty good heat on the conduct on hot summer day but winter time it don't have much effect at all.

to reficed [ repaired ] i increase the conductors upsized one notch and that slove that issue [ the old conductors were just plainjane TW conductors that why it nearly fritzed up ]

Merci, Marc
 
Dennis Alwon said:
I am not sure what you do at 300 degrees-- You would need a different type of insulation than those listed in T. 310.16. However if the ambient temp were 150 F and you were using #6 thhn wire we would normally use 65amps for this wire size. Now the correction factor.

Use the 90C column (Thhn) and then go to the 150 C row on the right side of the correction col. Follow it to the 90C and we get a correction factor of .58.

Now we must take the 65amps and multiply it by .58. That gives us 37.7 amps. Now the #6 wire is only good for 38 amps. Hope this helps
Click to expand...

I think that you meant 150 F.
 
I think the maximum ambient temperature is established by the AHJ. From the last code class I took (1996 NEC), the instructor said to use 113F for Phoenix, AZ.

Article 310.15(B)(2)(c) of the 2008 NEC will surely help wire manufacturers increase gross sales.
 
Ambient temperature

Ambient temperature

Ambient temp is the temperature of the environment in which the conductor is installed.

For instance - I took a Fluke digital thermometer into the attic a few summers ago....3rd day of daily temps at 88F ...around 3:00 PM. The Fluke measured 134F at the top of the joists and 143F at the bottom of the rafters....and actually that really wasn't ambient because the temps regularly reach into the upper 90s in the summer....so the ambient temp in the attic, running conductors over the tops of the joists would be around 143F [no , I didn't go into the attic when it got that hot]....and up over 150F if you installed the conductors on the bottom of the rafters.

But I haven't seen anyone apply derating to NM.
 
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