Ambient temperature

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elecold

elecold

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Conductors in an attic could well reach 90 degrees for a few weeks in the summer months, depending on where one lives. Do you need to make an adjustment in conductors size in the off chance it could reach above 86° F?
Is there I minimum time limit?
 
elecold said:
Conductors in an attic could well reach 90 degrees for a few weeks in the summer months, depending on where one lives. Do you need to make an adjustment in conductors size in the off chance it could reach above 86° F?
Is there I minimum time limit?
Click to expand...

There is no NEC minimum time limit.

In my opinion the NEC requires derating in most attic spaces yet no one, including myself does it.
 
big john said:
Equipment ratings are in Communist... er... Celcius.

90°C is 194°F. I hope your attic ain't getting that hot.
Click to expand...

Our conductor ampacity table is based off 30°C (86°F) ambient

The NEC would require derating the conductors in temps above that. If the derating is not that much it may not result in a larger conductor with small sizes.
 
To make things even more interesting if the structure has a basement the load center may be located there where the ambient temperature may be 60degF or just short of 16degC. Keep in mind that breakers arecalibrated to carry their rating at 40degC (104degF) continuously without tripping.
As such a 15a rated breaker in a 60degF ambient will now carry more current. My chart show at 25degC (77degF) will carry 17a which will even more at 60degF, 22a for a 20a rated breaker at 77degF.
Now, consider the cable run through a hot attic? You have to wonder how the cable can be protected at all. Thank goodness for the 75deg/90degC insulated THHN/NMB cable that is now being used today where the older stuff was 60degC.
The
 
templdl said:
To make things even more interesting if the structure has a basement the load center may be located there where the ambient temperature may be 60degF or just short of 16degC. Keep in mind that breakers arecalibrated to carry their rating at 40degC (104degF) continuously without tripping.
As such a 15a rated breaker in a 60degF ambient will now carry more current. My chart show at 25degC (77degF) will carry 17a which will even more at 60degF, 22a for a 20a rated breaker at 77degF.
Now, consider the cable run through a hot attic? You have to wonder how the cable can be protected at all. Thank goodness for the 75deg/90degC insulated THHN/NMB cable that is now being used today where the older stuff was 60degC.
The
Click to expand...

Much of the cable run thru an attic is thru or over the joists, the lowest possible point physically and thermally.

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and 90* for a few weeks? Try more like 130+ June-August here in homes with just soffit/ridge vent cooling.
 
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