Wire sizing

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Smart $

Esteemed Member
Location
Ohio
And I am saying that even the air immediately adjacent to the insulation must be lower than the insulation temperature for any heat transfer to take place across the interface.
It is... but by how much? IS that difference significant? A degree... two... when one molecule away?

The closest significantly lower temperature air may be on the outside of the conduit containing the referenced conductor. The air inside the conduit is much closer to the temperature of the conductor.

No matter what, it still amounts to the conductor operating temperature that is of issue here... not the ambient air temperature.
 

Smart $

Esteemed Member
Location
Ohio
Which is exactly why I took issue with your statement about the *ambient temperature* reaching 90F.
And what is the definition of ambient temperature?

Just ambient is: of or relating to the immediate surroundings of something.

Insulation temperature can very well be included in ambient with respect to temperature surrounding a conductor, and the surrounding bounds can also be the limit to the insulation, as the distance is ambiguous.

How far you can or will go to include something within the physical bounds of ambient anything is a figment of one's imagination until you get on the same page with cohorts. :lol:
 

Carultch

Senior Member
Location
Massachusetts
And what is the definition of ambient temperature?

Just ambient is: of or relating to the immediate surroundings of something.

Insulation temperature can very well be included in ambient with respect to temperature surrounding a conductor, and the surrounding bounds can also be the limit to the insulation, as the distance is ambiguous.

How far you can or will go to include something within the physical bounds of ambient anything is a figment of one's imagination until you get on the same page with cohorts. :lol:

I understand "ambient temperature" to mean the temperature of the surrounding air in the immediate environment of the circuit, which is the temperature that the immediate environment would be, where not subject to any part of the circuit/raceway/wiring method, direct sunlight or any other localized heat source or coolant device.

So air temperature, average temperature, temperature of the average surfaces surrounding the environment...etc.
 

Smart $

Esteemed Member
Location
Ohio
I understand "ambient temperature" to mean the temperature of the surrounding air in the immediate environment of the circuit, which is the temperature that the immediate environment would be, where not subject to any part of the circuit/raceway/wiring method, direct sunlight or any other localized heat source or coolant device.

So air temperature, average temperature, temperature of the average surfaces surrounding the environment...etc.
And where do you find this definition in the NEC?

You run a circuit into and across a room and the circuit powers a full time heater that maintains the room at a super balmy 68°C. Without this circuit or heater in the room, the temperature would average 34°C from heating effects which are external to the room. What is the ambient temperature of the room?
  1. 68°C
  2. 34°C
  3. 51°C
  4. None of the above
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
And where do you find this definition in the NEC?

You run a circuit into and across a room and the circuit powers a full time heater that maintains the room at a super balmy 68°C. Without this circuit or heater in the room, the temperature would average 34°C from heating effects which are external to the room. What is the ambient temperature of the room?
  1. 68°C
  2. 34°C
  3. 51°C
  4. None of the above

Um, 34 degrees C when the current through the circuit is shut off and the room and everything in it has cooled down. When you turn the heater on the ambient temperature goes to 68 degrees C and the wire gets hotter than that after things settle out, depending on how much current is going through the wire and assuming that the room is big enough so that the heating from the wire does not change the temperature of the room. Do I get a cookie?

Any amount of current you run through a conductor raises its temperature from what it already is when it has reached thermal equilibrium with its environment (its ambience, if you will). At 90 degrees C ambient temperature the allowable current through a 90 degree C rated conductor (its temperature adjusted ampacity) should be zero amps. I haven't done this, but it seems to me that if you plot and extrapolate the derate factors for elevated temperatures for a given temperature rated conductor, it should pass through zero when the ambient temp equals the rating of the conductor insulation.

Statics is easy, dynamics is hard.
 

Smart $

Esteemed Member
Location
Ohio
Um, 34 degrees C when the current through the circuit is shut off and the room and everything in it has cooled down. When you turn the heater on the ambient temperature goes to 68 degrees C and the wire gets hotter than that after things settle out, depending on how much current is going through the wire and assuming that the room is big enough so that the heating from the wire does not change the temperature of the room. Do I get a cookie?

Any amount of current you run through a conductor raises its temperature from what it already is when it has reached thermal equilibrium with its environment (its ambience, if you will). At 90 degrees C ambient temperature the allowable current through a 90 degree C rated conductor (its temperature adjusted ampacity) should be zero amps. I haven't done this, but it seems to me that if you plot and extrapolate the derate factors for elevated temperatures for a given temperature rated conductor, it should pass through zero when the ambient temp equals the rating of the conductor insulation.

Statics is easy, dynamics is hard.
Want to wait until Carultch replies before discussing... if you don't mind(?)
 

Carultch

Senior Member
Location
Massachusetts
Um, 34 degrees C when the current through the circuit is shut off and the room and everything in it has cooled down. When you turn the heater on the ambient temperature goes to 68 degrees C and the wire gets hotter than that after things settle out, depending on how much current is going through the wire and assuming that the room is big enough so that the heating from the wire does not change the temperature of the room. Do I get a cookie?

Any amount of current you run through a conductor raises its temperature from what it already is when it has reached thermal equilibrium with its environment (its ambience, if you will). At 90 degrees C ambient temperature the allowable current through a 90 degree C rated conductor (its temperature adjusted ampacity) should be zero amps. I haven't done this, but it seems to me that if you plot and extrapolate the derate factors for elevated temperatures for a given temperature rated conductor, it should pass through zero when the ambient temp equals the rating of the conductor insulation.

Statics is easy, dynamics is hard.

I would go by a worst case scenario approach, assuming that equipment operates as intended, and assuming equipment might fail.


If a room is heated to 68C, averaging throughout the walls and air, which is very extreme, then 68C would be the ambient temperature for circuit calculations. Although I could see this in industrial applications with high temperature furnaces operating in the room.


If a room is cooled to -10C, I would use the temperature that the room would be, if the cooling system were to fail. The outdoor ambient temperature. Unless the circuit is guaranteed not to operate until the room is at its low temperature, I wouldn't take credit for artificially low ambient temperatures.
 

Smart $

Esteemed Member
Location
Ohio
I would go by a worst case scenario approach, assuming that equipment operates as intended, and assuming equipment might fail.


If a room is heated to 68C, averaging throughout the walls and air, which is very extreme, then 68C would be the ambient temperature for circuit calculations. Although I could see this in industrial applications with high temperature furnaces operating in the room.


If a room is cooled to -10C, I would use the temperature that the room would be, if the cooling system were to fail. The outdoor ambient temperature. Unless the circuit is guaranteed not to operate until the room is at its low temperature, I wouldn't take credit for artificially low ambient temperatures.
That's the proper approach... you take the highest temperature scenario whether it be by design or by nature.

And a cookie to ggunn, too, BTW.

But you bring up a point in that there are temperature variations within the room where the circuit is run. The higher the run, usually the warmer it is... in a closed room. So ultimately it is not the average temperature which causes a fault. It's the hottest spot that will likely degrade first. So it doesn't matter if the room average temperature at 5'AFF is 68°C if it is 75°C at 10'AFF where the circuit is run. The whole point of this OT jaunt is that the operating temperature of the conductor is what matters... not the ambient temperature, as averaged at some point somewhere in a space that no one said is the proper design space to begin with.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Here's how I think it got to be this way...

Conditions of use are beyond the terminal enclosure... and where derating applies. The conductors must have the ability to carry the max calculated current, so no factoring for continuous loads.

In terminal enclosures, the NEC so-called conditions of use, other than termination temperature limitation, do not apply. Max' calculated current is padded to 125% continuous load to prevent the terminations from ever operating at the limit for an extended period under normal operating conditions.

You might say what establishes the normal operating conditions? In general the ambient temperature must be not more than the maximum operating temperature minus the rated temperature rise of the equipment. Some equipment provide details on how to derate ratings where extreme cases violate this approach, such as mounting a panelboard on a rooftop exposed to sunlight....

Which brings up another point...

After you establish the wire size based on the continuous use and conditions of use considerations (70 degree ampacity for continuous use and 90 degree ampacity for conditions of use for 90 degree wire), you then must verify that the conductors selected are adequately protected by the OCPD, which is the next size up from 125% of the inverter maximum output current. The conductor ampacity must be equal to or greater than the next size down OCPD. OK, but which ampacity?

It seems to me that you should use the conditions of use ampacity at 90 degrees. This is a real world case for me, if it's one I need to upsize the conductors and if it's the other I don't.
 

Smart $

Esteemed Member
Location
Ohio
Which brings up another point...

After you establish the wire size based on the continuous use and conditions of use considerations (70 degree ampacity for continuous use and 90 degree ampacity for conditions of use for 90 degree wire), you then must verify that the conductors selected are adequately protected by the OCPD, which is the next size up from 125% of the inverter maximum output current. The conductor ampacity must be equal to or greater than the next size down OCPD. OK, but which ampacity?

It seems to me that you should use the conditions of use ampacity at 90 degrees. This is a real world case for me, if it's one I need to upsize the conductors and if it's the other I don't.
Yes, you use the "field wiring" ampacity... i.e. the derated 90°C value. You'd have to rethink that if the wire never leaves the "box" though. :slaphead:
 
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