Conductor Limitations

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llmay

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How is the max. allowable current for a conductor determined? I'm guessing that temperature is it and if this is so, how can one find what the maximum temperature a bare conductor can tolerate before the copper material would become damaged. Even with ambient temperatures and various insulations considered, it still seems to me to boil down to temperature of the conductor.
Your input on this would be appreciated...
 
llmay said:
How is the max. allowable current for a conductor determined? I'm guessing that temperature is it and if this is so, how can one find what the maximum temperature a bare conductor can tolerate before the copper material would become damaged. Even with ambient temperatures and various insulations considered, it still seems to me to boil down to temperature of the conductor.
Your input on this would be appreciated...
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It "boils down" to the heat transfer through the insulation to the environment, and the temperature that the insulation must withstand to dissipate that much heat.

In the steady state all of the (I squared x R) must be dissipated to the environment so the difficult part is the heat transfer calculation. Most insulations (all polymers) will be destroyed before the copper is destroyed.
 
A conductor?s ampacity is based on the ability of the insulation system and its surrounding environment to dissipate the heat generated by current flowing through the conductor. The critical parameter is the temperature of the insulation system. A higher grade insulation material will survive a higher temperature. The ?H? in the name of conductor types, such as ?THHN,? means that that particular insulation material can withstand an additional 15 degrees of elevated temperature. The ?HH? in ?THHN? means it can withstand an extra 30 degrees.

But the ability of a conductor to dissipate heat through its insulation system also depends on where and how it is installed. Conductors in free air can dissipate heat to the surrounding atmosphere very easily, and thus have higher ampacity. Conductors in the company of other conductors have to dissipate their heat in the presence of other heat-generating sources (the other conductors), and thus must be derated.
 
I would generalize Charlie's response slightly.

A conductors ampacity is determined by its maximum allowed temperature, the thermal conductivity to the environment, and the amount of heat generated in the conductor.

The maximum allowed temperature is set by whatever is in contact with the conductor with the lowest permitted temperature. In general, the insulation will fail or suffer vastly accelerated ageing at temperatures well below what the copper itself can handle. But if the insulation system is suitably rated (say ceramic insulating tubes used for things like thermocouples), then the copper will fail before the insulator will fail, and the copper itself sets the maximum allowed temperature. Often the maximum allowed temperature is st not by the conductor insulation rating, but instead the rating of attached devices.

-Jon
 
charlie b said:
The ?H? in the name of conductor types, such as ?THHN,? means that that particular insulation material can withstand an additional 15 degrees of elevated temperature. The ?HH? in ?THHN? means it can withstand an extra 30 degrees.
Click to expand...
Degrees celsius, correct?
 
No, it's degrees F. (I'll not try to spell that word). :D

That is why, for example, Table 310.16 has columns for 60F, 75F, and 90F.

 
You also have to consider the terminations. It's not uncommon for a conductor to have a higher temp. rating than its termination.

Steve
 
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