This was discussed somewhere else recently, but I'll be damned if I can find the reference.
My hunch on this topic:
Insulation damage is caused by excessive temperature. When current flows in the conductor, heat is generated, and that causes the temperature to rise.
The thermal ampacity of a conductor (given in table 310.16, for example) is set by the combination of heat being produced in the conductor and heat being dissipated at the surface of the conductor. At the 'thermal ampacity' current level, the temperature of the conductor will rise to its design limit, with heat being produced equal to the heat being dissipated.
Conductors can tolerate a time limited overload, in that it takes _time_ for the generated heat to raise the temperature. The rate of temperature rise is set by how much heat is being produced, and the thermal mass of the conductors being heated. The greater the current density, the faster the heating. In the limit of severe overload, we can ignore the heat dissipation capability of the conductor, and simply consider the current density.
Now compare table 310.16 with the 'conductor properties' table. You will note that as conductors get larger, their permitted current density goes down. Examples:
14ga 20A 4100cmil 205 cmil/amp
10ga 30A 10380cmil 346 cmil/amp
6ga 55A 26250cmil 477 cmil/amp
2ga 95A 66370cmil 699 cmil/amp
For the same _percentage_ overload, smaller conductors will heat up and overheat faster than larger conductors.
My _hunch_ is that smaller conductors will reach thermal damage faster during overload, and thus breakers with lower trip settings are used to provide the necessary overload protection.
-Jon
My hunch on this topic:
Insulation damage is caused by excessive temperature. When current flows in the conductor, heat is generated, and that causes the temperature to rise.
The thermal ampacity of a conductor (given in table 310.16, for example) is set by the combination of heat being produced in the conductor and heat being dissipated at the surface of the conductor. At the 'thermal ampacity' current level, the temperature of the conductor will rise to its design limit, with heat being produced equal to the heat being dissipated.
Conductors can tolerate a time limited overload, in that it takes _time_ for the generated heat to raise the temperature. The rate of temperature rise is set by how much heat is being produced, and the thermal mass of the conductors being heated. The greater the current density, the faster the heating. In the limit of severe overload, we can ignore the heat dissipation capability of the conductor, and simply consider the current density.
Now compare table 310.16 with the 'conductor properties' table. You will note that as conductors get larger, their permitted current density goes down. Examples:
14ga 20A 4100cmil 205 cmil/amp
10ga 30A 10380cmil 346 cmil/amp
6ga 55A 26250cmil 477 cmil/amp
2ga 95A 66370cmil 699 cmil/amp
For the same _percentage_ overload, smaller conductors will heat up and overheat faster than larger conductors.
My _hunch_ is that smaller conductors will reach thermal damage faster during overload, and thus breakers with lower trip settings are used to provide the necessary overload protection.
-Jon
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