Amp rating for bare copper wire

[ggunn]

i'd be really interested to know how the voltage difference between a wire and another object affects its ampacity?
Why would the voltage difference affect the electrons flowing through the conductor?

You do understand, don't you, that ampacity relates to the temperature rating of an insulator on a conductor and not the metal wire itself? I don't think the word even has meaning for bare conductors.

 

[mivey]

You do understand, don't you, that ampacity relates to the temperature rating of an insulator on a conductor and not the metal wire itself? I don't think the word even has meaning for bare conductors.

Of course it does. With no insulation then we are concerned about metal damage instead of insulation damage. We usually stop short of the annealing temperature or some similar criteria.

 

[ggunn]

Of course it does. With no insulation then we are concerned about metal damage instead of insulation damage. We usually stop short of the annealing temperature or some similar criteria.

OK, then see the definition of ampacity in the NEC, and then tell me what the temperature rating is for bare copper. I couldn't find it.

Of course there is some current value for which the copper of a given gauge will melt, but the NEC definition of ampacity (and they made the word up so they can define it however they wish) explicitly refers to the temperature rating of the conductor. If there is no temperature rating, ampacity is meaningless.

 

[kwired]

OK, then see the definition of ampacity in the NEC, and then tell me what the temperature rating is for bare copper. I couldn't find it.

Of course there is some current value for which the copper of a given gauge will melt, but the NEC definition of ampacity (and they made the word up so they can define it however they wish) explicitly refers to the temperature rating of the conductor. If there is no temperature rating, ampacity is meaningless.

Definitions that appear in NEC are what the intended meaning of that word is when it appears in the document, or in some instances it is specified that a particular definition only applies to a specific article/section of the NEC.

The art 100 definition of ampacity does only reference temperature rating of the conductor. And ampacity tables in various locations are primarily concerned with not exceeding insulation temperature rating, they also take termination temperature into consideration and termination temp does effect size of conductor whether it is insulated, bare or covered. Insulation temperature is usually going to be higher then termination temperature on below 1000 volts anyway and conductors must be first sized to termination temp, then adjustments can be made for insulation temp if necessary. A bare conductor (in the rare case it could be used) would still need to be sized according to termination temp which will be well below conductor melting point, but the conductor possibly could need deration if it is expected to see such high temperatures - those kind of applications are going to be pretty rare and are just not covered in the NEC - designing such an application is what engineers are for:happyyes:

 

[fmtjfw]

OK, then see the definition of ampacity in the NEC, and then tell me what the temperature rating is for bare copper. I couldn't find it.

Of course there is some current value for which the copper of a given gauge will melt, but the NEC definition of ampacity (and they made the word up so they can define it however they wish) explicitly refers to the temperature rating of the conductor. If there is no temperature rating, ampacity is meaningless.

2014: 310.15(B)(21) Ampacity of Bare or Covered Conductors in Free Air

 

[kwired]

2014: 310.15(B)(21) Ampacity of Bare or Covered Conductors in Free Air

Good one - but that still is only the ampacity of the portion of conductor in free air. You still have termination temperature rating limiting what this conductor can be used for.

 

[MasterTheNEC]

Good one - but that still is only the ampacity of the portion of conductor in free air. You still have termination temperature rating limiting what this conductor can be used for.

As you would with any conductor listed in the associated tables of 310.15. However, much of the conductors used in T310.15(B)(21) would be used in Art. 399 applications and much of that lugging may be 100% rated. However, as stated in those applications where the limitations are present (as noted by kwired) the installer will adjust accordingly.

 

[mivey]

OK, then see the definition of ampacity in the NEC, and then tell me what the temperature rating is for bare copper. I couldn't find it.

The temperature rating is maximum temperature the conductor can reach without significantly affecting the tensile strength. Beyond that, it will begin to anneal after which it will sag and elongate more than the design limits.

Annealing occurs around 100C for copper and aluminum so specs will usually limit the max temperature to about 75C for a safety margin. There are high temperature conductors available if you need to go beyond that.

Of course there is some current value for which the copper of a given gauge will melt,

Long before that the conductor will have been damaged (softened).

but the NEC definition of ampacity (and they made the word up so they can define it however they wish) explicitly refers to the temperature rating of the conductor. If there is no temperature rating, ampacity is meaningless.

This is not the wild wild west. There are published standards, ASTM I believe.