Current capacity v Temperature

[phuct78]

Hi

I am trying to find out if there is a relationship between the current capacity of busbar and the ambient temperature.

I have a copper busbar of CSA 450mm² that has been calculated to have a current capacity of 570A, in free air, at 20^oC with a 30^oC temp rise. This runs on a 28V system.

What effect, if any, would there be on the current capacity when the ambient temperature is -56^oC or +58^oC.

[exit_failure]

Metals have a positive temperature coefficient. That means that their resistance rises with the temperature. i.e. their current carrying capability drops as the temperature rises. Roughly speaking, this is due to the structure of the metal becoming more disorganized as it heats up, hindering the electron flow.

The temperature coefficient of copper is α=0.00404*1/K (alpha).
Also every metal has a specific conductivity the one for copper is ρ0=1.68×10^(−8) at 20°C (293,15°K) (̥ρ is pronounced "ro")

from there you can calculate the corrected specific conductivity: ̥ρ(T)

Where T is the temperate you want want to know your conductors resistance of and T0 is your reference point (20°C in this case)


After that you can calculate the resistance of you conductor with this formula:

Where ρ is the specific resistivity we just calculated, l the length of your conductor in metres and A the cross-section area in m².

You can find all of this here:
https://en.wikipedia.org/wiki/Temperature_coefficient
https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

[exit_failure]

I almost forgot: The unit of ρ is Ωm.

Also: The temperature drift of copper is relatively high since it is a good thermal conductor and therefore has a higher temp. coeff. Compare that with Constantan, which hasn't very much drift at all - hence the name. Its temperature coefficient is only 0.000008.

[piguy101]

What sort of system runs on 28 V at 500 A? Is this some sort of low voltage motor/generator?

[JS]

Current carrying capability in wires is limited because of the temperature, as the insulation gets degraded, damaged or even melted if you go way out of the maximum range. The bare metal can handle the temperature, it could get red hot and still carry the current fine. As the resistance raises it has higher losses, so it's just about how much losses you can live with and how much temperature surrounding stuff can handle, solder joints melting(you probably don't have soldered connections as soldering such a bar is quite hard), mounting rig temps (insulators, as it's probably mounted to some insulators, if you use ceramic insulators that's a lot of ºK) and other mechanical constrains, expansion, thermal cycles, heat to surrounding components and at the last point melting the bar which is not likely and you are only talking 30ºC raise.

When air temp changes the resistance of the bar changes so the temp raise changes. For lower air temps you will have lower resistance hence lower temp raise, inverse also true. So you would be seeing a bit over 90ºC with 58ºC air temp, the bar won't melt there, are your losses within acceptable ranges? can your insulators handle that temperature? are your surrounding components ok with having this heat source? is your mounting rig compliant enough to handle the expansion? and the harder one, are the thermal cycles going to affect reliability? I think that's what will limit your decisions.

Metals have a positive temperature coefficient. That means that their resistance rises with the temperature. i.e. their current carrying capability drops as the temperature rises. Roughly speaking, this is due to the structure of the metal becoming more disorganized as it heats up, hindering the electron flow.
...

Just a detail, resistivity increases with temp because electrons are moving faster in a random way, so having more changes in directions as collisions are more likely so having a harder time to move around as they want, not because of metal structure changing with temperature. The rest of your post seems fine and instructive, I do disagree in about their current carrying capability dropping, it still can carry the current just with higher losses, which leads to higher temperatures but in free air it stabilizes at some point, as long as it doesn't melts metal is fine, as melting would be probably bad for mechanical reasons, other than that it could still carry the current in the liquid stage as long as it stays together, only once it becomes a gas it can't carry the current any more but I don't think that would be the case.

JS

[exit_failure]

[...]as melting would be probably bad for mechanical reasons[...]

Are you sure about that?