I was thinking about bus bar sizing and this occurred to me... when we size wires, we care about the cross section (i.e. diameter/radius) of the wire. For busbars all the ampacity tables are ALSO a function of cross section, a 2mm x 10mm busbar is 20mm2 or 40kcmils, and so on. But if one of the main issues around ampacity is the heat generation and dissipation, doesn't it matter A LOT how the busbar is actually connected? In a regular copper wire with lugs on either end, the current flows more or less uniformly from one end of the wire to the other, and as such the wire heats up more or less uniformly, so this all sort of makes sense. But if we consider a 6-inch copper busbar, the thermal behavior of that thing is going to be VERY different in these two scenarios:
1) we drill two 1/4" holes in the busbar, more or less in the middle, that are an inch apart. Attach lugs/wires/whatever at these points.
2) we drill two 1/4" holes in the busbar, but almost at opposite ends of the bar. Now those holes are more like 5" from each other. Attach lugs here.
In #1, the current is only flowing through a small portion of the bar, right? And if that's true, then the total resistivity of the bar is just a function of the cross section and the linear distance that the current travels, right?
In #2, it's still the same equation (cross section * distance) but the linear distance here is 5x as high, which means I'm generating 5x as much heat, right?
Also, copper is an incredibly good conductor of heat, so I think it's pretty safe to assume that within a few seconds the busbar is going to be more or less the same temperature throughout. In the case where the holes are really close, we're generating less heat, but we're dissipating that heat over a much larger surface of copper (let's assume the busbar is more or less exposed to free air).
So... with all those assumptions, shouldn't the effective current carrying capacity of a busbar with "close" holes be significantly higher than the same amount of current across the same cross-sectional bar with the holes spaced far apart?
There's definitely something I'm missing here - it might just be that no one wastes copper on longer bars than they need to, it might be that the code "just doesn't care if you do that" or any number of other mysteries. But I'd love someone to explain first whether or not I've got the basic physics wrong, and second whether that does or does not ever come into play when sizing these things.
And, just to be clear, I'm not about to try and "get away" with under-sizing anything and burning my house down. But this place seems to be the best combination I can find of people who are willing and able to explain both the theory and the practice, so I'm gonna keep asking ;-)
1) we drill two 1/4" holes in the busbar, more or less in the middle, that are an inch apart. Attach lugs/wires/whatever at these points.
2) we drill two 1/4" holes in the busbar, but almost at opposite ends of the bar. Now those holes are more like 5" from each other. Attach lugs here.
In #1, the current is only flowing through a small portion of the bar, right? And if that's true, then the total resistivity of the bar is just a function of the cross section and the linear distance that the current travels, right?
In #2, it's still the same equation (cross section * distance) but the linear distance here is 5x as high, which means I'm generating 5x as much heat, right?
Also, copper is an incredibly good conductor of heat, so I think it's pretty safe to assume that within a few seconds the busbar is going to be more or less the same temperature throughout. In the case where the holes are really close, we're generating less heat, but we're dissipating that heat over a much larger surface of copper (let's assume the busbar is more or less exposed to free air).
So... with all those assumptions, shouldn't the effective current carrying capacity of a busbar with "close" holes be significantly higher than the same amount of current across the same cross-sectional bar with the holes spaced far apart?
There's definitely something I'm missing here - it might just be that no one wastes copper on longer bars than they need to, it might be that the code "just doesn't care if you do that" or any number of other mysteries. But I'd love someone to explain first whether or not I've got the basic physics wrong, and second whether that does or does not ever come into play when sizing these things.
And, just to be clear, I'm not about to try and "get away" with under-sizing anything and burning my house down. But this place seems to be the best combination I can find of people who are willing and able to explain both the theory and the practice, so I'm gonna keep asking ;-)
