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bus bar heating / ampacity question


New Member
May 13, 2020
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 ;-)
Wires have ratings because of the insulation. The actual copper conductors can carry far more current than rated... you just set the insulation on fire.

Bus bars can carry more current than wires since there's no insulation to melt/ignite, and they can radiate to ambient faster.

If you're generating so much heat that you feel the need to add additional length to add as cooling fins, that copper would be much better spent increasing the cross sectional area between the lugs. Less heat due to 1/5th the resistance.
Yep that all makes sense but I guess I still dont understand how the length of the bus bar isn't a part of the ampacity calculation? When I find specs like "195A @ 50° rise" or whatever, how is the length of the bar assumed to impact that?
Yep that all makes sense but I guess I still dont understand how the length of the bus bar isn't a part of the ampacity calculation? When I find specs like "195A @ 50° rise" or whatever, how is the length of the bar assumed to impact that?

First, your scenario isn't done. It makes no sense. It's literally wasteful. Not sure if you've bought a lot of copper, but I haven't... because it was too expensive. Copper bus bars would have been a couple thousand for my battery. I went with Aluminum instead. Thicker with more conductivity than the planned copper, and about 40% the price. Easier to drill holes in too.

I digress.

The length isn't considered because it's offset by surface area.

Cross section, A

Two bus bars:

1" length of A = R
6" length of A = 6R

Additional heat of the 6" length is offset by the greater surface area.

It's not exact, but it's close enough, and it's easier to just look at a given A and temperature increase.
So the whole point was to figure out whether or not I was correctly understanding the physics, not whether the application has any actual relevant value. I don't know much, but I know putting two lugs/posts a foot apart makes a lot less sense than putting them 1" apart on a much thicker busbar. But the question was an example / thought experiment far more than any actual "should I do this". Somehow that got lost in the translation I think...

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