Quote:
Originally Posted by adamj12b
A 1/4" x 3/4" bus bar is good for about 35A continuous with a 30 degree C rise in temp. The same size bar is good for 500A continuous with a 65 degree C rise.
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Here's a chart that might help a bit. Ampacity Chart for Copper Bus Bar |
Keep in mind that this type of chart is of minimal usefulness when designing a motor controller.
The ampacity rating of a bare bus bar is typically rated in still air, with no other heat input. Even there, the rating leaves many cooling parameters unlisted (vertical, horizontal, or on-edge has a big effect on convective cooling). The rating of a wire is usually based on being in a conduit, with long-term heat build-up taken into account.
A motor controller environment is much different. There is a major heat input from the devices, combined with active cooling. You typically design around the device cooling rather than bus bar ampacity. In many cases short wires are significantly thinner than charts would suggest, with the heat being draw out by the surrounding structures.
Modern thermal design has moved from massive heatsinks to immediate dissipation. Fansinks use just enough copper to spread the heat to thin fins, where it's immediately transferred to moving air. Having extra thermal mass isn't bad in a motor controller, where there is often 10 seconds of full-current acceleration, followed by a much lower thermal load. But you still can't design around only thermal mass or you'll overheat on hills.