![]() The heading that applies to this in the ampacity chart is “DC Resistance At 20° C, MicroOhms/Ft.” This heading is saying, “At 20° C ambient temperature, you can expect the resistance to be this many microOhms per each foot of bus bar.” MicroOhm meaning one millionth of an Ohm, where an Ohm is a measure of impedance and thus resistance. This isn’t such a concern with short bus bars, but as the bar becomes longer, it can become a serious issue. In other words, we don’t want the resistance in the bus bar to be so high that there’s not enough electrical power at the other side. When dealing with DC power, the biggest concern is resistance per foot and the resulting voltage drop. For example, a 1/16 x 1/2 inch bus bar has an area of 0.0312 inches square, and the equivalent circular area of 39.7 circular mils. As a nod to those who continue to deal with wires, the ampacity chart includes a handy conversion between the square inch area of bus bars and the circular mils of wiring. Bus bars, however, are not circular, but square. Wires can be broken down into circular mils (a reference to the area of a circle in thousandths of an inch), where the greater a wire’s gauge/thickness means the more circular mils that are contained in that wire. Therefore, the ampacity chart is still based on the mathematical language developed around wiring. Before copper bus bar became the power transfer component of choice, wires were king in the the power distribution industry. ![]() The discussion of circular mils necessitates a quick history lesson. If you look under the “Area” column, you’ll see two different units of area: square inches and circular mils. So, you want to determine the appropriate bus bar thickness to carry a specific level of electrical current? First, what amperage does the conductor need to carry? What type of current: Direct Current (DC) or Alternating Current (AC)? What kind of conductor material do you want to use? For the material question, see this article that discusses the differences and potential cost savings when deciding between copper and aluminum bus bar. Instead of doing the math from scratch, the ampacity chart conveniently provides important data for each thickness, namely the expected resistance per foot and the amount of expected heat rise. So, as implied by the name, an ampacity chart (sometimes called an ampacity table) is a tool used by engineers to quickly calculate the maximum amperage capacity of a specific thickness of bus bar. An ampere is a unit that describes the level of electrical current carried by the conductor material. ![]() It turns out that ampacity is a combination of the words ampere and capacity. Also, as always, the engineers at Storm would like to remind bus bar designers that ampacity charts are a great guide, but one cannot guarantee that a part will perform as estimated until the part has undergone actual thermal testing and analysis.įor reference, please see Ampacities of Copper Alloy C11000 Bus Bars or Aluminum to Copper Ampacity Comparison Chart What is an Ampacity Chart? There are a number of considerations that need to be recognized beyond the figures given in the chart. However, these charts can only tell you so much. Wire ampacity chart how to#This article presents a brief overview of ampacity charts for both copper and aluminum bus bar, and shows how to interpret the data within. ![]()
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