Technical Corner
(written by Joshua Doores this month)
For the article this month we will take a look at overhead conductors, why one would decide to go overhead, and what conductor / cable to select. First, lets discuss why one would decide to go overhead in the first place. Depending on your area of expertise and the industry that you are in, designing a feeder or branch to be overhead may not be something that you do very often. As things often are, the cost is a major deciding factor, and running cables overhead vs underground is no different. Running your conductors and/or cables underground can very often cost 5x or more than going overhead. There are many factors that contribute to underground installations being expensive, but definitely not least among them are material costs. For reference, see the photo below which compares an underground conductor to that of an overhead conductor. Another important factor, however, is reliability. Reliability can also be looked at as the cost of maintenance and the cost of a failure to the infrastructure and how easily it can be repaired. The battle between material cost and reliability often boils down to length of run. At a certain distance, the added cost of an underground feed is too much for the added benefit of reliability. AAC?, AAAC?, ACSR?, Trapezoidal or round?, insulated or not?, triplex? Quadplex? Messenger? What does it all mean?
Figure 1: Photo taken from online PDF, 'Xcel Energy', 'Overhead vs Underground', Information Fact Sheet
When selecting an overhead conductor, it’s important to consider the following: the weight of the conductor / cable, diameter of conductor/cable, strength of conductor / cable as it relates to maximum span, clearances required, and the environment the overhead line will be located.
The weight, diameter, and strength all relate in a very mathematical way to determine the forces experienced by the utility poles and wires themselves. These calculations, along with clearance requirements determine utility pole class, height, and span. The environment can add safety factors from ice and wind in the region. These safety factors increase the overall load of the overhead conductors on the poles, winds can cause an effect known as ‘galloping’, ice can cover the lines adding additional stress, and rain / sleet / snow / sun can lead to corrosion and loss of mechanical strength overtime. Click this link for youtube.com video demonstrating the galloping effect. The National Electric Safety Code (NESC), Sections 25-27 is a great resource for understanding how to safely select utility poles and conductors for the environment. Aluminum is the common material used for overhead lines, granted every situation can call for a different product. ‘AAC’ stands for ‘All Aluminum Conductor’ and is considered to be the most economical solution for overhead distribution due to its low cost and high resistance to corrosion. However, its lower tensile strength does not make it suitable for large transmission or rural distribution. When it comes to shorter distribution, its hard to beat AAC. ‘ACSR’ stands for ‘Aluminum Conductor, Steel Reinforced’ and is considered the standard in most transmission lines. ACSR provides similar conductivity as AAC and similar cost, but with the mechanical strength of a steel core. ACSR can be selected with a wide range of steel content, ranging from 6% - 40%. The more steel the stronger the conductor and the longer it can be spanned across utility poles. The main downsides of ACSR is that its heavier than AAC and also susceptible to corrosion due to the steel component. ‘AAAC’ stands for ‘All Aluminum Alloy Conductor’ and this is one of the higher performing and costing overhead conductors available. AAAC is made from aluminum alloy 6201 which gives it good conductivity, good corrosion resistance, and good strength / weight ratio. And even though AAAC is more expensive than ACSR, trends continue to move in the direction of AAAC and in areas of higher corrosion, such as coastal areas, AAAC is the standard. ‘ACAR’ is another type of overhead conductor that stands for ‘Aluminum Conductor, Alloy 6 | The ROCHESTER ENGINEER MARCH 2021
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