WorldWide Drilling Resource

35 WorldWide Drilling Resource ® AUGUST 2014 Know the Whys and Wherefores by Britt Storkson Owner, P2FlowLLC The whys and wherefores are the reasons for doing something. I regularly come across people - intelligent people - who do not know why they are doing this instead of that, why they are using this product instead of that one, etc. Taken to the extreme, this unthinking conformity can be a problem. I think of lemmings jumping off of a cliff into the sea. They don’t think about what they are doing...they just do it. Parents have been known to ask their too-easily-influenced-children: “If every- body else jumped off a cliff, does that mean you should do it too?” Please don’t think I’m against con- formity of any kind. There are very good reasons almost everybody does or doesn’t do certain things. I’m just saying, know why you’re doing what you’re doing. It doesn’t mean you have to know the tiniest detail about what you’re doing, but you do need to understand the gen- eral concept and why you choose one product or method over another. The best example of this condition I can think of right now is wire or cable sizing. I am surprised at how many people do not understand wire and cable sizing - why you choose a certain size of wire for an electrical installation. Sure, they may be able to choose the correct size wire or cable from a chart, or by “plugging in” the values into a computer program which gives them the right answer, but they do not know why one chooses one wire size over another. Why we choose one size of wire over another has to do primarily with cost. Bigger (larger-diameter) wire is more expensive than smaller wire, so we want to use the smallest wire we can “get by with” to keep the costs low. But what does the phrase “get by with” mean? How do we know what is and isn’t acceptable when it comes to wire sizing? It all comes down to electrical resist- ance. Any material has a “resistance” to the flow of electrons, which is electrical current (amperage). Resistance causes a loss of voltage which is dissipated as heat. How much voltage is lost depends on the amount of current flowing through the material, and the resistance of the material. It’s Ohm’s law which states it takes 1 volt to push 1 amp of current through 1 Ohm of resistance. The objective here is to get as much energy through as little wire as possible to keep costs as low as possible. You don’t want to use too big a conductor because, while it doesn’t hurt anything, it greatly increases costs. But you cer- tainly don’t want to use too small a conduc- tor for a given load because it would reduce the voltage to the load too much, and could heat up and possibly start a fire. When pushing current (amperage) through a conductor, voltage is lost (or reduced), just like pressure is lost when pushing water through a pipe. It takes energy to push electrical current through a conductor, just like it takes energy to push water through a pipe. A 3% volt- age loss measured from source to load is considered acceptable because there is minimal heat rejection, and most loads can tolerate plus or minus 10% when it comes to voltage specifications. If you have 120 volts at the source, 3% would be about 3 volts acceptable loss. So 240 volts would be about 7 volts loss, and so on. You can see this voltage loss by measuring the voltage at the “load” (the electrical device like a fan, pump, etc.). First, turn off the load and measure the voltage going to the load. The voltage will be the same as it is at the electrical panel no matter how long the wire is, because there is no current flow. Then turn on the load and measure the voltage again. The voltage will be lower and, if you have sized the wire correctly for a 3% voltage drop, the voltage will be 3% lower when the load is on. How do we size the wire to get this 3% voltage loss? We know 1 circular mil (1/1000 of an inch circular diameter) of copper wire has a resistance of 10.7 Ohms per foot at room temperature of 68ºF (20ºC). So the formula to determine the voltage loss for copper wire is: volt- age loss = (21.4 x feet x amps)/circular mils. We use the constant 21.4 because it is double the resistance (10.7 x 2 = 21.4) and resistance is present as current flows both to and from the load. Any wire manufacturer will have tables that will give you the circular mil value for every American Wire Gauge (AWG) size. So that’s how we choose wire sizes. We need to know the voltage loss through conductor feeding the load, and for this we need to know the electrical load (amperage), length of run, conductor composition (typically copper or alumi- num), and diameter of the conductor. Now you know the whys and wherefores of wire sizing. 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