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Chapter 9—Wind Power Plants 9–43 a factor. Assuming the new starting current to be 15 A for the new motor, the voltage drop would be 19 V, which is less than 10% of the 240 V rating. Therefore 12 gauge wire would probably work for the 240 V case where it would not work for the 120 V situation. It should be mentioned that the above analysis is not completely accurate since it ignores the inductance in the wire. This increases the impedance and the voltage drop, so the analysis using only resistance is somewhat optimistic. Example. Suppose that we are forced to use the 120 V motor in the above example and that we want to select a wire size such the voltage drop during starting will be 10 V. The maximum resistance for the round trip would be R max = V I = 10 =0.333Ω/800ft = 0.417Ω/1000ft 30 The smallest conductor that has a resistance less than 0.417 Ω/ 1000 ft is 6 gauge, according to Table C.2. We need four times the volume of copper to meet the voltage drop requirement as we need to meet the thermal requirement. It is obvious from the table that copper is the better conductor per unit area. However, the specific gravity of aluminum is 2.70 and copper has a specific gravity of 8.96, so aluminum is the better conductor per unit mass. Copper tends to cost more per kg than aluminum, so if there were no other factors, aluminum would always be the conductor of choice. There are two factors that keep aluminum from this status. One is that aluminum oxide is not a conductor while copper oxide is. Copper conductors need no protection from the atmosphere at joints and splices, therefore. The second factor is that aluminum tends to cold flow under pressure. That is, when a screw is tightened onto an aluminum conductor, the aluminum tends to flow away from the high pressure point over a period of months or years so that the conductor becomes loose. The exposed aluminum forms a nonconducting layer of aluminum oxide so we have a high impedance connection. Current flowing through the poor connection heats up the surroundings and has been known to start fires. This fire hazard has caused virtually all household wiring to be made of copper. On the other hand, overhead and underground wiring outside of buildings tends to be mostly aluminum for cost reasons. Connections can be made of special compression fittings that both prevent cold flow and protect the joint from the atmosphere. If the connection is properly made, aluminum conductors are just as reliable as copper. Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 9—Wind Power Plants 9–44 Table C.2 Wire Size and Resistance (Adapted from Kloeffler and Sitz, Basic Theory in Electrical Engineering, Macmillan, 1955) Size Diameter Area Ohms per Ohms per AWG mils kcmil 1000 ft 1000 ft or at 20 o C at 20 o C kcmil copper aluminum 30 10.03 0.1005 103.2 169.2 28 12.64 0.1598 64.90 106.4 26 15.94 0.2541 40.81 69.90 24 20.10 0.4040 25.67 42.08 22 25.35 0.6424 16.14 26.46 20 31.96 1.022 10.15 16.63 18 40.30 1.624 6.385 10.47 16 50.82 2.583 4.016 6.581 14 64.08 4.107 2.525 4.139 12 80.81 6.530 1.588 2.603 10 101.9 10.38 0.9989 1.638 8 128.5 16.51 0.6282 1.030 6 162.0 26.25 0.3951 0.6476 4 204.3 41.74 0.2485 0.4073 2 257.6 66.37 0.1563 0.2561 1 289.3 83.69 0.1239 0.2031 1/0 324.9 105.5 0.09827 0.1611 2/0 364.8 133.1 0.07793 0.1277 3/0 409.6 167.8 0.06180 0.1013 4/0 460.0 211.6 0.04901 0.08034 250 500.0 250 0.04148 0.06800 350 591.6 350 0.02963 0.04857 500 707.1 500 0.02074 0.03400 750 866.0 750 0.01383 0.02267 1000 1000 1000 0.01037 0.01700 Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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