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Chapter 9—Wind Power Plants 9–9 Wire Sizes Once the circuit breaker is selected, we must choose underground conductors with adequate ampacity to go between the turbine and the transformer T 1 . Ampacity refers to the ability of a conductor to safely carry a given current. That is, the surrounding medium must be able to carry away the heat generated in the conductor without the conductor insulation getting too warm. The 1990 National Electrical Code (NEC) listed 27 different types of conductor insulation, ranging from an operating temperature of 60 o C for types TW and UF to 250 o C for types PFAH and TFE. Ampacity of a given size conductor increases with the temperature rating of the conductor insulation. The ampacity also varies with the surrounding medium and its ability to conduct heat away. The ampacity of a conductor buried in wet soil (swamps, coastal regions) is about double the ampacity of the same size conductor buried in very dry soil. The pattern of load flow also makes a significant difference. Starting at ambient temperatures, it requires a day or two of full rated current flow to raise the conductor temperature to its final temperature. This time period is called the thermal time constant. Windfarms rarely operate at rated conditions for more than 24 hours continuously, so only extended periods of high winds would be expected to raise the temperature of windfarm wiring to the thermal limit of the insulation, if the conductors were designed for 100 percent load factor. Given all these factors, it is very difficult to specify a fixed value of ampacity for a given conductor that will apply in every situation. The NEC recognizes this situation and allows engineers with the appropriate training in soils and heat transfer to choose wire sizes based on a detailed analysis for a given installation. Most students in this course do not have such a background, so we will use a NEC table (B-310-10 in the 1990 NEC) that has been prepared for average conditions. Selected values from this table are reproduced in Table 9.2. It applies to the case of three single insulated conductors rated at less than 2000 V which are direct buried in the earth. The ambient earth temperature is assumed to be 20 o C, the load factor is assumed to be 100 percent, and the thermal resistance is assumed to be 90. This value of thermal resistance applies to perhaps 90 percent of the soils in the U.S.A. It varies from about 60 for damp soil (coastal areas, high water table) to about 120 for very dry soil. This table shows ampacities for both copper and aluminum cables. Copper has a higher ampacity for a given wire size but aluminum cables of the same ampacity are perhaps 30 percent cheaper than copper cables. Aluminum cables require special attention at fittings because of the tendency of aluminum to cold flow under pressure. They also require special care because of oxidation problems. At the present time, copper is used for most indoor wiring, except perhaps for the very large sizes, and aluminum is used for most outdoor wiring, both overhead and underground. Cables are sized according to AWG (American Wire Gauge) or kcmil (thousands of circular mils). A circular mil is a measure of area, like m 2 or acres. It is determined by squaring the diameter of a cylindrical conductor when expressed in thousandths of an inch. That is, a cylindrical conductor of diameter 0.5 inch (or 500 mils) would have an area of (500) 2 = Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 9—Wind Power Plants 9–10 250,000 circular mils = 250 kcmil. Table 9.2. Ampacities of three single insulated conductors, 600 V class. COPPER ALUMINUM AWG one two one two kcmil circuit circuits circuit circuits 8 98 92 77 72 6 126 118 98 92 4 163 152 127 118 2 209 194 163 151 1 236 219 184 171 1/0 270 249 210 194 2/0 306 283 239 220 3/0 348 321 272 250 4/0 394 362 307 283 250 429 394 335 308 350 516 474 403 370 500 626 572 490 448 750 767 700 605 552 1000 887 808 706 642 1250 979 891 787 716 1500 1063 965 862 783 1750 1133 1027 930 843 2000 1195 1082 990 897 Older literature will show the units of thousands of circular mils as MCM rather than kcmil. The first ”M” in MCM stands for the Roman unit of one thousand, and would be a translation of the Greek kilo for the same quantity. Changing the name of the unit from MCM to kcmil is a big improvement because it allows a consistent use of multipliers. Using English units is bad enough without also using the Roman notation for multipliers! The AWG numbers get smaller as the cable diameter gets larger, which is always confusing to the newcomer. There are also four AWG sizes larger than the AWG number 1, which is also confusing. These are 0, 00, 000, and 0000, (or 1/0, 2/0, 3/0, and 4/0) pronounced one ought, two ought, three ought, and four ought. See Appendix C for a more detailed discussion of wire sizes and resistance. This ampacity table is for insulation rated at 75 o C. Higher temperature insulations can also be used, as mentioned earlier, but this is a commonly used value, mostly for economic reasons. The headings of the columns refer to “one circuit” and “ two circuits”. The one circuit case for a three-phase system requires three conductors, plus a neutral conductor of the same Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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