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9/27/01 AC 43.13-1B CHG 1 11-67. METHODS FOR DETERMINING CURRENT CARRYING CAPACITY OF WIRES. This paragraph contains methods for determining the current carrying capacity of electrical wire, both as a single wire in free air and when bundled into a harness. It presents derating factors for altitude correction and ex amples showing how to use the graphical and tabular data provided for this purpose. In some instances, the wire may be capable of carrying more current than is recommended for the contacts of the related connector. In this instance, it is the contact rating that dictates the maximum current to be carried by a wire. Wires of larger gauge may need to be used to fit within the crimp range of connector con tacts that are adequately rated for the current being carried. Figure 11-5 gives a family of curves whereby the bundle derating factor may be obtained. a. Effects of Heat Aging on Wire Insula- tion. Since electrical wire may be installed in areas where inspection is infrequent over ex tended periods of time, it is necessary to give special consideration to heat-aging character istics in the selection of wire. Resistance to heat is of primary importance in the selection of wire for aircraft use, as it is the basic factor in wire rating. Where wire may be required to operate at higher temperatures due either to high ambient temperatures, high-current load ing, or a combination of the two, selection should be made on the basis of satisfactory performance under the most severe operating conditions. b. Maximum Operating Temperature. The current that causes a temperature steady state condition equal to the rated temperature of the wire should not be exceeded. Rated temperature of the wire may be based upon the ability of either the conductor or the insulation to withstand continuous operation without deg radation. c. Single Wire in Free Air. Determining a wiring system’s current carrying capacity be gins with determining the maximum current that a given-sized wire can carry without ex ceeding the allowable temperature difference (wire rating minus ambient °C). The curves are based upon a single copper wire in free air. (See figures 11-4a and 11-4b.) d. Wires in a Harness. When wires are bundled into harnesses, the current derived for a single wire must be reduced as shown in fig ure 11-5. The amount of current derating is a function of the number of wires in the bundle and the percentage of the total wire bundle ca pacity that is being used. e. Harness at Altitude. Since heat loss from the bundle is reduced with increased al titude, the amount of current should be de rated. Figure 11-6 gives a curve whereby the altitude-derating factor may be obtained. f. Aluminum Conductor Wire. When aluminum conductor wire is used, sizes should be selected on the basis of current ratings shown in table 11-10. The use of sizes smaller than #8 is discouraged (Ref. AS50881A). Aluminum wire should not be attached to en gine mounted accessories or used in areas having corrosive fumes, severe vibration, me chanical stresses, or where there is a need for frequent disconnection. Use of aluminum wire is also discouraged for runs of less than 3 feet (AS50991A). Termination hardware should be of the type specifically designed for use with aluminum conductor wiring. 11-68. INSTRUCTIONS FOR USE OF ELECTRICAL WIRE CHART. a. Correct Size. To select the correct size of electrical wire, two major requirements must be met: Par 11-67 Page 11-25PDF Image | AFS-640
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