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9/27/01 AC 43.13-1B CHG 1 SECTION 5. ELECTRICAL WIRE RATING 11-66. GENERAL. Wires must be sized so that they: have sufficient mechanical strength to allow for service conditions; do not exceed allowable voltage drop levels; are protected by system circuit protection devices; and meet circuit current carrying requirements. a. Mechanical Strength of Wires. If it is desirable to use wire sizes smaller than #20, particular attention should be given to the me chanical strength and installation handling of these wires, e.g., vibration, flexing, and termi nation. Wire containing less than 19 strands must not be used. Consideration should be given to the use of high-strength alloy con ductors in small gauge wires to increase me chanical strength. As a general practice, wires smaller than size #20 should be provided with additional clamps and be grouped with at least three other wires. They should also have ad ditional support at terminations, such as con nector grommets, strain relief clamps, shrink able sleeving, or telescoping bushings. They should not be used in applications where they will be subjected to excessive vibration, re peated bending, or frequent disconnection from screw termination. b. Voltage Drop in Wires. The voltage drop in the main power wires from the genera tion source or the battery to the bus should not exceed 2 percent of the regulated voltage when the generator is carrying rated current or the battery is being discharged at the 5-minute rate. The tabulation shown in table 11-6 de fines the maximum acceptable voltage drop in the load circuits between the bus and the utili zation equipment ground. c. Resistance. The resistance of the cur rent return path through the aircraft structure is generally considered negligible. However, this is based on the assumption that adequate TABLE 11-6. Tabulation chart (allowable voltage drop between bus and utilization equipment ground). Nominal system voltage Allowable voltage drop continuous operation Intermittent operation 14 28 115 200 0.5 1 4 7 1 2 8 14 Par 11-66 Page 11-21 bonding to the structure or a special electric current return path has been provided that is capable of carrying the required electric cur rent with a negligible voltage drop. To deter mine circuit resistance check the voltage drop across the circuit. If the voltage drop does not exceed the limit established by the aircraft or product manufacturer, the resistance value for the circuit may be considered satisfactory. When checking a circuit, the input voltage should be maintained at a constant value. Ta bles 11-7 and 11-8 show formulas that may be used to determine electrical resistance in wires and some typical examples. d. Resistance Calculation Methods. Figures 11-2 and 11-3 provide a convenient means of calculating maximum wire length for the given circuit current. (1) Values in tables 11-7 and 11-8 are for tin-plated copper conductor wires. Be cause the resistance of tin-plated wire is slightly higher than that of nickel or silver- plated wire, maximum run lengths determined from these charts will be slightly less than the allowable limits for nickel or silver-plated copper wire and are therefore safe to use. Fig ures 11-2 and 11-3 can be used to derive slightly longer maximum run lengths for silver or nickel-plated wires by multiplying the maximum run length by the ratio of resistance of tin-plated wire, divided by the resistance of silver or nickel-plated wire.PDF Image | AFS-640
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