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AC 43.13-1B CHG 1 9/27/01 (1) The wire size should be sufficient to prevent an excessive voltage drop while car rying the required current over the required distance. (See table 11-6, Tabulation Chart, for allowable voltage drops.) (2) The size should be sufficient to pre vent overheating of the wire carrying the re quired current. (See paragraph 11-69 for al lowable current carrying calculation methods.) b. Two Requirements. To meet the two requirements (see paragraph 11-66b) in se lecting the correct wire size using figure 11-2 (4) The circuit has continuous opera tion. (5) Estimated conductor temperature is 20 °C or less. The scale on the left of the chart represents maximum wire length in feet to prevent an ex cessive voltage drop for a specified voltage source system (e.g., 14V, 28V, 115V, 200V). This voltage is identified at the top of scale and the corresponding voltage drop limit for continuous operation at the bottom. The scale (slant lines) on top of the chart represents am peres. The scale at the bottom of the chart rep resents wire gauge. STEP 1: From the left scale find the wire length, 50 feet under the 28V source column. STEP 2: Follow the corresponding horizontal line to the right until it intersects the slanted line for the 20-amp load. STEP 3: At this point, drop vertically to the bottom of the chart. The value falls between No. 8 and No. 10. Select the next larger size wire to the right, in this case No. 8. This is the smallest size wire that can be used without ex ceeding the voltage drop limit expressed at the bottom of the left scale. This example is plot ted on the wire chart, figure 11-2. Use figure 11-2 for continuous flow and figure 11-3 for intermittent flow. d. Procedures in Example No. 1 para graph 11-68c, can be used to find the wire size for any continuous or intermittent operation (maximum two minutes). Voltage (e.g. 14 volts, 28 volts, 115 volts, 200 volts) as in dicated on the left scale of the wire chart in figure 11-2 and 11-3. e. Example No. 2. Using figure 11-2, find the wire size required to meet the allowable voltage drop in table 11-6 for a wire carrying or figure 11-3, (1) The (2) The be carried. (3) The permitted. (4) The mittent current. the following must be known: wire length in feet. number of amperes of current to allowable voltage drop required continuous or inter (5) The estimated or measured con ductor temperature. (6) Is the wire to be installed in conduit and/or bundle? (7) Is the wire to be installed as a single wire in free air? c. Example No. 1. Find the wire size in figure 11-2 using the following known infor mation: (1) The wire run is 50 feet long, in cluding the ground wire. (2) Current load is 20 amps. (3) The voltage source is 28 volts from bus to equipment. Page 11-26 Par 11-68PDF Image | AFS-640
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