Circuit Protection and Electrical Systems

LEFT MAIN AC BUS RIGHT MAIN AC BUS AC GH BUS GND SVC BUS SYSTEM ARINC 629 BUSSES APBGCB BTB BTBTIE BUS GCB GHR GSTR GSSR PRI EP Analog control APU GCU BPCU RIGHT GCU LEFT GCU AUTO ISLN ON OFF L BUS TIE L GEN CTRL AUTO ISLN ON OFF R BUS TIE R GEN CTRL ON OFF APU GEN APU GEN RIGHT IDGLEFT IDG L TRU L XFR L MAIN AC L UTIL L DC HOT BAT BAT MAIN BAT CHARGER MAIN BATTERY L FCDC PSA C FCDC PSA R FCDC PSASTANDBY AC STATIC INVERTER BAT 2 CPT FIL INST F/0 FIL INST TRU C1 TRU C2 R TRU R DC GND SVC APU BAT CHARGER APU BATTERY BACKUP GEN CONVERTER R UTIL R XFR R MAIN AC GH DC GH AC GH TRU L IDG BU GEN R IDGBU GEN APU GEN RAT GEN L GCB L TBB PMG APB SEC EPC PRI EPC L BTB R BTB R GCB GHR L CCB R CCB L UB ELCU R UB ELCU R TBB GSTR DC BUS TIE RLY TRU C1 RLY TRU C2 RLY GSSR MAIN BAT. RLY BAT - CPT ISLN RLY CPT - F/0 BUS TIE RLY GND PWR BAT. RLYAC STBY PWR RLY PRIMARY EXT PWR PMG SECONDARY EXT PWR BAT PMG (L1) BAT PMG (R1) BAT PMG (L2, R2) 9-65 1. Two or more separately insulated conductors in the same jacket.

2. Two or more separately insulated conductors twisted together (twisted pair). 3. One or more insulated conductors covered with a metallic braided shield (shielded cable). 4. A single insulated center conductor with a metallic braided outer conductor (radio frequency cable). The term “wire harness” is used when an array of insulated conductors are bound together by lacing cord, metal bands, or other binding in an arrangement suitable for use only in specific equipment for which the harness was designed; it may include terminations. Wire harnesses are extensively used in aircraft to connect all the electrical components.

[Figure 9-111] For many years, the standard wire in light aircraft has been MIL-W-5086A, which uses a tin-coated copper conductor rated at 600 volts and temperatures of 105 °C. This basic wire is then coated with various insulating coatings. Commercial and military aircraft use wire that is manufactured under MIL-W-22759 specification, which complies with current military and FAA requirements. The most important consideration in the selection of aircraft wire is properly matching the wire’s construction to the application environment. Wire construction that is suitable for the most severe environmental condition to be encountered should be selected. Wires are typically categorized as being suitable for either open wiring or protected wiring application.

The wire temperature rating is typically a measure of the insulation’s ability to withstand the combination of ambient temperature and current-related conductor temperature rise. Conductor The two most generally used conductors are copper and aluminum. Each has characteristics that make its use advantageous under certain circumstances. Also, each has certain disadvantages. Copper has a higher conductivity; is more ductile; has relatively high tensile strength; and can be easily soldered. Copper is more expensive and heavier than aluminum. Although aluminum has only about 60 percent of the conductivity of copper, it is used extensively. Its lightness makes possible long spans, and its relatively large diameter for a given conductivity reduces corona (the discharge of electricity from the wire when it has a high potential). The discharge is greater when small diameter wire is used than when large diameter wire is used. Some bus bars are made of aluminum instead of copper where there is a greater radiating surface for the same conductance. The characteristics of copper and aluminum are compared in Figure 9-112.

ConductorsWire single solid conductor Solid conductor Stranded conductor A B Plating Bare copper develops a surface oxide coating at a rate dependent on temperature. This oxide film is a poor conductor of electricity and inhibits determination of wire. Therefore, all aircraft wiring has a coating of tin, silver, or nickel that has far slower oxidation rates. 1. Tin-coated copper is a very common plating material. Its ability to be successfully soldered without highly active fluxes diminishes rapidly with time after manufacture. It can be used up to the limiting temperature of 150 °C. 2. Silver-coated wire is used where temperatures do not exceed 200 °C (392 °F).

3. Nickel-coated wire retains its properties beyond 260 °C, but most aircraft wire using such coated 9-66 Tensile strength (lb-in) Tensile strength for same conductivity (lb) Weight for same conductivity (lb) Cross section for same conductivity (CM) Specific resistance (ohm/mil ft) Characteristic Copper Aluminum 55,000 100 10.6 25,000 40,000 48 160 17 strands has insulation systems that cannot exceed that temperature on long-term exposure. Soldered terminations of nickel-plated conductor require the use of different solder sleeves or flux than those used with tin- or silver-plated conductor. Insulation Two fundamental properties of insulation materials are insulation resistance and dielectric strength. These are entirely different and distinct properties.

Insulation resistance is the resistance to current leakage through and over the surface of insulation materials. Insulation resistance can be measured with a megohmmeter/ insulation tester without damaging the insulation, and data so obtained serves as a useful guide in determining the general condition of the insulation. However, the data obtained in this manner may not give a true picture of the condition of the insulation. Clean, dry insulation having cracks or other faults might show a high value of insulation resistance but would not be suitable for use. Dielectric strength is the ability of the insulator to withstand potential difference and is usually expressed in terms of the voltage at which the insulation fails because of the electrostatic stress. Maximum dielectric strength values can be measured by raising the voltage of a test sample until the insulation breaks down.

The type of conductor insulation material varies with the type of installation. Characteristics should be chosen based on environment, such as abrasion resistance, arc resistance, corrosion resistance, cut-through strength, dielectric strength, flame resistant, mechanical strength, smoke emission, fluid resistance, and heat distortion. Such types of insulation materials (e.g., PVC/nylon, Kapton®, and Teflon®) are no longer used for new aircraft designs, but might still be installed on older aircraft. Insulation materials for new aircraft designs are made of Tefzel ®, Teflon ®/Kapton ®/ Teflon® and PTFE/Polyimide/PTFE. The development of better and safer insulation materials is ongoing.

Since electrical wire may be installed in areas where inspection is infrequent over extended periods of time, it is necessary to give special consideration to heat-aging characteristics 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 loading, or a combination of the two, selection should be made on the basis of satisfactory performance under the most severe operating conditions.

Wire Shielding With the increase in number of highly sensitive electronic devices found on modern aircraft, it has become very important to ensure proper shielding for many electric circuits. Shielding is the process of applying a metallic covering to wiring and equipment to eliminate electromagnetic interference (EMI). EMI is caused when electromagnetic fields (radio waves) induce high frequency (HF) voltages in a wire or component. The induced voltage can cause system inaccuracies or even failure. Use of shielding with 85 percent coverage or greater is recommended. Coaxial, triaxial, twinaxial, or quadraxial cables should be used, wherever appropriate, with their shields connected to ground at a single point or multiple points, depending upon the purpose of the shielding.

[Figure 9-113] The airframe grounded structure may also be used as an EMI shield. Wire Substitutions When a replacement wire is required in the repair and modification of existing aircraft, the maintenance manual for that aircraft must first be reviewed to determine if the original aircraft manufacturer (OAM) has approved any substitution. If not, then the manufacturer must be contacted for an acceptable replacement. 9-67 Areas Designated as Severe Wind & Moisture Problem (SWAMP) SWAMP areas differ from aircraft to aircraft but are usually wheel wells, near wing flaps, wing folds, pylons, and other exterior areas that may have a harsh environment. Wires in these areas have often an exterior jacket to protect them from the environment. Wires for these applications often have design features incorporated into their construction that may make the wire unique; therefore, an acceptable substitution may be difficult, if not impossible, to find. It is very important to use the wire type recommended in the aircraft manufacturer’s maintenance handbook. Insulation or jacketing varies according to the environment. [Figure 9-114] Wire Size Selection Wire is manufactured in sizes according to a standard known as the American wire gauge (AWG). As shown in gauge numbers become larger. Typical wire sizes range from a number 40 to number 0000.

Gauge numbers are useful in comparing the diameter of wires, but not all types of wire or cable can be measured accurately with a gauge. A wire gauge tool may be used to determine the size of an unmarked wire. Larger wires are usually stranded to increase their flexibility. In such cases, the total area can be determined by multiplying the area of one strand (usually computed in circular mils when diameter or gauge number is known) by the number of strands in the wire or cable. Several factors must be considered in selecting the size of wire for transmitting and distributing electric power. 1. Wires must have sufficient mechanical strength to allow for service conditions.

2. Allowable power loss (I2 R loss) in the line represents electrical energy converted into heat. The use of large conductors reduces the resistance and therefore the I2 R loss. However, large conductors are more expensive, heavier, and need more substantial support. 3. If the source maintains a constant voltage at the input to the lines, any variation in the load on the line causes a variation in line current and a consequent variation in the IR drop in the line. A wide variation in the IR drop in the line causes poor voltage regulation at the load. The obvious remedy is to reduce either current or resistance. A reduction in load current lowers the amount of power being transmitted, whereas a reduction in line resistance increases the size and weight of conductors required. A compromise is generally reached whereby the voltage variation at the load is within tolerable limits and the weight of line conductors is not excessive.

4. When current is drawn through the conductor, heat is generated. The temperature of the wire rises until the heat radiated, or otherwise dissipated, is equal to the heat generated by the passage of current through the line. If the conductor is insulated, the heat generated in the conductor is not so readily removed as it would be if the conductor were not insulated. Thus, to protect the insulation from too much heat, the current through the conductor must be maintained below a certain value. When electrical conductors are installed in locations where the ambient temperature is relatively high, the heat generated by external sources constitutes an appreciable part of the total conductor heating. Allowance must be made for the influence of external heating on the allowable conductor current, and each case has its own specific limitations. The maximum allowable operating temperature of insulated conductors varies with the type of conductor insulation being used.

If it is desirable to use wire sizes smaller than #20, particular attention should be given to the mechanical strength and installation handling of these wires (e.g., vibration, flexing, and termination). Wires containing less than 19 strands must not be used. Consideration should be given to the use of high-strength alloy conductors in small-gauge wires to increase mechanical 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 additional support at terminations, such as connector grommets, strain relief clamps, shrinkable sleeving, or telescoping bushings. They should not be used in applications where they are subjected to excessive vibration, repeated bending, or frequent disconnection from screw termination. [Figure 9-116] Current Carrying Capacity In some instances, the wire may be capable of carrying more 9-68 0000 000 00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 460.0 410.0 365.0 325.0 289.0 258.0 229.0 204.0 182.0 162.0 144.0 128.0 114.0 102.0 91.0 81.0 72.0 64.0 57.0 51.0 45.0 40.0 36.0 32.0 28.5 25.3 22.6 20.1 17.9 15.9 14.2 12.6 11.3 10.0 8.9 8.0 7.1 6.3 5.6 5.0 4.5 4.0 3.5 3.1 212,000.0 168,000.0 133,000.0 106,000.0 83,700.0 66,400.0 52,600.0 41,700.0 33,100.0 26,300.0 20,800.0 16,500.0 13,100.0 10,400.0 8,230.0 6,530.0 5,180.0 4,110.0 3,260.0 2,580.0 2,050.0 1,620.0 1,290.0 1,020.0 810.0 642.0 509.0 404.0 320.0 254.0 202.0 160.0 127.0 101.0 79.7 63.2 50.1 39.8 31.5 25.0 19.8 15.7 12.5 9.9 0.166 0.132 0.105 0.0829 0.0657 0.0521 0.0413 0.0328 0.0260 0.0206 0.0164 0.0130 0.0103 0.00815 0.00647 0.00513 0.00407 0.00323 0.00256 0.00203 0.00161 0.00128 0.00101 0.000802 0.000636 0.000505 0.000400 0.000317 0.000252 0.000200 0.000158 0.000126 0.0000995 0.0000789 0.0000626 0.0000496 0.0000394 0.0000312 0.0000248 0.0000196 0.0000156 0.0000123 0.0000098 0.0000078 0.0500 0.0630 0.0795 0.100 0.126 0.159 0.201 0.253 0.319 0.403 0.508 0.641 0.808 1.02 1.28 1.62 2.04 2.58 3.25 4.09 5.16 6.51 8.21 10.40 13.10 16.50 20.80 26.20 33.00 41.60 52.50 66.20 83.40 105.00 133.00 167.00 211.00 266.00 335.00 423.00 533.00 673.00 848.00 1,070.00 0.0577 0.0727 0.0917 0.166 0.146 0.184 0.232 0.292 0.369 0.465 0.586 0.739 0.932 1.18 1.48 1.87 2.36 2.97 3.75 4.73 5.96 7.51 9.48 11.90 15.10 19.00 24.00 30.20 38.10 48.00 60.60 76.40 96.30 121.00 153.00 193.00 243.00 307.00 387.00 488.00 616.00 776.00 979.00 1,230.00 Gauge Number Diameter (mils) Circular (mils) Square inches 25 °C (77 °F) 65 °C (149 °F) Cross Section Ohms per 1,000 ft 9-69 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 contacts that are adequately rated for the current being carried. Figure 9-117 gives a family of curves whereby the bundle derating factor may be obtained.

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 degradation. Single Wire in Free Air Determining a wiring system’s current-carrying capacity begins with determining the maximum current that a given-sized wire can carry without exceeding the allowable temperature difference (wire rating minus ambient °C). The curves are based upon a single copper wire in free air.

[Figure 9-117] Wires in a Harness When wires are bundled into harnesses, the current derived for a single wire must be reduced, as shown in Figure 9-118. The amount of current derating is a function of the number of wires in the bundle and the percentage of the total wire bundle capacity that is being used. Harness at Altitude Since heat loss from the bundle is reduced with increased altitude, the amount of current should be derated. factor may be obtained. Aluminum Conductor Wire When aluminum conductor wire is used, sizes should be selected on the basis of current ratings shown in Figure 9-120.

The use of sizes smaller than #8 is discouraged. Aluminum wire should not be attached to engine mounted accessories or used in areas having corrosive fumes, severe vibration, mechanical stresses, or where there is a need for frequent disconnection. Use of aluminum wire is also discouraged for runs of less than 3 feet. Termination hardware should be of the type specifically designed for use with aluminum conductor wiring. Computing Current Carrying Capacity The following section presents some examples on how to calculate the load carrying capacity of aircraft electrical wire. The calculation is a step by step approach and several graphs are used to obtain information to compute the current carrying capacity of a particular wire.

Example 1 Assume a harness (open or braided) consisting of 10 wires, size 20, 200 °C rated copper, and 25 wires size 22, 200 °C rated copper, is installed in an area where the ambient temperature is 60 °C and the aircraft is capable of operating at a 35,000 foot altitude. Circuit analysis reveals that 7 of the 35 wires in the bundle (7⁄35 = 20 percent) are carrying power currents near or up to capacity. Step 1—Refer to the single wire in free air graph in to determine free air ratings. Since the wire is in an ambient temperature of 60 °C and rated at 200 °C, the change of the temperature is 200 °C – 60 °C = 140 °C. Follow the 140 °C temperature difference horizontally until it intersects with wire size line on Figure 9-117. The free air rating for size 20 is 21.5 amps, and the free air rating for size 22 is 16.2 amps.

Step 2—Refer to the bundle derating curves in Figure 9-118. The 20 percent curve is selected since circuit analysis indicate that 20 percent or less of the wire in the harness would be carrying power currents and less than 20 percent of the bundle capacity would be used. Find 35 (on the horizontal axis), since there are 35 wires in the bundle, and determine a derating factor of 0.52 (on the vertical axis) from the 20 percent curve. Step 3—Derate the size 22 free air rating by multiplying 16.2 by 0.52 to get 8.4 amps in harness rating. Derate the size 20 free air rating by multiplying 21.5 by 0.52 to get 11.2 amps in-harness rating.

Step 4—Refer to the altitude derating curve in Figure 9-119. Look for 35,000 feet (on the horizontal axis) since that is the altitude at which the aircraft is operating. Note that the wire must be derated by a factor of 0.86 (found on the vertical axis). Derate the size 22 harness rating by multiplying 8.4 amps by 0.86 to get 7.2 amps. Derate the size 20 harness rating by multiplying 11.2 amps by 0.86 to get 9.6 amps. Step 5—To find the total harness capacity, multiply the total number of size 22 wires by the derated capacity (25 × 7.2 = 180.0 amps) and add to that the number of size 20 wires multiplied by the derated capacity (10 × 9.6 = 96.8 amps) and multiply the sum by the 20 percent harness capacity factor. Thus, the total harness capacity is (180.0 + 96.0) × 0.20 = 55.2 amps. It has been determined that the total harness current should not exceed 55.2 A, size 22 wire should not carry more than 7.2 amps and size 20 wire should not carry more than 9.6 amps.

9-70 CONTINUOUS CIRCUIT VOLTAGE 200 115 28 14 800 200 100 600 150 75 700 400 100 50 630 360 90 45 560 320 80 40 490 280 70 35 420 240 60 30 350 200 50 25 280 160 40 20 210 120 30 15 175 100 25 12 140 80 20 10 112 64 16 8 98 56 14 7 84 48 12 6 63 36 9 4 56 32 8 49 28 7 42 24 6 3 35 20 5 2 7 4 1 0.5 VOLTAGE DROP 1 1.5 AMPERES 2 3 4 5 7 10 15 20 30 50 70 24 22 20 18 16 14 12 10 8 6 4 2 1 No. 8 wire at 20 amps 1/0 2/0 3/0 100 150 200 300 Wire length (ft) Wire length (ft) 4/0 100 150 200 300 Example 1 Example 2} WIRE SIZE NOTE Voltage drop chart Length (LI) is based on conductor temperature of 20 °C. L2 = (254.5) (L1) Continuous flow at 20° To determine length (L2) at a higher conductor temperature, (234.5) + (T2) Tin-plated MIL-W-27759 conductor use formula in which T2 = estimated conductor temperature °C.

Voltage drop example B No. 12 wire at 20 amps CIRCUIT VOLTAGE 200 115 28 14 1600 400 200 1200 300 150 1400 800 200 100 1260 720 180 90 1120 640 160 80 980 560 140 70 840 480 120 60 700 400 100 50 560 320 80 40 420 240 60 30 350 200 50 24 280 160 40 20 224 128 32 16 196 112 28 14 168 96 24 12 126 72 18 8 112 64 16 98 56 14 84. 48 12 6 70 40 10 4 14 8 2 1 VOLTAGE DROP Voltage drop example A 1 1.5 2 3 4 No. 14 wire at 20 amps 5 7 10 15 20 30 50 70 24 22 20 18 16 14 12 10 WIRE SIZE 8 6 4 2 1 1/0 2/0 3/0 4/0 9-71 Temperature difference (wire rating minus the ambient °C) 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100 300 200 100 90 80 70 60 50 40 30 1 Wire size 26 24 22 20 18 16 14 12 10 1 Temperature difference (wire rating minus the ambient °C) 40 50 60 70 80 90 100 200 300 400 500 600 700 800 900 1000 300 200 100 90 80 70 60 50 40 30 Current Amperes Wire size 8 6 4 1 1/0 2/0 3/0 4/02 NOT TO BE USED AS SINGLE WIRE1 9-72 Current derating factor 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0 Number of wires in bundle Bundle loading percent 20 40 60 80 100 Current derating factor 0 10 20 30 40 50 60 70 80 90 100 1.00 0.95 0.90 0.85 0.80 0.75 0.70 Altitude (x1,000 feet) 9-73 Continuous duty current Max.

(amp) wires in bundles, groups, resistance Wire or harnesses or conduits ohms/1000 feet size Wire conductor temperature rating @ 105 °C @ 150 °C @ 20 °C #8 30 45 1.093 #6 40 61 0.641 #4 54 82 0.427 #2 76 113 0.268 #1 90 133 0.214 #0 102 153 0.169 #00 117 178 0.133 #000 138 209 0.109 #0000 163 248 0.085 aluminum wire. Step 6—Determine the actual circuit current for each wire in the bundle and for the whole bundle. If the values calculated in step 5 are exceeded, select the next larger size wire and repeat the calculations. Example 2 Assume a harness (open or braided), consisting of 12 size 12, 200 °C rated copper wires, is operated in an ambient temperature of 25 °C at sea level and 60 °C at a 20,000-foot altitude. All 12 wires are operated at or near their maximum capacity.

Step 1—Refer to the single wire in free air curve in wire to determine free air ratings. Since the wire is in ambient temperature of 25 °C and 60 °C and is rated at 200 °C, the temperature differences are 200 °C – 25 °C = 175 °C and 200 °C – 60 °C = 140 °C, respectively. Follow the 175 °C and the 140 °C temperature difference lines on Figure 9-116 until each intersects wire size line. The free air ratings of size 12 are 68 amps and 59 amps, respectively. Step 2—Refer to the bundling derating curves in Figure 9-118. The 100 percent curve is selected because we know all 12 wires are carrying full load. Find 12 (on the horizontal axis) since there are 12 wires in the bundle and determine a derating factor of 0.43 (on the vertical axis) from the 100 percent curve.

Step 3—Derate the size #12 free air ratings by multiplying 68 amps and 61 amps by 0.43 to get 29.2 amps and 25.4 amps, respectively. Step 4—Refer to the altitude derating curve of Figure 9-119, look for sea level and 20,000 feet (on the horizontal axis) since these are the conditions at which the load is carried. The wire must be derated by a factor of 1.0 and 0.91, respectively. Step 5—Derate the size 12 in a bundle ratings by multiplying 29.2 amps at sea level and 25.4 amps at 20,000 feet by 1.0 and 0.91, respectively to obtain 29.2 amps and 23.1 amps. The total bundle capacity at sea level and 25 °C ambient temperature is 29.2 × 12 = 350.4 amps. At 20,000 feet and 60 °C ambient temperature, the bundle capacity is 23.1 × 12 = 277.2 amps. Each size 12 wire can carry 29.2 amps at sea level, 25 °C ambient temperature or 23.1 amps at 20,000 feet and 60 °C ambient temperature.

Step 6—Determine the actual circuit current for each wire in the bundle and for the bundle. If the values calculated in Step 5 are exceeded, select the next larger size wire and repeat the calculations. Allowable Voltage Drop The voltage drop in the main power wires from the generation 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 Figure 9-121 defines the maximum acceptable voltage drop in the load circuits between the bus and the utilization equipment ground.

The resistance of the current return path through the aircraft structure is generally considered negligible. However, this is based on the assumption that adequate bonding to the structure or a special electric current return path has been provided that is capable of carrying the required electric current with a negligible voltage drop. To determine 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. Figures 9-122 and 9-123 show formulas that may be used to determine electrical resistance in wires and some typical examples.

Nominal system voltage Allowable voltage drop during continuous operation Intermittent operation 14 28 115 200 1 2 8 14 0.5 1 4 7 bus and utilization equipment ground). 9-74 The following formula can be used to check the voltage drop. The resistance/ft can be found in Figures 9-122 and 9-123 for the wire size. Calculated voltage drop (VD) = resistance/ft × length × current Electric Wire Chart Instructions To select the correct size of electrical wire, two major requirements must be met: 1. The wire size should be sufficient to prevent an excessive voltage drop while carrying the required current over the required distance. [Figure 9-121] 2. The size should be sufficient to prevent overheating of the wire carrying the required current. (See Maximum Operating Temperature earlier in this chapter for computing current carrying capacity methods.) To meet the two requirements for selecting the correct wire Voltage drop Run lengths (feet) Circuit current (amps) Wire size from chart Check calculated voltage drop (VD) = (resistance/feet) (length) (current) 1 107 20 No. 6 VD = (0.00044 ohms/feet) (107 x 20) = 0.942 0.5 90 20 No. 4 VD = (0.00028 ohms/feet) (90 x 20) = 0.504 4 88 20 No. 12 VD = (0.00202 ohms/feet) (88 x 20) = 3.60 7 100 20 No. 14 VD = (0.00306 ohms/feet) (100 x 20) = 6.12 and checking voltage drop.

Maximum Voltage drop Wire size Circuit current (amps) Maximum wire run length (feet) Check calculated voltage drop (VD) = (resistance/feet) (length) (current) 1 No. 10 20 39 VD = (0.00126 ohms/feet) (39 x 20) = 0.98 0.5 --- 19.5 VD = (0.00126 ohms/feet) (19.5 x 20) = 0.366 4 --- 156 VD = (0.00126 ohms/feet) (156 x 20) = 3.93 7 --- 273 VD = (0.00126 ohms/feet) (273 x 20) = 6.88 length and checking voltage drop. size using Figure 9-116, the following must be known: 1. The wire length in feet. 2. The number of amperes of current to be carried. 3. The allowable voltage drop permitted. 4. The required continuous or intermittent current.

5. The estimated or measured conductor 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? Example A Find the wire size in Figure 9-116 using the following known information: 1. The wire run is 50 feet long, including the ground wire. 2. Current load is 20 amps. 3. The voltage source is 28 volts from bus to equipment. 4. The circuit has continuous operation. 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 excessive 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 amperes. The scale at the bottom of the chart represents 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 exceeding the voltage drop limit expressed at the bottom of the left scale. This example is plotted on the wire chart in and Figure 9-116 (bottom) for intermittent flow.

Example B Find the wire size in Figure 9-116 using the following known information: 1. The wire run is 200 feet long, including the ground wire. 2. Current load is 10 amps. 9-75 H215A20 WHITE H246A20 BLUE H217A20 ORANGE 3 inches max 3 inches max 15 inches max 3 inches max White Blue Orange White Blue Orange H215A20 H215A20H215A20 3 inches 3 inches15 inches15 inches A. Multiple wires in a sleeve B. Single wire without sleeve 3. The voltage source is 115 volts from bus to equipment. 4. The circuit has intermittent operation. Step 1—From the left scale, find the wire length of 200 feet under the 115V source column.

Step 2—Follow the corresponding horizontal line to the right until it intersects the slanted line for the 10-amp load. Step 3—At this point, drop vertically to the bottom of the chart. The value falls between No. 16 and No. 14. Select the next larger size wire to the right—in this case, No. 14. This is the smallest size wire that can be used without exceeding the voltage drop limit expressed at the bottom of the left scale. Wire Identification The proper identification of electrical wires and cables with their circuits and voltages is necessary to provide safety of operation, safety to maintenance personnel, and ease of maintenance. All wire used on aircraft must have its type identification imprinted along its length. It is common practice to follow this part number with the five digit/letter Commercial and Government Entity (CAGE) code identifying the wire manufacturer. You can identify the performance capabilities of existing installed wire you need to replace, and avoid the inadvertent use of a lower performance and unsuitable replacement wire.

Placement of Identification Markings Identification markings should be placed at each end of the wire and at 15-inch maximum intervals along the length of the wire. Wires less than 3 inches in length need not be identified. Wires 3 to 7 inches in length should be identified approximately at the center. Added identification marker sleeves should be located so that ties, clamps, or supporting devices need not be removed to read the identification. The wire identification code must be printed to read horizontally (from left to right) or vertically (from top to bottom). The two methods of marking wire or cable are as follows: 1. Direct marking is accomplished by printing the cable’s outer covering. [Figure 9-124B] 2. Indirect marking is accomplished by printing a heat- shrinkable sleeve and installing the printed sleeve on the wire or cables outer covering. Indirectly-marked wire or cable should be identified with printed sleeves at each end and at intervals not longer than 6 feet. [Figure 9-125] The individual wires inside a cable should be identified within 3 inches of their termination. [Figure 9-124A] Types of Wire Markings The preferred method is to mark directly on the wire without causing insulation degradation. Teflon-coated wires, shielded wiring, multiconductor cable, and thermocouple wires usually require special sleeves to carry identification marks.

There are some special wire marking machines available that can be used to stamp directly on the type wires mentioned H215A20 H215A20 H215A20 3 inches 6 ft 6 ft 3 inches marking). 9-76 above. Whatever method of marking is used, the marking should be legible and the color should contrast with the wire insulation or sleeve. Several different methods can be used to mark directly on the wire: hot stamp marking, ink jet printers, and laser jet printers. [Figure 9-126] The hot stamp method can damage the insulation of a newer type of wire that utilizes thin insulators. Fracture of the insulation wall and penetration to the conductor of these materials by the stamping dies have occurred. Later in service, when these openings have been wetted by various fluids or moisture, serious arcing and surface tracking have damaged wire bundles.

Identification sleeves can be used if the direct marking on the wire is not possible. [Figure 9-127] Flexible sleeving, either clear or opaque, is satisfactory for general use. When color-coded or striped component wire is used as part of a cable, the identification sleeve should specify which color is associated with each wire identification code. Identification sleeves are normally used for identifying the following types of wire or cable: unjacketed shielded wire, thermocouple wire, coaxial cable, multiconductor cable, and high temperature wire. In most cases, identification tape can be used in place of sleeving. For sleeving exposed to high temperatures (over 400 °F), materials, such as silicone fiberglass, should be used. Polyolefin sleeving should be used in areas where resistance to solvent and synthetic hydraulic fluids is necessary. Sleeves may be secured in place with cable ties or by heat shrinking. The identification sleeving for various sizes of wire is shown in Figure 9-128.

Wire Installation & Routing Open Wiring Interconnecting wire is used in point-to-point open harnesses, normally in the interior or pressurized fuselage, with each wire providing enough insulation to resist damage from handling and service exposure. Electrical wiring is often installed in aircraft without special enclosing means. This practice is known as open wiring and offers the advantages of ease of maintenance and reduced weight. Wire Groups & Bundles & Routing Wires are often installed in bundles to create a more organized installation. These wire bundles are often called wire harnesses. Wire harnesses are often made in the factory or electrical shop on a jig board so that the wire bundles could be 24 22 20 18 16 14 12 10 8 6 4 2 1 0 00 000 0000 8 6 4 2 1 0 00 000 0000 Wire size Sleeving size AN # AL # No. Nominal ID (inch) 12 11 10 9 8 7 6 4 2 0 3 /8 inch 1 /2 inch 5 /8 inch 3 /4 inch 0.085 0.095 0.106 0.118 0.113 0.148 0.166 0.208 0.263 0.330 0.375 0.500 0.625 0.750 9-77 preformed to fit into the aircraft. [Figure 9-129] As a result, each harness for a particular aircraft installation is identical in shape and length. The wiring harness could be covered by a shielding (metal braid) to avoid EMI. Grouping or bundling certain wires, such as electrically unprotected power wiring and wiring going to duplicate vital equipment, should be avoided. Wire bundles should generally be less than 75 wires, or 11⁄2 to 2 inches in diameter where practicable. When several wires are grouped at junction boxes, terminal blocks, panels, etc., identity of the groups within a bundle can be retained.

Slack in Wire Bundles Wiring should be installed with sufficient slack so that bundles and individual wires are not under tension. Wires connected to movable or shock-mounted equipment should have sufficient length to allow full travel without tension on the bundle. Wiring at terminal lugs or connectors should have sufficient slack to allow two reterminations without replacement of wires. This slack should be in addition to the drip loop and the allowance for movable equipment. Normally, wire groups or bundles should not exceed 1⁄2 inch deflection between support points. [Figure 9-130] This measurement may be exceeded if there is no possibility of the wire group or bundle touching a surface that may cause abrasion. Sufficient slack should be provided at each end to permit replacement of terminals and ease of maintenance; prevent mechanical strain on the wires, cables, junctions, and supports; permit free movement of shock- and vibration- mounted equipment; and allow shifting of equipment, as necessary, to perform alignment, servicing, tuning, removal of dust covers, and changing of internal components while installed in aircraft.

Twisting Wires

When specified on the engineering drawing, or when accomplished as a local practice, parallel wires must sometimes be twisted. The following are the most common examples: 1. Wiring in the vicinity of magnetic compass or flux valve 2. Three-phase distribution wiring 3. Certain other wires (usually radio wiring) as specified on engineering drawings Twist the wires so they lie snugly against each other, making approximately the number of twists per foot as shown in twisting. If the insulation is torn or frayed, replace the wire.

Spliced Connections in Wire Bundles

Splicing is permitted on wiring as long as it does not affect the reliability and the electromechanical characteristics of the wiring. Splicing of power wires, coaxial cables, multiplex bus, and large-gauge wire must have approved data. Splicing of electrical wire should be kept to a minimum and avoided 1/2" maximum with normal hand pressure 9-78