Par 11-69 Page 11-33
11-69. COMPUTING CURRENT CARRYING CAPACITY.
a. 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, will be installed in an area where the ambient temperature is 60 °C and the vehicle is capable of operating at a 60,000-foot altitude. Circuit analysis reveals that 7 of the 35 wires in the bundle (7/35 = 20 percent) will be carrying power currents nearly at or up to capacity.
STEP 1: See the “single wire in free air” curves in Figure 11-4a. Determine the change of temperature of the wire to determine free air ratings. Since the wire will be in an ambient of 60 °C and rated at 200 °C, the change of temperature is 200 °C − 60 °C = 140 °C. Follow the 140 °C temperature difference horizontally until it intersects with wire size line on Figure 11-4a. 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: See the “bundle derating curves” in Figure 11-5, the 20-percent curve is selected since circuit analysis indicates 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 abscissa) since there are 35 wires in the bundle, and determine a derating factor of 0.52 (on the ordinate) 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: See the “altitude derating curve” of Figure 11-6, look for 60,000 feet (on the abscissa) since that is the altitude at which the vehicle will be operating. Note that the wire must be derated by a factor of 0.79 (found on the ordinate). Derate the size #22 harness rating by multiplying 8.4 amps by 0.79 to get 6.6 amps. Derate the size #20 harness rating by multiplying 11.2 amps by 0.79 to get 8.8 amps.
STEP 5: To find the total harness capacity, multiply the total number of size #22 wires by the derated capacity (25 × 6.6 = 165.0 amps) and add to that the number of size #20 wires multiplied by the derated capacity (10 × 8.8 = 88 amps) and multiply the sum by the 20-percent harness capacity factor. Thus, the total harness capacity is (165.0 + 88.0) × 0.20 = 50.6 amps. It has been determined that the total harness current should not exceed 50.6 A, size #22 wire should not carry more than 6.6 amps and size #20 wire should not carry more than 8.8 amps.
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.
b. Example 2. Assume a harness (open or braided), consisting of 12, size #12, 200 °C-rated copper wires, will be operated in an ambient of 25 °C at sea level and 60 °C at a 20,000-foot altitude. All 12 wires will be operated at or near their maximum capacity.
Par 11-69 Page 11-34 STEP 1: See the “single wire in free air” curve in Figures 11-4a and b. Determine the temperature difference of the wire to determine free air ratings. Since the wire will be in ambient 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 11-4a until each intersects wire size line, the free air ratings of size #12 are 68 amps and 61 amps, respectively.
STEP 2: See the “bundling derating curves” in Figure 11-5, the 100-percent curve is selected because we know all 12 wires will be carrying full load. Find 12 (on the abscissa) since there are 12 wires in the bundle and determine a derating factor of 0.43 (on the ordinate) 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 26.2 amps, respectively.
STEP 4: See the “altitude derating curve” of Figure 11-6, look for sea level and 20,000 feet (on the abscissa) since these are the conditions at which the load will be 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 rating by multiplying 29.2 amps at sea level and 26.6 amps at 20,000 feet by 1.0 and 0.91, respectively, to obtain 29.2 amps and
23.8 amps. The total bundle capacity at sea level and 25 °C ambient is
29.2x12 = 350.4 amps. At 20,000 feet and 60 °C ambient the bundle capacity is
23.8x12 = 285.6 amps. Each size #12 wire can carry 29.2 amps at sea level, 25 °C ambient or 23.8 amps at 20,000 feet, and 60 °C ambient.
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.
FIGURE 11-2. CONDUCTOR CHART, CONTINUOUS FLOW
CONTINTOUS
CIRCUIT AMPERES
VOLTAGE 200 115 28 14 800 200 100 100 600 150 75 700 400 100 630 360 50 45 560 320 80 40 490 250 70 35 420 240 60 30 350 200 50 25 WIRE LENGTH IN FEET 280 160 40 20 15 20 10 24 22 20 18 16 1/0 2/0 3/0 4/0
VOLTAGE DROP WIRE SIZE NOTE , EXAMPLE # 2 NO. & WIRE AT 20 AMPS EXAMPLE # 1 VOLTAGE DROP CHART TIN-PLATED MIL-W-27759 CONTINUOUS FLOW AT 20° CONDUCTOR LENGTH (LI) IS BASED ON CONDUCTOR TEMPERATURES TEMPERATURE USE FORMULA WHERE T2 = ESTIMATED CONDUCTOR TEMPERATURE *e OF 20 °C TO DETERMINE LENGTH (L2) AT HIGHER CONDUCTOR NO. 12 WIRE AT 20 AMPS NO. 14 WIRE AT 20 AMPS L2 = 25454(1) Par 11-69 Page 11-35
FIGURE 11-3. CONDUCTOR CHART, INTERMITTENT FLOW
CIRCUIT
VOLTAGE AMPERES 200 115 | 28 | 14 1600 400 200 100 1200 300 150 1400 800 200 100 1260 720 180 90 200 1120 640 160 80 980 560 140 70 840 480 120 60 700 400 100 50 560 320 40 240 60 30 200 50 24 160 40 20 16 = 18 16 14 48 12 70 40 10 24 22 14 2 1 1/0 2/0 3/0 4/0 14 8 2
VOLTAGE DROP NOTE LENGTH (LI) IS BASED ON CONDUCTOR TEMPERATURES OF 20° C
WIRE SIZE TO DETERMINE LENGTH (LZ) AT HIGHER CONDUCTOR TEMPERATURE USE FORMULA WHERE 12 - ESTIMATED CONDUCTOR TEMPERATURE °C VOLTAGE DROP CHART (254.5) (L1) INTERMITTENT FLOW AT 20° (234.5) + (T2) TIN-PLATED MIL-W-27759
CONDUCTOR Par 11-69 Page 11-36
FIGURE 11-4A. SINGLE COPPER WIRE IN FREE AIR 300 WIRE SIZE 26 200 TEMPERATURE DIFFERENCE (WIRE RATING MINUS THE AMBIENT = T IN C) 30 10 30 60 70 80 90100
CURRENT AMPERES
A NOT TO BE USED AS SINGLE WIRE Par 11-69 Page 11-37
FIGURE 11-4B. SINGLE COPPER WIRE IN FREE AIR 300 g WIRE SIZE 8 3/0- & 200 TEMPERATURE DIFFERENCE (WIRE RATING MINUS THE AMBIENT = T IN 30 50 60 70 80 90 100 200 300 400 50 600 700 800 9001000
CURRENT AMPERES Par 11-69 Page 11-38
FIGURE 11-5. BUNDLE DERATING CURVES
1.0 CURRENT DERATING FACTOR 0.9 BUNDLE LOADING,
PERCENT -20 40
0.6
0.5
0.4
0.3
0.2
0.1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
NUMBER OF WIRES IN BUNDLE Par 11-69 Page 11-39
Par 11-69 Page 11-40 FIGURE 11-6. ALTITUDE DERATING CURVE 11 -70.–11-75. [RESERVED.]
Par 11-76 Page 11-41 SECTION 6. AIRCRAFT ELECTRICAL WIRE SELECTION
11-76. GENERAL. Aircraft service imposes severe environmental condition on electrical wire.
To ensure satisfactory service, inspect wire annually for abrasions, defective insulation, condition of terminations, and potential corrosion. Grounding connections for power, distribution equipment, and electromagnetic shielding must be given particular attention to ensure that electrical bonding resistance has not been significantly increased by the loosening of connections or corrosion.
a. Wire Size. Wires must have sufficient mechanical strength to allow for service conditions. Do not exceed allowable voltage drop levels. Ensure that the wires are protected by system circuit protection devices, and that they meet circuit current carrying requirements. 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. When used in interconnecting airframe application, #24 gauge wire must be made of high-strength alloy.
b. Installation Precautions for Small Wires. As a general practice, wires smaller than size #20 must be provided with additional clamps, grouped with at least three other wires, and 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 will be subjected to excessive vibration, repeated bending, or frequent disconnection from screw terminations.
c. Identification. 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. Existing installed wire that needs replacement can thereby be identified as to its performance capabilities, and the inadvertent use of a lower performance and unsuitable replacement wire avoided.
(1) In addition to the type identification imprinted by the original wire manufacturer, aircraft wire also contains its unique circuit identification coding that is put on at the time of harness assembly. The traditional “Hot Stamp” method has not been totally satisfactory in recent years when used on modern, ultra-thin-walled installations. 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, serious arcing and surface tracking have damaged wire bundles.
(2) Extreme care must be taken during circuit identification by a hot stamp machine on wire with a 10 mil wall or thinner. Alternative identification methods, such as “Laser Printing” and “Ink Jet,” are coming into increasing use by the industry. When such modern equipment is not available, the use of stamped identification sleeving should be considered on thin-walled wire, especially when insulation wall thickness falls below 10 mils.
11-77. AIRCRAFT WIRE MATERIALS. Only wire specifically designed for airborne use must be installed in aircraft.
Par 11-77 Page 11-42
a. Authentic Aircraft Wire. Most aircraft wire designs are to specifications that require manufacturers to pass rigorous testing of wires before being added to a Qualified Products List (QPL) and being permitted to produce the wire. Aircraft manufacturers who maintain their own wire specifications invariably exercise close control on their approved sources. Such military or original equipment manufacturer (OEM) wire used on aircraft should only have originated from these defined wire mills. Aircraft wire from other unauthorized firms, and fraudulently marked with the specified identification, must be regarded as “unapproved wire,” and usually will be of inferior quality with little or no process control testing. Efforts must be taken to ensure obtaining authentic, fully tested aircraft wire.
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 have 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 strands have 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.
c. Conductor Stranding. Because of flight vibration and flexing, conductor round wire should be stranded to minimize fatigue breakage.
d. Wire Construction Versus Application. 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” applications. MIL-W-5088L (replaced by SAE AS50881, Wiring Aerospace Vehicle), Appendix A, Table A-I lists wires considered to have sufficient abrasion and cut-through resistance to be suitable for open-harness construction. MIL-W-5088L (replaced by SAE AS50881, Wiring Aerospace Vehicle), Appendix A, Table A-II lists wires for protected applications. These wires are not recommended for aircraft interconnection wiring unless the subject harness is covered throughout its length by a protective jacket. 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.
e. Insulation. There are many insulation materials and combinations used on aircraft electrical wire. 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. An explanation of many of the abbreviations is identified in the glossary.
Par 11-78 Page 11-43
11-78. SUBSTITUTIONS. In the repair and modification of existing aircraft, when a replacement wire is required, 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 OAM must be contacted for an acceptable replacement.
a. MIL-W-5088L (replaced by SAE AS50881, Wiring Aerospace Vehicle) Appendix A lists wire types that have been approved for military aerospace applications in open and protected wiring applications. These wires could potentially be used for substitution when approved by the OAM.
b. Areas designated as severe wind and moisture problem (SWAMP) areas differ from aircraft to aircraft but generally are considered to be areas such as wheel wells, near wing flaps, wing folds, pylons, and other exterior areas that may have a harsh 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.
c. The use of current military specification, multiconductor cables in place of OEM installed constructions may create problems such as color sequence. Some civilian aircraft are wired with the older color sequence employing “Red-Blue-Yellow” as the first three colors.
Current military specification, multiconductor cables, in accordance with MIL-DTL-27500 (NEMA WC27500), use “White-Blue-Orange” for the initial three colors. Use of an alternative color code during modification without adequate notation on wiring diagrams could severely complicate subsequent servicing of the aircraft.
TABLE 11-1 2. COMPARABLE PROPERTIES OF WIRE INSULATION SYSTEMS 11 -79.–11-84. [RESERVED.]
Par 11-85 Page 11-44 SECTION 7. TABLE OF ACCEPTABLE WIRES
11-85. AIRCRAFT WIRE TABLE. Tables 11-13 and 11-14 list wires used for the transmission of signal and power currents in aircraft. It does not include special purpose wires such as thermocouple, engine vibration monitor wire, fiber optics, data bus, and other such wire designs. Fire resistant wire is included because it is experiencing a wider application in aircraft circuits beyond that of the fire detection systems.
a. All wires in Tables 11-13 and 11-14 have been determined to meet the flammability requirements of 14 CFR part 25, § 25.1713(c) and the applicable portion of part 1 of appendix F part 25.
b. The absence of any wire from Tables 11-13 and 11-14 are not to be construed as being unacceptable for use in aircraft. However, the listed wires have all been reviewed for such use and have been found suitable, or have a successful history of such usage.
c. Explanations of the various insulation materials mentioned in Table 11-13, by abbreviations, can be found in Appendix 1.
TABLE 11-13. OPEN WIRING Document Voltage Rating (maximum) Rated Wire Temperature (°C) Insulation Type Conductor Type M22759/1 600 200 Fluoropolymer insulated TFE and TFE coated glass Silver coated copper M22759/2 600 260 Fluoropolymer insulated TFE and TFE coated glass Nickel coated copper M22759/3 600 260 Fluoropolymer insulated TFE-glass TFE Nickel coated copper M22759/4 600 200 Fluoropolymer insulated TFE-glass FEP Silver coated copper M22759/5 600 200 Fluoropolymer insulated extruded TFE Silver coated copper M22759/6 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper M22759/7 600 200 Fluoropolymer insulated extruded TFE Silver coated copper M22759/8 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper M22759/9 1000 200 Fluoropolymer insulated extruded TFE Silver coated copper