Par 11-67 Page 11-508 Harness at Altitude. Since heat loss from the bundle is reduced with increased altit ude, the amount of current should be de-rated. Figure 11-6 gives a curve whereby the altitude derating 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 table 11-10. 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 wh ere there is a need for frequent disconnection. Use of aluminum wire is also discouraged for runs of less than three feet.
Termination hardware should be of the type specifically designed for use with aluminum conductor wiring.
INSTRUCTIONS FOR USE OF ELECTRICAL WIRE CHART.
Correct Size. To select the correct size of electrical wire, two major requirements must be met:
The wire size should be sufficient to prevent an excessive voltage drop while carrying the required current over the required distance.
(See table 11-6, Tabulation Chart, for allowable voltage drops.)
The size should be sufficient to prevent overheating of the wire carrying the required current. (See paragraph 11-69 for allowable current carrying calculation methods.)
Two Requirements. To meet the two requirements (see paragraph 11-66b) in selecting the correct wire size using figure 11-2 or figure 11-3, the following must be known:
The wire length in feet.
The number of amperes of current to be carried.
The allowable voltage drop permitted.
The required continuous or intermittent current.
The estimated or measured conductor temperature.
Is the wire to be installed in conduit and/or bundle?
Is the wire to be installed as a single wire in free air?
Example No. 1. Find the wire size in figure 11-2 using the following known information:
The wire run is 50 feet long, including the ground wire.
Current load is 20 amps.
The voltage source is 28 volts from bus to equipment.
The circuit has continuous operation.
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., 12V, 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.
Par 11-68 Page 11-509 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 and select the next wire size to the right. 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, figure 11-2. Use figure 11-2 for continuous flow and figure 11-3 for intermittent flow.
Procedures in Example No. 1. The procedures in example No. 1, paragraph 11-68c, can be used to find the wire size for any continuous or intermittent operation (maximum two minutes). Voltage (e.g.
12 volts, 28 volts, 115 volts, 200 volts) as indicated on the left scale of the wire chart in figure 11-2 and 11-3.
Example No. 2. Find the wire size required to meet the allowable voltage drop in table 11-6 for a wire carrying current at an elevated conductor temperature using the following information:
The wire run is 15 feet long, including the ground wire.
Circuit current is 20 amps, continuous.
The voltage source is 28 volts.
The wire type used has a 200 °C conductor rating and it is intended to use this thermal rating to minimize the wire gauge. Assume that the method described in paragraph 11-66d(6) was used and the minimum wire size to carry the required current is #14.
Ambient temperature is 50 °C under hottest operating conditions.
STEP 1: Assuming that the recommended load bank testing described in paragraph 11-66d(5) is unable to be conducted, then the estimated calculation methods outlined in paragraph 11-66d(6) may be used to determine the estimated maximum current (Imax). The #14 gauge wire mentioned a bove can carry the required current at 50 °C ambient (allowing for altitude and bundle derating ).
Use figure 11-4a to calculate the Imax a #14 gauge wire can carry.
Find the temperature differences (Tr-Ta) = (200° C-50° C) = 150 °C.
Follow the 150 ° C line to intersect with #14 wire size and reads 47 Amps at bottom of chart (current amperes).
Use figure 11-6, left side of chart reads 0.91 for 20,000 feet, multiple
0.91 x 47 Amps = 42.77 Amps.
Use figure 11-5, find the derate factor for 8 wires in a bundle at 60 percent. First find the number of wires in the bundle (8) at bottom of graph and intersect with the 60 percent curve meet. Read derating factor, (left side of graph) which is 0.6. Multiply
0.6 x 42.77 Amps = 26 Amps.
T 2 = estimated conductor temperature
T 1 = 50 °C ambient temperature
T R = 200 °C maximum conductor rated temperature
I 2 = 20 amps circuit current, continuous
Par 11-68 Page 11-510
FIGURE 11-4a. Single copper wire in free air.
I max = 26 amps (this is the maximum current the #14 gauge wire could carry at 50 °C ambient
L 1=15.5 feet maximum run length for size #14 wire carrying 20 amps from figure 11-2
STEP 2: From paragraph 11-66d (5) and (6), determine the T2 and the resultant maximum wire length when the increased resistan ce of the higher temperature conductor is taken into account.
( )( )TTTT I I R21 1 2=+ − / max
T2 =+ −50 200 50 20 26oo oCC C A A() ( / = 50°C+(150°C)(.877) T2 = 182°C
L= (254.5 C)(L )(234.5 C) + (T )2 1 2°° L 2 = (254.5 C)(15.5F t)(234.5 C) + (182 C)°°° L 2 = 9.5 ft
Par 11-68 Page 11-511
FIGURE 11-4b. Single copper wire in free air.
The size #14 wire selected using the methods outlined in paragraph 11-66d is too small to meet the voltage drop limits from figure 11-2 for a 15 feet long wire run.
STEP 3: Select the next larger wire (size #12) and repeat the calculations as follows:
L 1=24 feet maximum run length for 12 gauge wire carrying 20 amps from figure 11-2
I max = 34.4 amps (this is the maximum current the size #12 wire can carry at 50 °C ambient using calculation methods outlined in paragraph 11-69) T C + (200 C - 50 C) ( 20A / 34.4 C + (150 C)(.762) = 164.4 C L C(LC+( T L C)(24ft)(234.5 C) + (164.4 C) L C)(24ft)398.9 ft 2 oo 2 o 1 o 2 2 o oo 2 o == = = == 50 50 254 5 234 5 254 5 254 5 15 3 oo o o .)
.)
(.
(. .
Par 11-68 Page 11-512
FIGURE 11-5. Bundle derating curves.
Par 11-68 Page 11-513
FIGURE 11-6. Altitude derating curve.
Par 11-68 Page 11-514 The resultant maximum wire length, after adjusting downward for the added resistance associated with running the wire at a higher temperature, is 15.3 feet, which will meet the original 15-foot wire run length requirement without exceeding the voltage drop limit expressed in figure 11-2
COMPUTING CURRENT CARRYING CAPACITY .
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: Refer to 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 to 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: Refer to the “bundle derating curves” in figure 11-5, 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 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 airrating by multiplying
21.5 by 0.52 to get 11.2 amps in-harness rating.
STEP 4: Refer to 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 multiply ing 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 x 6.6 = 165.0 amps) and add to that the number of size #20 wires multiplied by the derated capacity (10 x 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) x 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.
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 11-76 Page 11-515 STEP 1: Refer to the “single wire in free air” curve in figure 11-4a, 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 inters ects wire size line, the free air ratings of size #12 are 68 amps and 61 amps, respectively.
STEP 2: Refer to 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 absci ssa) 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 rati ngs by multiplying 68 amps and 61 am ps by 0.43 to get 29.2 amps and
26.2 amps, respectively.
Par 11-76 Page 11-516 STEP 4: Refer to 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 rati ngs by multiplying 29.2 amps at sea level and 26.6 amps at 20,000 feet by 1.0 and 0.91, respectively, to obtained 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, 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.
11-70.11-75. [RESERVED.]
Par 11-76 Page 11-517 SECTION 6. AIRCRAFT ELECTRICAL WIRE SELECTION
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.
Wire Size. Wires must have sufficient mechanical strength to allow for servi ce 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 ca rrying 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.
Installation Precautio ns for Small Wires. A s 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.
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 C.A.G.E. 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.
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-thinwalled 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.
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.
AIRCRAFT WIRE MATERIALS. Only wire, specifically designed for airborne use, must be installed in aircraft.
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 (Q PL) and being permitted to produce the wire. Aircraft manufacturers who maintain their own wire specifications invariably exercise close control on their approved
Par 11-78 Page 11-518 (and 11-38) 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.
Platings. Bare copper develops a surface oxide coating at a rate dependent on temperature. This oxide film is a poor conductor of electricity and inhibits retermination of wire. Therefore, all airc raft wiring has a coating of either tin, silver, or nick el, that have far slower oxidation rates.
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.
Silver-coated wire is used where temperatures do not exceed 200 °C (392 °F).
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 longterm 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.
Conductor Stranding. Because of fli ght vibration and flexing, conductor round wire should be stranded to minimize fatigue breakage.
Wire Construction Versus Application. T h e m o s t 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 typica lly categorized as being suitable for either “open wiring” or “protected wiring” applications. MIL-W-5088L, 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, Appendix A table A-II lists wires for protected app lications. 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.
Insulations. There are many insulation materials and combinations used on aircra ft electrical wire. An explanation of many of the abbreviations are identified in the glossary.
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.
MIL-W-5088L 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.
Areas designated as severe wind and moisture problem (SWAMP) areas differ
Par 11-78 Page 11-519 (and 11-38) 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 uni que; 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.
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 fi rst three colors. Current military specification, multi-conductor cables, in accordance with MIL-C- 27500, 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 servici ng of the aircraft. At the time of this writing, MIL-C-27500 is being revised to include the older color sequence and could eliminate this problem in the future.
11-79.11-84. [RESERVED.]