Par 11-53 Page 11-501 insulation of the electrically “live” components) is especially vulnerable to catastrophic failure as a result of overtorquing.
When replacing relays in alternating current (ac) applications, it is essential to maintain proper phase sequencing. For any application involving plug-in relays, proper engagement of their retaining mechanism is vital.
The proximity of certain magnetically permanent, magnet assisted, coil operated relays may cause them to have an impact on each other. Any manufacturer’s recommendations or precautions must be closely followed.
LOAD CONSIDERATIONS.
When switches or relays are to be used in applications where current or voltage is substantially lower than rated conditions, additional intermediate testing should be performed to ensure reliable operation. Contact the manufacturer on applications different from the rated conditions.
OPERATING CONDITIONS FOR SWITCHES AND RELAYS. Switches and relays should be compared to their specification rating to ensure that all contacts are made properly under all conditions of operation, including vibration equivalent to that in the area of the aircraft in which the switch or relay is to be installed.
11-57.11-65. [RESERVED.]
Par 11-66 Page 11-502
SECTION 5. ELECTRICAL WIRE RATING
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.
Mechanical Strength of Wires. 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. Wire 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 will be subjected to excessive vibration, repeated bending, or frequent disconnection from screw termination.
Voltage Drop in Wires. 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 table 11-6 defines the maximum acceptable voltage drop in the load circuits between the bus and the utilization equipment ground.
Resistance. The resistance of the current 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
12 28 115 200
0.5 1 4 7
1 2 8 14
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. Wh en checking a circuit, the input voltage should be maintained at a constant value.
Tables 11-7 and 11-8 show formulas that may be used to determine electrical resi stance in wires and some typical examples.
Resistance Calculation Methods. Figures 11-2 and 11-3 provide a convenient means of calculating maximum wire length for the given circuit current.
Values in tables 11-7 and 11-8 are for tinplated copper conductor wires. Because 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. Figures 11-2 and 11-3 can be used to derive slightly longer maximum run lengths for silver or nickelplated 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.
TABLE 11-7. Examples of determining required wire size using figure 11-2.
Voltage drop
Run Lengths
Circuit Current Wire size from chart
Checkcalculated voltage drop = (Resistance /Ft) (Length) (Current) = 1 volt
100 feet
20 amps No. 6
(.000445 ohms/ft)
0.5 volt
4 volt
50 feet
100 feet
40 amps
No. 2
No. 12 (100 ft) (20 amps)= .89 volts
(.000183 ohms/ft) (50 ft) (40 amps)= .366 volts
(.00202
Par 11-66 Page 11-503
7 volt
100 feet amps
20 amps
No. 14 See Note 1 ohms/ft) (100 ft) (20 amps)=
4.04 volts
(.00304 ohms/ft) (100 ft) (20 amps)=
6.12 volts
Note #1: #14 Wire should be routed separately for this current rating.
TABLE 11-8. Examples of determining maximum run length using figure 11-3.
Maximum Voltage drop
Wire Size
Circuit Current Maximum Wire Run Length Check-calculated voltage drop = (Resistance/Ft) (Length)(Current)= 1 volt
0.5 volt
4 volt
7 volt No. 10
- - - -
- - - -
- - - - 20 amps
- -
- -
- - 39 feet
19.5 feet
156 feet
273 feet
(.00126 ohms/ft) (39 ft)(20 amps)= .98 volts
(.00126 ohms/ft)
(19.5 ft) (20 amps)= .366 volts
(.00126 ohms/ft) (156 ft) (20 amps)=
3.93 volts
(.00126 ohms/ft) (273 ft) (20 amps)=
6.88 volts As an alternative method or a means of checking results from figure 11-2, resistance for a given wire size can be read from table 11-9 and multiplied by wire run length and circuit current.
Voltage drop calculations for aluminum wires can be accomplished by multiplying the resistance for a given wire size, defined in table 11-10, by the wire run length and circuit current.
When the estimated or measured conductor temperature (T 2) exceeds 20 °C, such as in areas having elevated ambient temperatures or in fully loaded power-feed wires, the maximum allowable run length (L 2), must be shortened from L 1 (the 20 °C value) using the following formula for copper conductor wire:
L CLCT2 1 2254 5234 5= °°+(.) ( )(.) ( )
For aluminum conductor wire, the formula is:
L C LCT2 1 2258 1238 1= °°+(. ) ( )(. ) ( )
These formulas use the reciprocal of each material’s resistivity temperature coefficient to take into account increased conductor resist ance resulting from operation at elevated temperatures.
To determine T 2 for wires carrying a high percentage of their current carrying capability at elevated temperatures, laboratory testing using a load bank and a high-temperature chamber is recommended. Such tests should be run at anticipated worse case ambient temperature and maximum current-loading combinations.
1 1/0 2/0 3/0 4/0 LENGTH (LI) IS BASED ON CONDUCTOR TEMPERATURES OF 20 °C TO DETERMINE LENGTH (L2) AT HIGHER CONDUCTOR TEMPERATURE USE FORMULA WHERE T2 = ESTIMATED L2= (254.5) (LI) (234.5) + (12) AMPERES 10 12 NOTE VOLTAGE DROP CHART CONTINOUS FLOW AT 20° CONDUCTOR TEMPERATURE °C TIN-PLATED MIL-W-27759 18 16 WIRE SIZE CONDUCTOR 20 EXAMPLE # 1 EXAMPLE # 2 NO 8 WIRE AT 20 AMPS NO 12 WIRE AT 20 AMPS NO 14 WIRE AT 20 AMPS CONTINOUS CIRCUIT VOLTAGE 30 20 001 910010 1O.5 VOLTAGE DROP
WIRE LENGTH IN FEET FIGURE 11-2. Conductor chart, continuous flow.
Par 11-66 Page 11-504
AMPERES 1 1/0 2/0 3/0 4/0 NOTE LENGTH (LI) IS BASED ON CONDUCTOR TEMPERATURES OF 20° C TO DETERMINE LENGTH (L2) AT HIGHER CONDUCTOR TEMPERATURE USE FORMULA WHERE T2 = ESTIMATED CONDUCTOR TEMPERATURE °C (254.5) (LI) L2= (234.5) + (T2) 18 16 20 VOLTAGE DROP CHART INTERMITTENT FLOW AT 20° TIN-PLATED MIL-W-27759
WIRE SIZE CONDUCTOR 00 CIRCUIT VOLTAGE 115 240 112 48 8 2 VOLTAGE DROP
WIRE LENGTH IN FEET FIGURE 11-3. Conductor chart, intermittent flow.
Par 11-66 Page 11-505
Par 11-66 Page 11-506 TABLE 11-9. Current carrying capacity and resistance of copper wire.
Wire Size Continuous duty current (amps)-Wires in bundles, groups, harnesses, or conduits. (See Note #1) Max. resistance ohms/1000ft@20° C Nominal conductor Wire Conductor Temperature Rating tin plated conductor area - 105°C 150°C 200°C (See Note #2) circ.mils 24 22 20 18 16 14 12 10 8 6 4 2 1 0 00 000 0000
2.5 3 4 6 7 10 13 17 38 50 68 95 113 128 147 172 204 4 5 7 9 11 14 19 26 57 76 103 141 166 192 222 262 310 5 6 9 12 14 18 25 32 71 97 133 179 210 243 285 335 395
28.40
16.20
9.88
6.23
4.81
3.06
2.02
1.26
0.70
0.44
0.28
0.18
0.15
0.12
0.09
0.07
0.06 475 755 1,216 1,900 2,426 3,831 5,874 9,354 16,983 26,818 42,615 66,500 81,700 104,500 133,000 166,500 210,900 Note #1: Rating is for 70°C ambient, 33 or more wires in the bundle for sizes 24 through 10, and 9 wires for size 8 and larger, with no more t han 20 percent of harness current carrying capacity being used, at an operating altitude of 60,000 feet. For rating of wires under other conditions or configurations see paragraph 11-69.
Note #2: For resistance of silver or nicke l-plated conductors see wire specifications.
TABLE 11-10. Current carrying capacity and resistance of aluminum wire.
Wire Size Continuous duty current (amps) Wires in bundles, groups or harnesses or conduits (See table 11-9 Note #1)
Max. resistance ohms/1000ft Wire conductor temperature rating @20°C 105°C 150 °C 8 6 4 2 1 0 00 000 0000 30 40 54 76 90 102 117 138 163 45 61 82 113 133 153 178 209 248
1.093
0.641
0.427
0.268
0.214
0.169
0.133
0.109
0.085 Note: Observe design practices described in paragraph 11-67 for aluminum conductor
Par 11-66 Page 11-507 Approximate T 2 can be estimated using the following formula:
TT TT I I R21 1=+ − () ( / max2 )
Where:
T1 = Ambient Temperature
T2 = Estimated Conductor Temperature
TR = Conductor Temperature Rating
I2 = Circuit Current (A=Amps)
Imax = Maximum Allowable Current
(A=Amps) at TR
This formula is quite conservative and will typically yield somewhat higher estimat ed temperatures than are likely to be encountered under actual operating conditions.
METHODS FOR DETERMINING CURRENT CARRYING CAPACITY OF WIRES. This paragraph contains methods for determining the current carrying capacity of electrical wire, both as a single wire in free air and when bundled into a harness. It presents derating factors for altitude correction and examples showing how to use the graphical and tabular data provided for this purpose. In some instances, the wire may be capable of carrying more 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 11-5 gives a family of curves whereby the bundle derating factor may be obtained.
Effects of Heat Aging on Wire Insulation. Since electrical wire may be installed in areas where inspection is infrequent over extended periods of time, it is necessary to give speci al consideration to heataging 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.
Maximum Operating Temperature. The current that causes a temperature steady st ate condition equal to the rated temperature of the wi re should not be exceeded.
Rated temperature of the wire may be based upon the ability of either the conduc tor or the insulation to withstand continuous operation without degradation.
Single Wire in Free Air. Determining a wiring system’s current carryi ng 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. (See figures 11-4a and 11-4b.)
Wires in a Harness. When wires are bundled into harnesses, the current derived for a single wire must be reduced as shown in figure 11-5. 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.
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.