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. 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 is manufactured in sizes according to a standard  known as the American wire gauge (AWG). As shown in  Figure 9-115 , the wire diameters become smaller as the  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. 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. 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  Figure 9-114.  Wire harness with protective jacket. 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. 9-69 Wire Substitutions Areas Designated as Severe Wind and Moisture  Problem (SWAMP)  Wire Size Selection