3.1 Aircraft Wire Selection, Sizing, and Stripping Standards

3.1 Aircraft Wire Selection, Sizing, and Stripping Standards

Core Aviation Standard: In certified aircraft, electrical wiring is an airborne primary system. Federal Aviation Administration (FAA) Advisory Circular AC 43.13-1B Chapter 11 dictates that every conductor installed on an aircraft must withstand severe environmental vibration, wide thermal cycling, fluid exposure, and mechanical stress without degrading electrical or structural integrity.

Aircraft wiring errors represent severe flight safety hazards. A single chafed insulation jacket, nicked conductor strand, or undersized conductor can trigger in-flight electrical fires, cockpit smoke events, or loss of primary flight instruments. Avionics technicians must master the engineering standards governing wire selection, conductor sizing, bundle derating, and precision stripping.

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FAA Regulatory Foundation: AC 43.13-1B Chapter 11

FAA Advisory Circular AC 43.13-1B, titled Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair, serves as the technician's primary technical benchmark. Chapter 11 (Aircraft Electrical Systems) describes acceptable methods for wire selection, routing, clamping, termination, and inspection when the manufacturer's Instructions for Continued Airworthiness (ICA) or Supplemental Type Certificate (STC) data do not specify a procedure. It is acceptable data, not approved data, and it does not override the manufacturer's instructions.

Under 14 CFR Part 43, wiring modifications must maintain airframe airworthiness by ensuring:

• Dielectric Integrity: Wire insulation must not cold-flow under clamp pressure or emit toxic fumes when overheated.
• Conductor Ampacity: Operating currents must never raise conductor temperature beyond the insulation's maximum rated continuous thermal limit.
• Mechanical Durability: Conductor stranding must flex under flight vibration without work-hardening or fracturing.

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Aircraft Wire Specifications: MIL-W-22759 (SAE AS22759)

AC 43.13-1B paragraph 11-77 says only wire specifically designed for airborne use must be installed in aircraft, so commercial building wire or automotive primary wire, often insulated with polyvinyl chloride (PVC), does not belong in a certified aircraft. Under flame or severe electrical arcing, PVC insulation releases dense black smoke and lethal, highly corrosive hydrogen chloride ($HCl$) gas that blinds flight crews and destroys avionics circuit boards.

Aircraft wire is usually built to military or airframe-manufacturer specifications, most commonly SAE AS22759 (formerly MIL-W-22759), with stranded copper or copper-alloy conductors and fluoropolymer insulation. AC 43.13-1B warns that wire from unauthorized sources that is fraudulently marked with a specification number is unapproved wire.

Ratings shown are from the wire tables in AC 43.13-1B (Tables 11-11 and 11-12). Note that /34 is the tin-plated, 150°C normal-weight XL-ETFE version.

Conductor Metallurgy and Protective Platings

Bare, unplated copper oxidizes rapidly at elevated temperatures, leading to high contact resistance and embrittlement. Certified aircraft conductors utilize finely stranded copper treated with protective metal platings:

1. Tin Plating: Rated up to +150°C. It provides economical oxidation protection and excellent solderability. At continuous temperatures above +150°C, tin diffuses into copper, forming a brittle intermetallic boundary layer.
2. Silver Plating: Rated up to +200°C. Silver exhibits exceptional electrical conductivity and elevated thermal endurance. However, technicians must protect silver-plated wire from moisture ingress: pinholes or scratches in the silver plating expose underlying copper, which, in the presence of water and oxygen, generates an aggressive galvanic corrosion known as red plague (cuprous oxide).
3. Nickel Plating: Rated up to +260°C. Nickel provides the highest oxidation resistance in extreme thermal zones (such as turboprop and jet engine nacelles). AC 43.13-1B notes that soldered terminations on nickel-plated conductors need different solder sleeves or flux than those used on tin- or silver-plated wire.

Conductor alloys also matter. AC 43.13-1B paragraph 11-76 requires 24 AWG wire used in interconnecting airframe applications to be made of high-strength alloy, and paragraph 11-66 suggests considering high-strength alloy conductors in other small gauges to add mechanical strength.

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American Wire Gauge (AWG) System and Circular Mils

Aircraft electrical systems quantify conductor size using the American Wire Gauge (AWG) system. The AWG scale operates on an inverse logarithmic relationship: as the gauge number increases, the physical conductor diameter and cross-sectional area decrease.

Circular Mils Calculation

In electrical engineering, cross-sectional area is measured in circular mils. One mil equals one one-thousandth of an inch ($0.001\ \text{inch}$). The area of a solid circular conductor in circular mils ($A_{\text{cmil}}$) is simply the diameter in mils squared:

$A_{\text{cmil}} = d^2$

For stranded aircraft conductors, the total circular mil area equals the sum of the circular mil areas of the individual strands. Conductor direct-current resistance ($R$) is directly proportional to conductor length ($L$) and material resistivity ($\rho$), and inversely proportional to circular mil area:

$R = \rho \frac{L}{A_{\text{cmil}}}$

Source: AC 43.13-1B Table 11-9. The current ratings are for wires in bundles: 70°C ambient, 33 or more wires in the bundle for sizes 24 to 10, no more than 20 percent of harness capacity in use, and 60,000 ft altitude. For other conditions, compute ratings with paragraph 11-69 and Figures 11-4a through 11-6.

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Continuous vs. Intermittent Loading and Voltage Drop Limits

When sizing an aircraft conductor, two distinct engineering criteria must be evaluated independently, and the larger wire size dictated by either test must be selected:

1. Current-Carrying Capacity (Ampacity): The wire must carry the circuit current without exceeding the temperature limit of the insulation.
2. Allowable Voltage Drop: Conductor electrical resistance over the entire circuit length must not reduce operating voltage below the minimum threshold required by the avionics equipment.

Load Definitions

• Continuous Load: An electrical load that operates for more than 2 minutes continuously (e.g., flight management systems, VHF radios, pitot heat, navigation lighting, fuel boost pumps).
• Intermittent Load: An electrical load that operates for 2 minutes or less (e.g., landing gear drive motors, flap actuators, engine starter relays, pitch trim motors).

Allowable Voltage Drop Limits (AC 43.13-1B Table 11-6)

Calculating Circuit Voltage Drop

To determine the voltage drop ($V_{\text{drop}}$) across a conductor run of one-way length $L$ (in feet) carrying current $I$ (in amperes):

$V_{\text{drop}} = \frac{2 \times L \times R_{1000} \times I}{1000}$

(Note: The factor of 2 covers a two-wire circuit in which current returns on a wire of the same length. When current returns through bonded airframe structure, AC 43.13-1B says the structure's resistance is generally considered negligible, so only the wire run is counted. The AC's wire charts use run length including any ground wire. The separate 2 percent limit in paragraph 11-66b applies to the main power wires from the source to the bus.)

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Wire Bundle Derating Factors

A single wire suspended horizontally in free air dissipates internal resistive ($I^2 R$) heat efficiently via ambient convection and radiation. However, when multiple conductors are bound together into a dense harness bundle, inner conductors are insulated by surrounding wires, trapping heat and raising bundle core temperatures dramatically.

AC 43.13-1B handles this in three steps (paragraphs 11-67 and 11-69):

1. Single wire in free air (Figures 11-4a and 11-4b): find the free-air rating from the allowable temperature rise (wire temperature rating minus ambient).
2. Bundle derating (Figure 11-5): multiply by a factor that depends on both the number of wires in the bundle and the percentage of total bundle capacity actually in use.
3. Altitude derating (Figure 11-6): multiply again, because thinner air removes less heat.

The AC's own worked example (paragraph 11-69): a harness of 35 wires (10 at 20 AWG and 25 at 22 AWG, all rated 200°C) sits in a 60°C ambient on a vehicle that flies at 60,000 ft, and no more than 20 percent of bundle capacity is used.

Figure 11-6 gives about 0.91 at 20,000 ft and 0.79 at 60,000 ft. There is no single bundle factor that depends only on wire count, so read the curves for the actual bundle.

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Mechanical Wire Stripping Standards

Precision wire stripping is the foundation of electrical termination. Damage inflicted on conductor strands during stripping cannot be reversed and creates an immediate failure point.

Calibrated Die-Type Strippers vs. Hand Strippers

• Tools to Avoid: Serrated pliers, diagonal cutters, and knife blades should not be used to strip aviation wire, because they scrape, nick, or cut strands unpredictably. AC 43.13-1B paragraph 11-157 says cutting tools must be sharp and stripping tools must be adjusted to avoid nicking, cutting, or otherwise damaging strands.
• Preferred Tooling: Precision die-type strippers (e.g., MIL-SPEC M22520 frame with precision-ground die blades, or Ideal Stripmaster with matched AS22759 dies) or thermal strippers. Thermal strippers utilize electrically heated nichrome elements to cleanly melt fluoropolymer insulation without allowing metal-to-metal blade contact with the conductor.

Allowable Strand Damage (AC 43.13-1B Table 11-13)

Every nick removes conductor area and creates a stress riser where vibration can start a fatigue crack, so the goal is no damage at all. AC 43.13-1B sets the maximum it will accept:

• Inspection: Look at every stripped end under magnification. If damage exceeds these limits (or the stricter limit in the manufacturer's data), cut the wire back and strip it again.
• Remove no more insulation than necessary (paragraph 11-157d).

PROPER STRIP DIMENSIONS (AC 43.13-1B):

       Insulation Clearance Gap:
       [per contact or terminal spec]
              |---|
===================       _____________________
INSULATION JACKET  |     |   BARE STRANDED     |  (Table 11-13 limits)
===================       ---------------------
                   |-----|
                      ^
                      |--- Bare Conductor Exposure

Stripping Length and Insulation Clearance

• Contact Barrel Depth: Conductor strip length must equal the contact wire barrel depth plus an insulation clearance gap.
• Insulation Clearance Gap: Leave the small, visible gap between the end of the insulation and the contact or terminal barrel that the contact or terminal manufacturer specifies. AC 43.13-1B does not set one universal number.
  • If the gap is too small, insulation can enter the crimp barrel, so the crimp grips insulation instead of copper and resistance rises.
  • If the gap is too large, bare conductor is exposed outside the barrel, creating a short-circuit risk to adjacent pins or chassis metal.

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Avionics Shop Worked Scenario: Sizing a 12 A Load Feed

Scenario: A technician is wiring a new heater load on a 28 VDC aircraft. The load draws a continuous current of 12 amperes. The one-way wire run from the bus circuit breaker to the load connector is 25 feet, and the load returns through bonded structure. The wire will be routed inside a main wire harness bundle containing 35 current-carrying conductors. What is the correct wire size?

Step 1: Check Current Rating in the Bundle

• Continuous current: $I = 12\ \text{A}$.
• AC 43.13-1B Table 11-9 already rates wires for a bundle of 33 or more wires at a 70°C ambient, with no more than 20 percent of harness capacity in use.
• For 150°C wire (M22759/16), 16 AWG is rated 11 A, which is too small for 12 A, and 14 AWG is rated 14 A, which passes.
• If the real bundle runs hotter or carries a larger share of its capacity than the table assumes, rerate it with paragraph 11-69 and Figures 11-4 through 11-6.

Step 2: Check Allowable Voltage Drop (28 VDC Continuous)

• Maximum allowable voltage drop: $V_{\text{drop}} \le 1.0\ \text{V}$.
• Calculate for AWG 14 ($R_{1000} = 3.06\ \Omega / 1000\ \text{ft}$): $V_{\text{drop}} = \frac{25\ \text{ft} \times 3.06\ \Omega / 1000\ \text{ft} \times 12\ \text{A}}{1000} = 0.918\ \text{volts}$
• Since $0.918\ \text{V} \le 1.00\ \text{V}$, AWG 14 meets both the thermal ampacity and voltage drop requirements. (If an isolated two-wire return loop of 50 total feet were required, $V_{\text{drop}}$ would be $1.836\ \text{V}$. AWG 12 would still drop $50 \times 2.02 / 1000 \times 12 = 1.21\ \text{V}$, so AWG 10 ($0.76\ \text{V}$) would be needed to stay within $1.0\ \text{V}$).
• Engineering Choice: Select M22759/16-14 (AWG 14, ETFE insulation, tin-plated copper).