Wire Sizing, Ampacity, and Bundling Rules per AC 43.13-1B

Every wire in an aircraft carries current, and every wire generates heat as a result. Unlike home wiring buried safely inside walls, aircraft wiring is routed through tight spaces near fuel lines, flight control cables, and structure — all while enduring vibration, temperature swings, and decades of service. Choosing the wrong wire size or bundling wires carelessly can start an in-flight fire or cause a silent electrical failure at the worst possible moment. AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, is the FAA's primary advisory document guiding maintenance technicians through correct wire selection, ampacity determination, and bundling practice. Understanding these rules is essential for the AMT Airframe written test and, far more importantly, for turning out airworthy electrical work.

Wire Gauge, Conductor Material, and Construction

Aircraft wire is specified by American Wire Gauge (AWG). The AWG scale runs counterintuitively: the larger the AWG number, the smaller the wire. A 22 AWG wire is physically much thinner than a 4 AWG wire. Most aircraft use stranded copper conductors rather than solid conductors because stranded wire is far more resistant to the flex fatigue caused by vibration. Aluminum conductors are occasionally found in large transport-category aircraft for weight savings, but they require special termination techniques and are generally avoided in light aircraft repair due to their susceptibility to oxidation and fatigue cracking at connections.

The insulation type matters as well. MIL-spec wire types commonly found in certificated aircraft include MIL-W-22759 (crosslinked ETFE or PTFE insulation, suitable for high-temperature and fire-resistant applications) and older types such as MIL-W-5086. AC 43.13-1B guidance and the aircraft's wiring diagrams together determine which insulation type is appropriate for a given replacement.

Ampacity: Current-Carrying Capacity

Ampacity is the maximum continuous current a wire can carry without its insulation overheating and degrading. AC 43.13-1B provides ampacity tables that list the continuous-duty current limit for each wire gauge, but these values are not simple fixed numbers — they are derated depending on context. Two major factors drive derating: bundling and conduit routing.

When a wire is run in free air, it can dissipate heat efficiently and carry its full rated ampacity. When the same wire is bundled with other wires or run inside a conduit, heat builds up because the surrounding wires and the conduit wall restrict airflow and create a thermal blanket effect. AC 43.13-1B specifies derating factors — expressed as percentages — that must be applied to the free-air ampacity when wires are bundled. For example, a bundle of two or three wires may require the individual wire ampacity to be derated to approximately 70% of its free-air value; a large bundle of many wires may push that derating to 50% or lower. The technician must look up the specific derating table in AC 43.13-1B and apply it honestly — never assume the free-air value is acceptable inside a bundle.

The practical consequence is important: if your circuit requires 15 amperes of continuous current and the wire will live inside a large bundle, you may need to select a wire two or three gauges heavier than the simple ampacity table suggests, just to satisfy the derated limit. Always work backward from the derated ampacity, not from the free-air value.

Voltage Drop: The Second Sizing Criterion

Ampacity keeps the wire from catching fire. Voltage drop keeps the load device working correctly. Even a wire that is large enough not to overheat can have enough resistance to drop significant voltage along its length, starving sensitive avionics, motors, or lights of the voltage they need to operate properly.

AC 43.13-1B provides guidance on acceptable voltage drop limits. The general rule is that the total voltage drop in a circuit — from the bus to the load and back through the return path — should not exceed a small fraction of system voltage. For most 28-volt aircraft systems, a drop of no more than approximately 1 volt on the supply conductor (and 1 volt on the return) is the target for continuous-duty circuits; intermittent or motor-starting circuits may allow slightly more. For 14-volt systems, the allowable drop is proportionally smaller in absolute terms.

To check voltage drop, the technician calculates (or looks up) the resistance of the candidate wire, multiplies by the load current, and compares the result to the allowable limit. If the run is long and the allowable drop is tight, the wire must be upsized even if it already meets the ampacity requirement. This means final wire selection is always driven by whichever criterion — ampacity or voltage drop — demands the larger conductor. You pick the larger of the two and that is your minimum acceptable gauge.

Circuit Protection Sizing

A circuit breaker or fuse must protect the wire, not just the load. This means the protective device must be rated at or below the wire's derated ampacity. AC 43.13-1B is explicit: if the circuit breaker is rated higher than the wire can carry, the wire can overheat and ignite before the breaker ever trips. The protective device rating, the wire ampacity, and the load current must form a logical hierarchy — load current is lowest, wire ampacity is next, and the breaker rating must not exceed the wire's safe capacity.

Wire Routing and Mechanical Protection

Proper routing is as important as proper sizing. AC 43.13-1B sets out the following key routing rules:

Separation from fuel and hydraulic lines: Electrical wires must be routed above fuel and hydraulic lines wherever possible so that any fluid leak drips away from the wiring, not onto it. Where crossing is unavoidable, it should be done at right angles to minimize contact exposure.
Separation from flight controls: Wire bundles must be kept clear of control cables, pulleys, bell cranks, and rod ends. There must be no possibility of a bundle jamming or chafing against a moving control surface mechanism.
Chafe protection: Wherever a wire or bundle passes through a bulkhead, firewall, or structural member, it must be protected by a grommet or a properly installed clamp with a cushioned lining. Bare metal edges will saw through insulation over time, creating a short-circuit risk.
Clamping intervals: AC 43.13-1B specifies maximum spacing between support clamps — generally no more than 24 inches for most installations, with closer spacing required near connectors and in areas with high vibration.
Slack and strain relief: Wire runs should have enough slack to allow for vibration movement and thermal expansion without placing tension on connectors, but not so much slack that the wire can rub against structure.

Bundling Rules

Bundling wires together is standard practice, but the composition of a bundle matters. AC 43.13-1B guidance calls attention to these bundling considerations:

Do not bundle ignition wires with other circuits. High-energy ignition leads radiate electromagnetic interference that can corrupt avionics or sensor signals and must be routed separately.
Separate power wiring from sensitive signal wiring wherever possible to prevent induced noise.
Maximum bundle diameter should be manageable with standard aircraft clamps. Excessively large bundles become difficult to inspect, replace, and protect, and they impose high derating requirements.
Coaxial cables (such as antenna feedlines) should not be tightly bundled with power leads. They may be loosely routed alongside other wire groups but generally run as separate routed items.
Apply derating for every bundled wire, even if only a few wires in the bundle carry high current. The thermal effect of neighbors must be accounted for.

Why These Rules Matter

Electrical fires are among the most dangerous threats in aviation because they can be difficult to detect, impossible to extinguish completely in flight, and capable of disabling multiple systems simultaneously. The rules in AC 43.13-1B were developed from decades of accident investigation, operational experience, and materials science. Undersized or over-bundled wires that run warm for thousands of flight hours gradually degrade their own insulation, and eventually a vibration crack or a moisture intrusion triggers an arc. Poorly routed wires that chafe against structure do the same thing — just faster.

Key Numbers and Rules

AWG scale: Higher number = smaller wire; 22 AWG is thinner than 4 AWG.
Derating: Apply AC 43.13-1B tables — bundled wire ampacity may be 50–70% of free-air value depending on bundle size.
Voltage drop target: Approximately 1 volt maximum on supply run for continuous-duty 28V circuits; select the wire gauge that satisfies both ampacity and voltage drop.
Circuit protection: Breaker/fuse rating must not exceed the wire's derated ampacity.
Clamp spacing: Generally no more than 24 inches between supports.
Routing priority: Wires above fluid lines; isolated from ignition leads; grommeted through structure.
Wire type: Stranded conductors required for vibration resistance; insulation type must match application and OEM specification.

Common Test Traps

Using free-air ampacity for a bundled installation. The test will often describe a bundled wire and ask which gauge is correct. If you ignore derating and select the gauge from the free-air column, you pick a wire that is dangerously undersized.
Forgetting voltage drop as a sizing criterion. Students often memorize ampacity tables and forget that a long wire run may need to be upsized to keep voltage drop within limits, even if it already passes the ampacity check.
Assuming the circuit breaker protects the load, not the wire. The breaker must be sized to the wire's derated ampacity ceiling, not simply to the load's rated current draw.
Confusing AWG direction. Exam questions sometimes describe increasing gauge numbers as meaning larger wire — the opposite of reality. Always remember that AWG increases as the conductor gets smaller.
Believing bundling rules are optional for short runs. AC 43.13-1B derating requirements apply regardless of bundle length. A short but heavily loaded bundle still traps heat and must be derated accordingly.