Learn how to choose aircraft wire gauge using ampacity, voltage drop, run length, derating, circuit protection and approved aviation data.
For aircraft electrical wiring, choose an approved conductor size that meets both current-carrying capacity and permitted voltage drop after accounting for run length, ambient temperature, bundling and duty cycle. Then verify insulation, mechanical strength and circuit-protection coordination against the aircraft’s approved data. Never select aircraft wire by amperage alone.
For Aviation & Real-World Flying, the aircraft’s wiring manual, maintenance data, approved drawing or alteration instructions take precedence over a calculated answer. On an existing certified aircraft, do not downsize or substitute wire simply because another gauge appears adequate. New circuits and alterations must be designed and approved by an appropriately authorised person under the applicable national rules.
Wire sizing depends on where electrical power originates and how the circuit is distributed. Our overview of aircraft voltage generation and distribution explains the battery, generator, alternator and bus arrangements behind those calculations.
What determines the correct aircraft wire size?
The correct size is determined by the worst electrical and installation conditions the wire will experience, not just the equipment’s nominal current.
| Criterion | What must be checked | Why it affects gauge |
|---|---|---|
| Load current | Maximum continuous current, intermittent load, start-up surge and abnormal operating cases | The conductor must remain within its permitted temperature and ampacity limits. |
| System voltage | Nominal and minimum operating voltage | Low-voltage systems tolerate less absolute voltage drop, while constant-power equipment may draw more current as voltage falls. |
| Circuit length | The complete electrical path, using the length definition specified by the approved sizing method | Longer conductors have more resistance and voltage loss. |
| Installation | Ambient temperature, bundles, conduit, ventilation, altitude corrections and proximity to hot equipment | These conditions can reduce the wire’s allowable current. |
| Duty cycle | Continuous, intermittent or momentary operation | A starter feeder and a continuously powered avionics circuit cannot automatically use the same sizing assumptions. |
| Mechanical limits | Vibration, unsupported length, connector contact range and terminal strength | A very small conductor may pass the electrical calculation but still be unsuitable mechanically. |
AWG numbering runs backwards: a lower AWG number means a larger conductor. Gauge alone does not establish capacity; conductor material, stranding, insulation temperature rating and installation conditions also matter.
How is aircraft wire gauge calculated?
Aircraft wire gauge is selected by checking ampacity and voltage drop separately, then using the larger conductor required by either check.
- Identify the real load. Establish maximum normal current, simultaneous loads, start-up or motor inrush, duty cycle and the minimum expected bus voltage. Use equipment installation data rather than an estimated wattage where available.
- Select the approved wire family. Determine conductor material, insulation specification, temperature rating and environmental requirements before consulting a sizing table. A copper-wire table cannot be applied unchanged to aluminium cable.
- Apply the correct ampacity table. Start with authority-accepted or manufacturer-approved data, then apply its corrections for ambient temperature, bundled loaded wires, conduit and other installation conditions.
- Calculate voltage drop. Use the resistance at the design temperature and the complete current path. Compare the result with the permitted drop for that circuit and duty cycle.
- Check circuit protection and terminations. Confirm that the fuse or circuit breaker protects the installed wire and that every contact, splice and terminal is approved for the selected gauge.
- Increase conductor size if either test fails. Move to a larger conductor, which means a lower AWG number, and repeat the checks. Also confirm that the larger wire fits the connector and routing without an unapproved modification.
For a familiar light-aircraft example, our explanation of the Cessna 172 battery, alternator and bus arrangement shows why the source point, protective device and return path must be identified before sizing a circuit.
How do you calculate aircraft wire voltage drop?
For a simple DC circuit, voltage drop is current multiplied by total circuit resistance:
Voltage drop = current × total circuit resistance
For a dedicated two-wire circuit, include both supply and return conductors. If each conductor is 12 feet long, the resistive path is normally 24 feet. With an airframe return, include the resistance of ground leads, structural bonds and connections rather than assuming the return has zero resistance. Some approved charts define length differently, so follow the chart’s instructions instead of doubling the distance automatically.
As an illustration, an 8-amp circuit with 24 feet of total conductor at 0.004 Ω per foot has 0.096 Ω resistance and approximately 0.77 V drop. If the circuit permits only 0.5 V, that conductor fails even if its ampacity is adequate. Select the next suitable larger size and recalculate.
For AC circuits, especially long or high-current 400 Hz feeders, impedance and power factor may also be significant. Use the approved AC sizing method rather than relying only on DC resistance.
Does circuit-breaker size determine wire gauge?
No—circuit-breaker size is one input, but it does not determine aircraft wire gauge by itself.
A fuse or breaker primarily protects the wiring from excessive current. The installed conductor must be permitted behind that protective-device rating after all installation derating is applied, while the breaker’s trip characteristics must also tolerate legitimate start-up or transient current.
A mistake we see constantly is increasing the breaker rating to stop nuisance trips. That can leave the existing wire inadequately protected. Find the overload, fault or inrush problem first; if the breaker and wire genuinely need to be enlarged, the entire circuit must be reassessed and approved.
Why is a generic AWG ampacity chart insufficient?
A generic AWG chart usually omits the aircraft-specific conditions that decide whether a conductor is safe.
- Automotive and household charts may assume different insulation temperatures, ventilation and acceptable voltage drop.
- A free-air rating can be too generous for a tightly packed aircraft wire bundle.
- Using only the one-way distance understates voltage drop in a dedicated-return circuit.
- Calculating at nominal voltage can miss higher current at the minimum operating voltage.
- Engine-bay temperature and nearby ducts or exhaust components can require substantial derating or rerouting.
- Small wires may fail vibration, terminal or minimum mechanical-size requirements despite carrying the current electrically.
- A cable shield must not be treated as the normal load return unless the approved design specifically uses it that way.
Can automotive wire be used in an aircraft?
Automotive wire should not be substituted merely because its copper cross-section has the same AWG number.
Aircraft wire specifications cover insulation temperature, fluid resistance, abrasion, smoke and flame behaviour, conductor plating, flexibility and compatible termination methods. Certified aircraft require the wire type specified by approved data. Experimental or amateur-built aircraft may operate under different rules, but the designer still needs an aviation-suitable wire specification and accepted installation practice.
What should be checked after selecting the wire?
The finished circuit should be inspected and tested against its approved design under representative electrical load.
Verify routing, clamps, bend radius, chafe protection, separation from heat and fluid lines, crimp quality, connector contact size and ground-bond condition. Confirm voltage at the load and investigate unexpected drop, heating or breaker operation. A successful operational test does not cure an undersized or unapproved conductor; the calculation, protection and installation all have to be correct.