A technician is selecting wire gauge for a new 28-volt DC circuit that will carry 15 amps continuously through a 40-foot wire run from the bus to the equipment. Which factor, in addition to current-carrying capacity, must be considered to properly size this wire according to standard aircraft wiring practices?
- AThe ambient cabin temperature during ground operations only
- BThe total voltage drop over the length of the wire run
- CThe color of the wire insulation for circuit identification
- DThe number of other circuit breakers on the same bus bar
Show answer & explanationAnswer & explanation
Correct answer: B. The total voltage drop over the length of the wire run
Wire sizing must account for both current-carrying capacity (ampacity) and allowable voltage drop over the length of the run. Longer wire runs at higher currents can create excessive voltage drop, so wire size charts factor in both current and length to keep voltage drop within acceptable limits (typically no more than a few percent of system voltage) for proper equipment operation.
Why the other options are wrong
- A. Ambient temperature does affect ampacity ratings, but the specific consideration here relative to wire length is voltage drop, and it applies in flight as well as on ground.
- C. Insulation color is for identification purposes and does not affect ampacity or voltage drop calculations.
- D. While bus loading matters for overall electrical load planning, it is not the direct factor considered per-wire in gauge selection.
Wire Size Selection (Voltage Drop)
Aircraft wire gauge must be selected based on both current-carrying capacity and allowable voltage drop, which increases with wire length and current draw.
- Wire size charts factor in current, length, and allowable voltage drop
- Longer wire runs require larger gauge wire to limit voltage drop
- Excessive voltage drop can cause equipment malfunction or dim lighting
- Ambient temperature also affects a wire's continuous current rating
Memory trick: Think of a long garden hose — the longer it is, the more pressure (voltage) is lost by the time water (current) reaches the end.