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Voltage drop limit in main power cables

Aviation Mechanic Powerplant (AMP) practice question 1 of 13 on Engine Electrical Systems. ACS element AM.III.F.K7 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a 28-volt system, what is the maximum voltage drop permitted in the main power cable from the generator to the bus while the generator is carrying rated current?

  1. A2.8 volts, or 10 percent
  2. B0.56 volt
  3. C1 volt, the load circuit limit
Show the answer and explanation

Correct answer: B. 0.56 volt

  • A — incorrect Ten percent is far beyond the allowance; the limit for main power cables is stated as 2 percent of the regulated voltage.
  • B — correct The drop in the main power cables from the generation source or the battery to the bus must not exceed 2 percent of the regulated voltage, and 2 percent of 28 volts is 0.56 volt.
  • C — incorrect One volt is the figure used for a continuously operated load circuit between the bus and the equipment, not for the main power cable feeding the bus.

BACKGROUND

The voltage drop in the main power cables from the aircraft generation source or the battery to the bus should not exceed 2 percent of the regulated voltage when the generator is carrying rated current or the battery is being discharged at a five-minute rate. A five-minute rate means the battery should last at least five minutes in an emergency with all battery operated equipment running. Separate allowances apply to the load circuits between the bus and the utilization equipment; for a 28-volt system, a 1-volt drop is permissible for continuous operation. The resistance of the return path through aircraft structure is treated as negligible provided bonding is adequate, and a measurement of 0.005 ohm from the generator or battery ground point to a device ground terminal is considered satisfactory.

STUDY TIP

Keep the two allowances apart: 2 percent applies to the feed from the source to the bus, while the load circuit tables cover the run from the bus to the equipment.

KEY POINTS

  • Main power cable drop is limited to 2 percent of regulated voltage.
  • Two percent of 28 volts works out to 0.56 volt.
  • Load circuit allowances from the bus are a separate figure.

SOURCES

  • amp-powerplant/ch04.txt "Voltage Drop in Aircraft Wire & Cable"
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MORE ON ENGINE ELECTRICAL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.