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Voltage drop limit to the bus

Aviation Mechanic Airframe (AMA) practice question 6 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.R5 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONThe voltage drop in the main power wires from the generation source to the bus must not exceed 2 percent of the regulated voltage. In a 28-volt system, what is that limit?

  1. A2 volts
  2. B0.56 volt
  3. C1.4 volts, or 5 percent
Show the answer and explanation

Correct answer: B. 0.56 volt

  • A — incorrect Two volts is the allowable drop between the bus and utilization equipment ground in a 28-volt system during continuous operation, a different limit.
  • B — correct Two percent of 28 volts is 0.56 volt, the maximum drop allowed in the main power wires to the bus.
  • C — incorrect This applies 5 percent instead of the 2 percent stated for the main power wires.

BACKGROUND

Wire size must be large enough to prevent an excessive voltage drop while carrying the required current over the required distance, and also large enough to prevent overheating. The voltage drop in the main power wires from the 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 the 5-minute rate. A separate tabulation defines the maximum acceptable drop in the load circuits between the bus and the utilization equipment ground, which for a 28-volt system is 2 volts during continuous operation and 1 volt for intermittent operation. Circuit resistance can be checked by measuring the voltage drop across the circuit with the input voltage held constant. Calculated voltage drop equals resistance per foot times length times current.

STUDY TIP

Keep the two limits separate: a percentage rule applies from the source to the bus, and a table of volts applies from the bus to the equipment ground.

KEY POINTS

  • Source-to-bus drop is limited to 2 percent of regulated voltage.
  • On a 28-volt system that works out to 0.56 volt.
  • Bus-to-equipment drop uses its own table, 2 volts continuous at 28 volts.

SOURCES

  • sources/ama-airframe/ch09.txt "Allowable Voltage Drop"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.