How would you determine the correct fuse rating for a new accessory wire?

• A. Use a fuse rated at half the accessory current to ensure protection.
• B. Use the same rating as the generator output.
• C. Select a fuse rating well above the accessory current to avoid nuisance blows.
• D. Determine the accessory's maximum expected current, then choose a fuse just above that value but below wiring ampacity and per protection scheme.

Answer: (D)

Sizing a fuse for a new accessory wire is about protecting the wiring while allowing normal operation. A fuse should not trip during normal use, but it must open if a fault causes current well above what the wire can safely carry. To do this, determine the accessory’s maximum expected current during worst‑case operation, including any startup or inrush current if the device draws a surge when it starts. Then pick a fuse rating just above that maximum, but below the wiring’s ampacity and in line with the aircraft’s protection scheme. This ensures the circuit can handle normal loads without nuisance trips, while still protecting the wire from overheating in a fault. Choosing a fuse rating well above the accessory current defeats protection, while choosing one too small would cause nuisance trips or prevent normal operation. If the accessory has a significant inrush, a time‑delay fuse may be appropriate so the startup surge is tolerated without blowing, while still protecting the wire during a sustained fault.