Adding a Transponder to an Already-Loaded 28 V Circuit
A shop is asked to add a 5 A continuous avionics load (a new transponder) to an existing 28 V DC circuit that already carries a 9 A continuous load. The circuit is 14 AWG wire in a loom with nine other wires, in an equipment bay where ambient reaches 70 C, protected by a 15 A thermal breaker. A mechanic proposes tapping the new unit into the existing circuit at the nearest terminal strip because, in his words, 14 A on a 15 A breaker leaves a 1 A margin. The aircraft has a 100 A alternator with 22 A of measured spare capacity at cruise, and the electrical load analysis has not been updated since a previous modification.
1. Evaluate the proposed tap. Is the breaker adequate? Is the wire adequate?
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Neither is adequate.
- Breaker: 14 A continuous on a 15 A thermal breaker is 93 percent of rating. Thermal breakers trip lower as ambient rises, and the bay is at 70 C. Normal practice keeps continuous load at or below about 80 percent of the breaker rating - 12 A here. The mechanic's 1 A margin is not margin, it is the trip band.
- Wire: 14 AWG is nominally about 15 A in free air at room temperature. Apply two derates: bundle factor for a 10-wire loom, and the high ambient correction. Both push the usable ampacity well below 14 A. The wire, not just the breaker, is being overloaded.
- The failure mode is not a clean trip; it is a slowly cooked loom and nuisance trips on hot days.
Conclusion: the existing circuit cannot legally or safely absorb this load.
2. What is the correct installation approach, and what data must be produced?
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A dedicated circuit.
- Size the wire with the derates applied - bundle, ambient, altitude - which for 5 A in that bay likely lands on 12 AWG with margin for inrush.
- Protect with a breaker sized to the wire, not to the load: a 20 A breaker on 12 AWG, coordinated so the wire is protected under all conditions.
- Route separately from the existing loom where practical, with its own clamps and chafe protection, and terminate on the correct bus with a properly crimped, inspected terminal.
- Confirm bus bar and terminal-stud ratings and physical space at the panel before committing.
Data: revise the electrical load analysis, and revise weight and balance for the new unit and hardware. Both are approval documents, not paperwork afterthoughts.
3. The alternator shows 22 A of spare capacity and the new load is 5 A. What system-level checks still have to be made before signoff?
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Spare capacity at cruise is the easy case; the design cases are the hard ones.
- Check the worst-case configuration, not cruise: single-alternator operation, ground idle with the battery charging, and night/icing loads all running. That is where a 5 A addition disappears the 22 A margin.
- Check the transponder's inrush/pulsed current. A transponder draws short high-current bursts on reply; verify the breaker will not nuisance-trip and that the bus transient is acceptable to other equipment.
- Confirm the essential/emergency bus allocation is not degraded and that the new unit sheds correctly if required.
- Verify the amended electrical load analysis is approved and on board, and that the weight and balance reflects the change.
- Do a functional check at the end: the unit powers up, the breaker holds through a full transmit cycle, and no other equipment on the bus resets.