Electrical Load and Circuit Protection After Adding EFB Outlets
A Cessna 208B operating under a commercial certificate is in for avionics upgrades. The installer proposes to add two USB-C PD outlets, each rated 60 W, for pilot and copilot EFBs, and to replace the existing 5 A cigar-lighter circuit breaker with a 15 A breaker to feed both outlets. The aircraft has a 28 V DC alternator rated at 150 A; the most recent electrical load analysis shows a worst-case continuous load of 118 A and a peak load of 132 A. The new outlets would add up to 10 A continuous. The proposed wiring would run alongside existing COM and nav antenna coax for 1.5 m and through a floor conduit. The installer has no approved load analysis or wire-sizing calculation, and the circuit breaker panel has no spare positions. The aircraft is scheduled for a return to service tomorrow.
1. Evaluate the electrical load and circuit protection proposal. What is wrong with it?
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The proposal is not supported by the existing load analysis. Adding 10 A continuous raises the worst-case continuous load to 128 A, or about 85% of the 150 A alternator rating, and the peak load could reach about 142 A, or about 95%. Those figures may leave insufficient margin for battery charging, transients, and alternator cooling, depending on the approved load-analysis criteria. A new load analysis is required.
Replacing the 5 A breaker with a 15 A breaker is also unsafe unless the wire, terminals, and connectors are rated for 15 A and the voltage drop is acceptable. The existing cigar-lighter wiring is unlikely to be sized for that load. Two 60 W PD outlets can draw high current, especially if both are used at once, and the circuit protection must be sized to protect the wire, not just the outlets. A shared 15 A breaker may not provide adequate protection or allow independent fault isolation.
The proposed coax routing alongside COM/nav coax also raises an EMI concern and should be reviewed.
2. What approvals, calculations, and tests should be completed before return to service?
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Obtain approved or acceptable installation data covering the outlets, wiring, circuit protection, and routing. Complete an electrical load analysis that includes the new continuous and peak loads and verifies alternator capacity and battery charging margins. Calculate wire gauge, breaker size, and voltage drop for each circuit, and select circuit protection that protects the smallest wire in the circuit. Provide independent protection for each outlet or a justified shared circuit.
Address EWIS requirements: routing, clamping, chafe protection, separation from other wiring and coax, and identification. Perform bonding and grounding checks, an EMI/EMC check with the outlets loaded and the aircraft's COM/nav systems operating, and a functional load test. Update the weight and balance if required. Record all calculations and test results in the work package. The return-to-service entry must reference the approved data and state that the tests were satisfactory.
3. The installer says the aircraft must fly tomorrow and asks the AME to sign a 'minor alteration' entry. How should the AME decide?
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The AME should not sign until the alteration is properly classified and supported. The fact that the work is urgent does not change the approval requirement. Replacing a breaker and adding new outlets may be a minor alteration if it meets the applicable definition and is supported by acceptable data, but the absence of a load analysis and wire-sizing calculation means the basis for that classification is missing.
If the alteration is minor, the AME still needs acceptable data and must ensure the work is within the licence scope. If it is major, approved data and the applicable approval process are required. The AME should tell the installer what is missing and offer a compliant path: complete the calculations, obtain approval if needed, perform the tests, and then release the aircraft. If the operator insists on flight before that, the AME should decline to certify and escalate the issue. Signing an unsupported entry would misrepresent the aircraft's airworthiness.