Electrical Wire Sizing

Electrical Wire Sizing (vehicle-design/sizing/electrical-wire-sizing)

Use when you must size an aircraft electrical wire run at the feeder level: converting a continuous load current, ambient temperature, conductor temperature and run length into a gauge selection by derated bundled ampacity, then checking the round-trip voltage drop at the load current against the bus tolerance. This leaf implements the standard conductor ampacity derating and resistance model in pure Python (stdlib only), over a documented copper conductor reference data set for gauges 22 AWG down to 6 AWG. It pairs with vehicle-design/sizing/aircraft-electrical-load-analysis, which builds the per-bus consumer load rollup that supplies the continuous load used here, and with vehicle-design/sizing/battery-sizing, whose pack circuit drop check is not a wire run sizing. Does NOT do: EWIS installation and inspection quality (manufacturing-quality/assembly/ewis-installation- quality); avionics thermal management (vehicle-design/sizing/avionics- bay-cooling-sizing).

Domain quick reference

• Ampacity model: A(gauge, T_amb) = A_base * BUNDLE_DERATE * TEMP_DERATE(T_amb), with A_base the free-air 30 C base ampacity from the module constant set (5.0 A at 22 AWG rising to 60.0 A at 6 AWG), BUNDLE_DERATE = 0.60 for a bundle of 5 or more wires, and the linear temperature derate 1.0 - 0.006 * (T_amb - 30), floored at 0.5.
• Temperature derate values: 1.0 at 30 C, 0.94 at 40 C, 0.91 at 45 C. The 0.94 module constant is the documented value at the 40 C calibration point of the linear slope; the worked example runs at 45 C with the 0.91 factor as anchored.
• Selection rule: the smallest gauge whose derated ampacity meets the continuous load is selected; the model covers 22 AWG up to 6 AWG and raises ValueError when the load exceeds the 6 AWG derated capacity.
• Resistance model: R = rho * (1 + alpha * (T - 20)) / A_x, with rho = 1.72e-8 ohm m at 20 C for copper, alpha = 0.00393 per C, and A_x the conductor cross section in m^2 from the area constant set. At 45 C, 8 AWG gives 2.257e-3 ohm/m and 6 AWG gives 1.420e-3 ohm/m.
• Voltage drop (round trip): V_drop = 2 * L * I * R, then the percent drop is 100 * V_drop / V_bus; the run passes when the percent drop is at or below MAX_PERCENT_DROP = 3.0 percent of the bus voltage.
• SAE AS50881 frames the EWIS derating context; the relations above are standard engineering methodology, summary-only.

Workflow

1. Fix the run inputs: continuous load current I, run length L, bus voltage V_bus, ambient temperature T_amb, conductor temperature T_c.
2. Check candidate ampacities at the ambient with ampacity(gauge, T_amb); at 45 C the derate multiplies the free-air base by 0.60 * 0.91, which is the 0.546 effective factor.
3. Select the conductor with select_gauge(I, T_amb), the smallest gauge whose derated ampacity meets the load.
4. Get the conductor resistance at the operating temperature with resistance_per_meter(gauge, T_c).
5. Compute the round-trip drop with voltage_drop(I, L, gauge, T_c) and its percentage of the bus with percent_drop(V_drop, V_bus).
6. Run the full bundle with wire_size_review(I, L, V_bus, T_amb, T_c), which returns the selected gauge, ampacity, margin, drop, percent drop and the pass or fail verdict against the 3 percent bus tolerance.
7. When the verdict is fail, upsize one gauge and re-check the drop with the primitive functions until the percent drop clears the bus tolerance.
8. Confirm the deterministic checks with the contract test scripts/test_electrical_wire_sizing.py.

Worked example

A 25 A continuous load on a 10 m run fed from a 28 V DC bus, ambient 45 C and conductor temperature 45 C.

• Derated ampacity check: ampacity("10", 45.0) = 18.018 A, below the 25 A load; ampacity("8", 45.0) = 25.116 A meets it, so select_gauge(25.0, 45.0) = "8".
• Resistance: resistance_per_meter("8", 45.0) = 2.257e-3 ohm/m.
• Drop on 8 AWG: voltage_drop(25.0, 10.0, "8", 45.0) = 1.128 V, which is percent_drop = 4.03 percent of the 28 V bus, above the 3.0 percent tolerance, verdict fail.
• Upsize to 6 AWG: ampacity("6", 45.0) = 32.76 A, resistance 1.420e-3 ohm/m, voltage_drop(25.0, 10.0, "6", 45.0) = 0.710 V, percent 2.54 percent, verdict pass. Final selection 6 AWG.
• Review bundle: wire_size_review(25.0, 10.0, 28.0, 45.0, 45.0) returns gauge "8", ampacity 25.116 A, margin 0.116 A, drop 1.128 V, percent 4.03, verdict "fail", which flags the drop-limited upsize to 6 AWG above. Over a short 2 m run the same 8 AWG selection drops 0.806 percent and passes.

Verification

• Confirm select_gauge(25.0, 45.0) returns "8" and select_gauge(5.0, 30.0) returns "18" (the truth table smallest gauge meeting the load).
• Confirm the 45 C ampacity anchors: 18.018 A at 10 AWG, 25.116 A at 8 AWG, 32.76 A at 6 AWG (bounds 18.0 / 25.1 / 32.8 A within 0.2 A).
• Confirm resistance_per_meter("8", 45.0) = 2.257e-3 ohm/m within 5 percent and the 6 AWG value 1.420e-3 ohm/m.
• Confirm the drop anchors: 1.128 V on 8 AWG and 0.710 V on 6 AWG within 0.05 V, with percent drops 4.03 and 2.54.
• Confirm the identities: doubling the run length doubles the drop, and percent_drop equals 100 * V_drop / V_bus exactly.
• Confirm every non-physical input raises ValueError: unknown gauge, load at or below zero or beyond the 6 AWG capacity, negative length, bus voltage at or below zero, ambient outside the 30 to 100 C band.
• Confirm determinism: repeated review calls return identical dicts with exactly the documented keys.
• Run the contract test offline: python3 scripts/test_electrical_wire_sizing.py (35 tests, deterministic).

Related leaves

• vehicle-design/sizing/aircraft-electrical-load-analysis: the per-bus consumer load rollup that supplies the continuous load current to the conductor sizing here.
• vehicle-design/sizing/battery-sizing: traction energy storage with pack cell count and an internal pack circuit drop check, not a wire run sizing.
• vehicle-design/sizing/avionics-bay-cooling-sizing: the thermal management side of the avionics installation.
• manufacturing-quality/assembly/ewis-installation-quality: EWIS installation and inspection quality, downstream of the gauge choice.

Pitfalls

• Sizing on the undated free-air table: the base ampacity of 10 AWG is 33 A free air, but in a 5+ wire bundle at 45 C the derated value is 18.0 A, so an undated selection under-sizes the feeder.
• Using one-way resistance as one-way drop: the drop must cover the round trip, so the 2 * L factor applies; a 1.128 V drop is the two conductor total, not the one-way value.
• Stopping at the gauge that meets ampacity: in the worked example 8 AWG meets the 25 A load but its 4.03 percent drop fails the 3 percent bus tolerance, so the drop verdict drives the upsize to 6 AWG.
• Reading the review verdict as the final selection: wire_size_review reports the ampacity-selected gauge with its drop verdict; a fail verdict means iterate the primitive functions one gauge up.
• Ignoring the ambient band: the model derate is defined for 30 to 100 C ambient and raises ValueError outside it; derating below 30 C ambient is not part of this model.
• Confusing neighbors: the load rollup leaf, the battery pack leaf and the EWIS install quality leaf each own adjacent questions, and none of them selects a wire run gauge by derated ampacity.

Behavior contract (gate 3)

Run the deterministic contract test (stdlib unittest, offline):

python3 scripts/test_electrical_wire_sizing.py

The test covers the sizing contract: the 45 C ampacity anchors (18.0 / 25.1 / 32.8 A bounds at 10 / 8 / 6 AWG), the select_gauge truth table and the beyond-table rejection, resistance anchors for 8 and 6 AWG at 45 C with the temperature ratio identity, the voltage-drop anchors 1.128 V and 0.710 V, the length-doubling and percent-drop identities, the wire_size_review dict key contract with the fail and pass verdicts, determinism, and ValueError rejection of unknown gauge, non-positive load, negative length, non-positive bus voltage and out-of-band ambient.

Compliance

• Standards referenced, not reproduced: FAR 25 Subpart H frames the EWIS certification context and SAE AS50881 is named as the derating reference only; the ampacity derating and resistance relations above are standard engineering methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.