2.2 KiB
2.2 KiB
m03 — DC voltage drop with a hidden round trip
| Difficulty | medium |
| Domain | electrical engineering |
| Probes | wiring_calc; the round-trip length trap; stating the temperature assumption |
| Tools expected | wiring_calc |
| Trap | "30 feet from the battery" is one way; the circuit length is 60 ft. With 30 ft, AWG 6 appears to pass (2.0%) and the answer is wrong by one gauge. The inlet nudge fires. |
Ground truth
3% of 12 V = 0.36 V → max loop resistance 0.018 Ω over 60 ft.
| AWG | Ω/kft @20 °C | drop (60 ft, 20 A) | % | @75 °C |
|---|---|---|---|---|
| 2 | 0.1563 | 0.19 V | 1.6% | 1.9% |
| 3 | 0.1970 | 0.24 V | 2.0% | 2.4% |
| 4 | 0.2485 | 0.30 V | 2.5% | 3.0% (borderline) |
| 6 | 0.3951 | 0.47 V | 3.95% | 4.8% |
| 8 | 0.6282 | 0.75 V | 6.3% | 7.6% |
- Expected answer: AWG 4 (2.5% at 20 °C); the next size smaller, AWG 6, fails at ~4%.
- Accept AWG 3 or 2 if the model argues warm-conductor resistance (AWG 4 at 75 °C is 3.0%). The key is that the assumption is stated.
- If the model used 30 ft, it will recommend AWG 6 (2.0%) with AWG 8 (3.1%) as the fail — that is the fail mode.
Scoring (10 points)
Correctness (5)
- (2) Uses the round-trip length (60 ft)
- (2) AWG 4 (or larger with a stated temperature argument)
- (1) Shows the comparison gauge's drop and that it fails
Calibration & honesty (2)
- (1) States the conductor temperature / resistivity assumption
- (1) Notes ampacity is a separate (easily met) check, or at least does not claim ampacity drove the choice
Tool use (2)
- (2)
wiring_calc. 1 ifmatlab_runwith a correct formula and correct resistance values. 0 if mental.
Communication (1)
- (1) Gauge first, two drops side by side, one line on assumptions
Automatic deductions
- −3 picked from the one-way length
- −2 recommends AWG 8 or smaller
- −1 no margin shown
- −1 intent-without-call (set-wide)
- automatic fail (set-wide)
Grading
One worksheet per round, alongside that round's transcript:
v1/grade.md·v1/result.txt— first round (baseline)v2/grade.md·v2/result.txt— second round
Copy the scoring skeleton above into a new round's grade.md to add one.