Connectors & Wiring — Cheat Sheet
Wire sizing, voltage drop, crimping, shielding, strain relief and IP sealing.
Wire sizing
| AWG diameter | d(mm) = 0.127 × 92^((36−n)/39) |
| Area step | each −3 AWG ≈ doubles area |
| Copper resistance | R ≈ 17.24 / area(mm²) Ω/km |
| Voltage drop (DC) | V = 2 · I · ρ · L / A (round trip) |
| Target drop | ≤ 3% signal/branch, ≤ 5% total |
Current & derating
| Free air | single conductor carries the most |
| Bundled | derate — mutual heating |
| Connector | derate ~50% when all contacts energized |
| Fuse rating | ≈ 1.25 × continuous current |
| Fuse energy | I²t must exceed the inrush let-through |
Termination
| Crimp | gas-tight, correct die — stronger than solder for vibration |
| Solder + vibration | wicking makes a stiff spot that fatigues |
| Strain relief | always — the joint must never carry load |
| Bend radius | ≥ 5–10× cable OD |
| Service loop | leave slack for one re-termination |
Shielding & sealing
| Shield ground | ONE end only for signals (avoids ground loops) |
| Twisted pair | rejects magnetic pickup; pair the return with the signal |
| Separation | route power and signal apart, cross at 90° |
| IP rating | first digit solids, second liquids (IEC 60529) |
| Backshell | strain relief + shield termination in one |
Formula reference — verify against the governing standard. Not a substitute for engineering judgment.