Purpose
Heat Kills Wires
Current through resistance generates heat (I^2*R). In a wire bundle, center wires are hottest -- surrounded by other hot wires with no cooling path. This simulation shows the thermal gradient in a harness cross-section.
The Physics
Joule Heating
P = I^2 * R (Watts per meter)
Bundle Derating (AS50881)
1-3 wires: factor 1.0 | 4-7: 0.8 | 8-19: 0.7 | 20-39: 0.5 | 40+: 0.4
Wire max temp = ambient + rise. Must stay below insulation rating (150C for M22759/34).
Key Insight: Thermal analysis is one of Artifact's key differentiators over legacy tools. Most ECAD tools DON'T compute harness thermal -- they just check wire gauge. Artifact computes bundle center temperature for every harness segment.
Historical
James Prescott Joule (1818-1889)
Discovered P=I^2*R (1841). Also established conservation of energy. In space, there's NO convective cooling -- wires can only radiate heat (Stefan-Boltzmann law).
References
Spacecraft Thermal Control Handbook (Gilmore), Ch 1-3 | SAE AS50881 Section 3 | NASA-STD-8739.4 Section 5