PCB traces, vias, and wiring

Trace width — IPC-2221: I = k·ΔT^0.44·A0.725, with the cross-section A in square mils and k = 0.048 for outer layers, 0.024 for inner layers. Copper weight sets the thickness (1 oz/ft² ≈ 35 µm). Give a length to see resistance, voltage drop, and power at the trace’s operating temperature (ρ = 1.724 × 10⁻⁸ Ω·m at 20 °C, +0.393%/°C).

Vias — the plating ring (drill diameter, plating thickness inward) treated as a conductor with the same IPC-2221 formula, its resistance through the board, and how many vias a current needs.

Wire gauges — AWG 0000 to 40 (diameter 0.127 mm × 92^36 − n)/39, area, resistance per metre, and rule-of-thumb ampacity for a single wire in free air and for bundled runs; plus metric sizes with their nearest AWG.

Cable drop & fuses — two-way DC voltage drop for a run, the percentage of the supply it costs, the smallest metric or AWG size within a drop budget, and a fuse rating at about 125% of the load that also protects the chosen wire.

Microstrip impedance — IPC-2141: Z₀ ≈ 87/√(εr + 1.41)·ln(5.98h/(0.8w + t)), and the inverse for a target impedance. It is an approximation: use your fabricator’s stack-up calculator for controlled-impedance boards.