Wiring, connectors and PCBs

Why this matters

Wires and connectors are the most frequent failure points of a drone’s electrical system. Undersized wires heat up and drop voltage; loose or under-rated connectors get hot enough to melt their plastic; and high-current wires near the compass make the drone misjudge its heading and circle. These details look small, but they decide whether a well-designed system actually works.

Choosing wire size

The resistance of copper wire at 20 °C from NBS Handbook 100 (annealed copper wire tables) is, for example, 3.277 Ω/km for 10 AWG, 5.21 Ω/km for 12 AWG, 8.28 Ω/km for 14 AWG and 13.2 Ω/km for 16 AWG. The lower the AWG number, the larger the wire. In a DC circuit, count the length of both the positive and negative wires.

ohm_per_km = {10: 3.277, 12: 5.21, 14: 8.28, 16: 13.2}   # NBS Handbook 100, copper at 20 °C
amps, length_m, limit_v = 80, 0.6, 0.01 * 22.2
for awg, r_km in ohm_per_km.items():
    r = r_km * length_m / 1000
    drop, heat = amps * r, amps ** 2 * r
    verdict = "ok" if drop <= limit_v else "too small"
    print(f"{awg} AWG: {r * 1000:.2f} mΩ, drop {drop:.3f} V, heat {heat:.1f} W -> {verdict}")

10 AWG: 1.97 mΩ, drop 0.157 V, heat 12.6 W -> ok
12 AWG: 3.13 mΩ, drop 0.250 V, heat 20.0 W -> too small
14 AWG: 4.97 mΩ, drop 0.397 V, heat 31.8 W -> too small
16 AWG: 7.92 mΩ, drop 0.634 V, heat 50.7 W -> too small

Connectors

A connector’s current rating must come from the manufacturer’s datasheet, not the number in the model name or the advertising. Amass’s datasheets, for example, rate the XT60H-M (2025 edition) at 35 A with 12 AWG wire, and the XT90H-M at 45 A (90 A momentary) with 10 AWG. The “90” in XT90 is therefore not a continuous current. A survey drone with a peak near 100 A needs connectors rated for it, or its peak current limited below the rating.

PCB traces

Distribution boards and add-on boards use copper traces instead of wires. The IPC-2221 formula used by PCB makers’ calculators is $I=k\Delta T^{0.44}{A}^{0.725}$, where $A$ is the cross-section (square mils), $\Delta T$ the temperature rise (°C) and $k=0.048$ for outer layers. IPC-2152 (2009) is the dedicated standard for trace current capacity and should be used for real work.

k, delta_t, amps = 0.048, 10, 5                    # outer layer, °C, A
area_mil2 = (amps / (k * delta_t ** 0.44)) ** (1 / 0.725)
thickness_mil = 1.378                              # 1 oz/ft² copper
width_mil = area_mil2 / thickness_mil
print(f"{amps} A, +{delta_t} °C: area {area_mil2:.0f} mil², width {width_mil:.0f} mil = {width_mil * 0.0254:.2f} mm")

5 A, +10 °C: area 150 mil², width 109 mil = 2.77 mm

In KiCad (version 10, GPL licence), ERC checks the schematic, for example for unconnected power pins, and DRC checks the board, for example for traces narrower than the rules or too close together. Set the width rule for power nets to match the calculation.

Reducing compass interference

High current in wires creates magnetic fields that disturb the compass. Keep high-current leads as short as possible, run positive and negative together so their fields cancel, and mount the compass on a mast away from the PDB and battery. ArduPilot’s CompassMot procedure measures interference from motors and power wires: under 30% is acceptable, 31–60% is a grey zone, and above 60% the unit should be moved or an external compass used.

Module lab

Common mistakes

Summary

• Choose wire size from resistance per length, voltage drop and heat, counting both directions
• Connector ratings must come from the manufacturer’s datasheet
• Estimate traces with the IPC-2221 formula, check against IPC-2152, and verify designs with ERC and DRC
• High-current leads should be short, run together and away from the compass; CompassMot under 30% is acceptable

Check your understanding

1. What is the resistance of 12 AWG wire 1 m long in total?
2. That wire carries 40 A. What are the voltage drop and heat?
3. Why is the number in the name XT90 not the continuous current to design with?
4. Which KiCad check finds traces that are too close together?
5. What should you do if CompassMot shows 70%?