How it works
Wire has resistance. Resistance causes a voltage drop proportional to current. For DC systems this is Ohm’s law applied to the conductor:
R = ρ × L / A
V_drop = I × R
P_loss = I² × RWhere:
ρ— resistivity of copper at 20°C: 1.724 × 10⁻⁸ Ω·mL— wire length in meters (use round-trip length for both conductors)A— cross-sectional area in m²
For AWG 22 copper wire (0.326 mm²):
R/m = 1.724e-8 / (0.326e-6) = 52.9 mΩ/mA 2-meter round-trip (4 m total) at 0.5 A:
R = 0.0529 × 4 = 211 mΩ
V_drop = 0.5 × 0.211 = 106 mV106 mV on a 3.3 V rail is 3.2% — right at the warning threshold.
Temperature correction
Copper resistance increases with temperature. The correction factor uses the temperature coefficient α = 0.00393/°C:
R_T = R_20 × (1 + 0.00393 × (T − 20))At 85°C (inside an enclosure in direct sun), resistance is 26% higher than at 20°C. If your voltage drop budget is tight, verify at maximum operating temperature.
Why round-trip matters
Every circuit requires two conductors: the supply wire and the return (ground). Both contribute resistance. A 2-meter cable has 4 meters of total conductor length. Most engineers remember the supply wire and forget the return. Always use round-trip length.
AWG vs mm²
AWG is American Wire Gauge — a logarithmic scale where smaller numbers = larger wire (counterintuitively). AWG 10 is much thicker than AWG 26. The relationship to mm²:
A (mm²) = 0.012668 × 92^((36−AWG)/39)European IEC 60228 uses nominal mm² areas (0.5, 0.75, 1.0, 1.5, 2.5…). These don’t always map exactly to an AWG equivalent.
Common mistakes for embedded/IoT
Using the same wire gauge for power and signals. Signal wires (SPI, UART, GPIO) can be AWG 28–30 with no voltage drop concern. Power wires to sensors, actuators, and radio modules need to be sized for peak current, not just idle current. An nRF52 transmitting at +8 dBm peaks at 7 mA — a 1-meter AWG 30 cable drops 30 mV under that load, which may trip an LDO into dropout.
Ignoring connector resistance. A standard 0.1” pin header contact adds ~20–50 mΩ per contact. Five connectors in series on a 12 V automotive harness can add 250 mΩ — comparable to several meters of AWG 22 wire. Crimp connectors (Molex, JST) have lower contact resistance than press-fit headers.
Not derating for bundled cables. The current ratings in the table assume a single wire in open air at 30°C ambient. In a cable loom of 10 wires, thermal dissipation is reduced and current ratings drop by 40–50%. Check IEC 60364-5-52 tables for bundled cable correction factors.
5V USB power runs over thin cable. USB 2.0 spec allows 100 mA, but many devices draw 500 mA during charging. AWG 28 internal USB cable (common in many cables) drops 160 mV per meter round-trip at 500 mA. A 2-meter USB cable can drop 320 mV, leaving only 4.68 V at the device — below some LDO minimum input thresholds.
Not accounting for battery internal resistance. A CR2032 has 15–25 Ω internal resistance. At 5 mA pulse (nRF52 transmit), the internal drop is 75–125 mV before the wire even contributes. For coin cell–powered designs, battery internal resistance dominates wire resistance by an order of magnitude.