Wire Gauge, Resistance, and Current Capacity

Wire Gauge, Resistance, and Current Capacity Guide

Every wire has resistance that drops voltage and produces heat under load. Copper conductors are not perfect: their finite resistivity converts some of the electrical energy into heat, and the voltage at the far end of a cable run is always lower than the voltage at the source. The resistance per unit length depends on cross-sectional area, which the American Wire Gauge (AWG) system encodes as a single number.¹ Larger AWG numbers mean thinner wire and higher resistance per metre. For DC power runs, two separate limits apply: the maximum current before insulation temperature rises past its rating, and the maximum current before voltage drop at the load becomes unacceptable. Both require Ohm's Law to evaluate, and both must pass before the gauge is appropriate for the application.

Calculating voltage drop with V = I × R

Voltage drop across a wire run uses V = I × R, where R is the total conductor resistance for the length of the run.² Standard AWG resistances per metre at 20°C: 26 AWG is roughly 130 mΩ/m; 24 AWG is roughly 84 mΩ/m; 22 AWG is roughly 53 mΩ/m; 20 AWG is roughly 33 mΩ/m.¹ For a complete circuit, the return conductor adds the same resistance, so multiply by two for the total loop resistance.

For 5 m of 24 AWG at 2 A: loop resistance = 2 × 5 × 0.084 = 0.84 Ω, voltage drop = 2 × 0.84 = 1.68 V. On a 5 V bus, that is a 33 percent drop, which will cause most loads to malfunction because the remaining 3.32 V at the load terminals falls well below the minimum operating voltage of typical 5 V digital ICs and microcontroller boards.

Checking the load minimum supply voltage

Either a heavier gauge or a shorter run is necessary to keep the load within its minimum supply voltage specification, and voltage drop per wire gauge decides which fix is cheaper. A practical rule of thumb for 5 V systems: keep the total wire and connector drop below 250 mV, which means the loop resistance must stay below 0.125 Ω at 2 A.³ For 24 AWG at 84 mΩ/m one-way, the maximum round-trip length is about 0.75 metres. For 20 AWG at 33 mΩ/m, the limit extends to about 1.9 metres. These distances explain why USB cables longer than 1.5 metres often cause voltage-drop problems for power-hungry peripherals, and why active USB-C cables with built-in voltage regulation exist for runs beyond 2 metres.

Current-carrying capacity and thermal insulation limits

Ampacity tables tell you how much current a wire can carry before its insulation overheats, but that single number cannot confirm whether the load at the far end of the cable receives enough voltage to operate reliably.⁴ They capture only the thermal limit of the installation. A thin wire may survive thermally at a given current while the voltage at the load has already fallen below the minimum operating threshold, leaving the circuit malfunctioning without ever tripping a breaker or triggering an overtemperature alarm.

For DC power runs, check both limits because neither one alone gives you the complete picture. Start with the ampacity table for the installation environment to confirm the wire stays within its thermal rating, then calculate voltage drop for the actual cable length and current with V = I × R to verify the load receives adequate supply voltage at maximum draw. Thermal ratings depend on ambient temperature and bundling: a wire running inside a conduit with four other current-carrying conductors must be derated, sometimes by 30 to 40 percent, because the bundle traps heat that would dissipate freely in open air.

Applying both limits to the same wire decision

The stricter of the two requirements sets the minimum gauge, so the same wire may pass the ampacity check and still fail the voltage-drop check if the run is long enough. A 2 m LED strip cable and a 10 m remote sensor cable may need different gauges even when they carry the same current, because the longer run accumulates more total resistance and hits the voltage-drop limit well before the thermal limit becomes a concern.

In automotive wiring, this dual-limit check is standard practice: a 5 A accessory on a 3-metre run needs 18 AWG for ampacity, but if the run extends to 6 metres, the voltage drop requires 16 AWG even though the current has not changed. Failing to check both limits is the most common wiring mistake in aftermarket automotive and marine installations, where the cost of a callback or a warranty claim far exceeds the modest price difference between the correct gauge and the underestimated one.

Power loss in cable runs and the cost of undersized wire

Power wasted as heat in a cable run is P = I² × R_loop.⁵ For 2 A through a loop resistance of 0.84 Ω (5 m of 24 AWG return), the power loss is 2² × 0.84 = 3.36 W. That heat dissipates in the conductors and raises their temperature, which in turn raises their resistance slightly and increases the loss further. On a multi-hour duty cycle in a battery-powered device, even a modest fraction of a watt drawn from a small cell accumulates into significant total energy loss over the course of a day.

Switching to 20 AWG for the same run reduces loop resistance to 2 × 5 × 0.033 = 0.33 Ω, cutting the loss to 2² × 0.33 = 1.32 W. The heavier gauge also cuts the voltage drop to 0.66 V, bringing the load closer to its rated supply range.

This means the load sees more of the nominal voltage and draws its design current rather than starving under a sag caused by undersized conductors. In low-voltage battery-fed systems the improvement from going one gauge heavier is often surprisingly visible: a dim LED brightens, a sensor stops returning outlier readings, and a motor spins at closer to its rated speed simply because the wiring no longer wastes power before it reaches the load.

Power loss in cable runs and the cost of undersized wire

Choosing wire by voltage drop rather than ampacity alone results in more energy-efficient installations and avoids the marginal-supply failure mode where a circuit works at room temperature but fails when the cable warms up under load, because the increase in conductor resistance at elevated temperature causes additional voltage drop that further reduces the voltage available at the load in a self-reinforcing cycle.

For long-term field installations where rewiring is difficult, adding one gauge size as insurance against future load growth keeps the cable cooler and extends insulation life. Document the calculated voltage drop at first commissioning as a baseline for future troubleshooting if a connector contacts begin to oxidise. Over a 10-year service life, the energy saved by using one gauge heavier than the minimum can exceed the cost of the wire itself, making the heavier gauge the economical choice in addition to the reliable one.

When to use this

Use this guide when selecting wire gauge for a DC power run before cutting and crimping. Reference it when calculating the voltage drop from a battery to a remote load, or when verifying that an existing harness can carry an increased current without unacceptable voltage loss or overheating.

Examples