DC resistance is the opposition a material offers to the flow of direct current (steady, one-direction electrical current). It’s measured in ohms (Ω) and defined by a straightforward relationship: resistance equals voltage divided by current. If you apply 10 volts across a wire and 2 amps flow through it, that wire has a DC resistance of 5 ohms.
The term “DC” distinguishes this value from AC resistance, which behaves differently due to effects that only appear when current alternates direction. DC resistance represents the pure, baseline resistance of a conductor using its entire cross-section.
What Determines DC Resistance
Four physical properties control how much DC resistance a conductor has: its length, its cross-sectional area, the material it’s made from, and its temperature.
Longer conductors have more resistance. A 100-meter copper wire resists current more than a 10-meter copper wire of the same thickness, simply because electrons have farther to travel and encounter more atomic collisions along the way. Thicker conductors have less resistance because there’s more room for current to flow, the same way a wider pipe carries water more easily.
The material matters because different metals allow electrons to move with varying degrees of ease. This property is called resistivity. Silver has the lowest resistivity of any common metal at 1.59 × 10⁻⁸ ohm-meters, followed closely by copper at 1.68 × 10⁻⁸. Aluminum comes in at 2.65 × 10⁻⁸, roughly 60% higher than copper, which is why aluminum wiring needs to be thicker to carry the same current safely. The relationship ties together neatly: resistance equals resistivity multiplied by length, divided by cross-sectional area.
How Temperature Changes Resistance
DC resistance isn’t fixed. It shifts with temperature, and for most metals, it rises as the conductor heats up. The atoms in a warm metal vibrate more, creating additional obstacles for flowing electrons.
Each metal has a temperature coefficient that tells you how much its resistance changes per degree Celsius. Copper’s coefficient is about 3.9 × 10⁻³ per °C, meaning for every degree above the reference temperature (usually 20°C), copper’s resistance increases by roughly 0.39%. That sounds small, but it adds up. A copper motor winding running at 80°C has noticeably higher resistance than the same winding at room temperature, which translates to more wasted energy as heat.
Not all metals respond equally. Iron’s coefficient is 5.0 × 10⁻³ per °C, making it more temperature-sensitive. Nichrome, the alloy used in heating elements, has a coefficient of just 0.4 × 10⁻³ per °C, so its resistance stays nearly constant whether it’s warm or extremely hot. That stability is exactly why it’s chosen for applications like toasters and industrial heaters.
DC Resistance vs. AC Resistance
When direct current flows through a wire, it uses the entire cross-section evenly. AC resistance is always equal to or higher than DC resistance because of a phenomenon called the skin effect: alternating current crowds toward the outer surface of a conductor, effectively shrinking the usable cross-section. Since less area is available for current to flow through, resistance goes up.
The skin effect gets worse at higher frequencies. At 60 Hz (standard household power), the effect is mild in typical wiring. At radio frequencies or in high-speed digital circuits, it becomes significant enough that engineers use special techniques like stranded wire bundles or hollow tubing to keep AC resistance manageable. DC resistance serves as the baseline, the lowest possible resistance a given conductor can have. Any AC application will see resistance at or above that number.
Why DCR Matters in Electronic Components
In electronics, you’ll often see “DCR” listed on datasheets for inductors, transformers, and coils. This is the DC resistance of the wire wound inside the component, and it directly affects performance. Every milliohm of DCR wastes power as heat when current passes through, reducing efficiency and warming up the component.
For power inductors used in voltage converters (the circuits that regulate power in phones, laptops, and servers), DC losses often dominate over AC losses. Lower DCR means less wasted energy and less heat to manage. The tradeoff is physical size: thicker wire lowers DCR but makes the inductor bigger. Engineers constantly balance these competing demands, especially in compact devices where space is tight. When you see two inductors with the same inductance value but different DCR ratings, the one with lower DCR will generally run cooler and more efficiently, but it will also be larger or more expensive.
Speaker cables, long power runs, and PCB traces all have DC resistance that matters in practice. A speaker cable with too much resistance absorbs power that should be reaching the speaker, reducing volume and damping control. A long power cable feeding a remote sensor can drop enough voltage across its own resistance to cause the sensor to malfunction.
Measuring DC Resistance Accurately
For most everyday purposes, a standard multimeter in resistance mode works fine. You connect two leads to the component and read the value. This two-wire method is adequate when the resistance you’re measuring is well above 100 ohms, because the small resistance of the test leads themselves (typically 10 milliohms to 1 ohm) is negligible by comparison.
Low-resistance measurements are a different story. If you’re measuring a 500-milliohm resistor with test leads that have a combined resistance of 100 milliohms, those leads add a 20% error to your reading. For anything at or below about 1,000 ohms where accuracy matters, a four-wire (Kelvin) measurement is the standard approach. One pair of leads pushes current through the component while a separate pair measures the voltage directly across it, eliminating the lead resistance from the reading entirely. This is the method used in calibration labs, on production lines, and anywhere precise low-resistance values are needed.
Temperature control also matters during measurement. Since resistance shifts with temperature, professional measurements are referenced to 20°C. If you measure a copper winding at 35°C, you’ll need to correct the reading back to the reference temperature using the temperature coefficient to get a meaningful comparison against specifications.