Resistance for copper wire is the inherent opposition the metal presents to electron flow, converting a small fraction of electrical energy into heat and causing a measurable voltage drop along the conductor's length.

When you pull wire for a new branch circuit, you are likely focused on ampacity—making sure the wire can handle the breaker size without melting. But ignoring the actual resistance of the copper run is one of the most common reasons DIY circuits underperform, motors burn out, and smart home devices fail to boot. This guide breaks down the physics, the math, and the exact decision path you need to size copper wire correctly for any residential or low-voltage application.

What Resistance for Copper Wire Actually Means (and What It Changes)

In a real circuit, wire resistance changes two critical variables: the voltage available at the load and the ambient heat generated inside the wall or conduit. Every foot of copper wire acts as a tiny, low-value resistor in series with your load. As current flows through this resistance, energy is lost as heat (calculated via I²R), and the voltage at the destination drops below the source voltage.

Safety Warning: Any time you are verifying resistance, continuity, or pulling wire near a live panel, you must de-energize the circuit at the main breaker, apply a lockout/tagout if possible, and verify the wires are dead using a non-contact voltage tester and a tested digital multimeter. Local codes may require a licensed electrician for panel terminations.

The physical resistance of a copper conductor is fixed by three factors:

  • Length: Resistance scales linearly with the total out-and-back distance of the circuit.
  • Cross-sectional area (AWG): Thicker wire (lower AWG number) provides more parallel paths for electrons, reducing resistance.
  • Temperature: Copper has a positive temperature coefficient; as the wire heats up under load, its resistance increases, which in turn generates slightly more heat.

The Worked Example: Calculating Real-World Voltage Drop

Let's move past theory and look at a real jobsite scenario. You are wiring a 120V dedicated outlet for a high-draw appliance in a detached garage. The one-way distance from the subpanel to the outlet is 100 feet. You plan to use standard 12 AWG solid copper wire on a 15A breaker.

According to standard copper wire tables, the resistance of 12 AWG uncoated copper at an operating temperature of 75°C is approximately 1.98 ohms per 1,000 feet (Georgia State University HyperPhysics).

Step 1: Calculate Total Wire Length
A 100-foot run requires 100 feet of hot wire and 100 feet of neutral wire. Total conductor length = 200 feet.

Step 2: Calculate Total Resistance (R)
R = (1.98 Ω / 1000 ft) × 200 ft = 0.396 ohms.

Step 3: Calculate Voltage Drop (V_drop)
Using Ohm's Law (V = I × R), with a 15A load:
V_drop = 15A × 0.396 Ω = 5.94 volts.

Step 4: Calculate Percentage Drop
(5.94V / 120V) × 100 = 4.95%.

The Verdict: A 4.95% voltage drop exceeds the 3% maximum recommended by the National Electrical Code (NEC) for branch circuits. Your 120V appliance will only see 114.06V under full load, which can cause motors to overheat and draw excess current.

To fix this, you must step up to 10 AWG copper wire, which drops the resistance to roughly 1.24 Ω/kft at 75°C, bringing the voltage drop down to an acceptable 2.06%.

Where You Meet This in Practice (and Where It Bites You)

You rarely notice wire resistance on a 20-foot run to a bedroom receptacle. The resistance bites you on long, high-current, or low-voltage runs where the math turns against you.

  • Level 2 EV Chargers: A 40A continuous load on a 240V circuit pulled 150 feet through a crawlspace. If you use 8 AWG wire (rated for 50A), the resistance will cause a massive voltage drop that can trigger the EVSE's internal undervoltage protection, halting the charge midway through the night.
  • Solar DC Strings: Solar panels output low voltage (e.g., 40V) and high current (e.g., 10A). Because P = I²R, the high current makes DC wire resistance incredibly destructive to your harvest. This is why solar installers use thick 10 AWG or 8 AWG PV wire even for relatively short roof-to-inverter runs.
  • PoE Cameras and Smart Hubs: Power over Ethernet (PoE) operates at 48V DC over tiny 24 AWG or 23 AWG copper conductors. If you exceed 300 feet, the resistance of the copper starves the camera's internal switching regulator, causing it to boot-loop continuously.

Resistance vs. Ampacity: The Most Common Confusion

The most frequent mistake DIYers make is confusing resistance with ampacity. They are entirely different concepts governed by different physics.

Ampacity is the maximum continuous current a wire can carry before its insulation degrades or melts. It is dictated by the NEC (specifically Table 310.16) and depends on the insulation type (THHN, XHHW, NM-B), the ambient temperature, and how many current-carrying conductors are bundled in the conduit. Ampacity does not care how long the wire is; a 1-foot piece of 14 AWG copper and a 500-foot piece of 14 AWG copper both have an ampacity of 15A (in the 60°C column).

Resistance, conversely, is a property of the conductor metal itself. It cares deeply about length and cross-section, but it doesn't care about your insulation type.

Bench Tip: You can have a wire with perfectly safe ampacity but terrible resistance. If you run 200 feet of 12 AWG NM-B cable for a 20A compressor, the wire won't melt (it's within its 20A ampacity limit), but the high resistance will drop the voltage at the compressor to 108V. The compressor motor will stall, draw locked-rotor current, and burn out its windings. Always check ampacity first for safety, then check resistance (voltage drop) for performance.

Decision Tree: Sizing Copper Wire for Your Next Run

Use this decision path to select the correct wire gauge for your next project. Do not skip the voltage drop verification step for any run exceeding 50 feet.

Condition / Measurement Action Required Concrete Pick / Value
Run is under 50 ft, standard 15A/20A 120V circuit Size purely by NEC ampacity limits (60°C column for NM-B). 14 AWG for 15A; 12 AWG for 20A.
Run is 50–100 ft, 15A/20A 120V circuit Step up one AWG size to defeat resistance-induced voltage drop. 12 AWG for 15A; 10 AWG for 20A.
Run is > 100 ft on a 20A 240V circuit (e.g., EV charger) Calculate exact V-drop. Step up until drop is < 3%. Default Pick: Southwire 10 AWG THHN Copper (or 8 AWG if >150 ft).
Low voltage DC (12V/24V/48V) over 20+ ft Ignore standard AC ampacity tables. Size strictly for < 2% V-drop. Use a dedicated DC wire calculator; expect to use 8 AWG or 6 AWG.

Frequently Asked Questions About Copper Wire Resistance

Does stranded copper wire have more resistance than solid copper?

Technically, yes. A stranded wire of a given AWG has slightly more resistance than a solid wire of the same AWG. This is because the spiraling of the strands makes the actual path of the electrons slightly longer than the physical length of the wire, and there are microscopic air gaps between the strands. However, for standard residential wiring at 60Hz, this difference is negligible (usually less than 2%) and does not affect your voltage drop calculations or breaker sizing.

How much does temperature affect copper resistance?

Copper's resistance increases by about 0.393% for every 1°C rise in temperature. This is why the NEC and engineering tables reference specific temperature columns (like the 75°C or 90°C columns). If you are sizing wire for a hot attic where ambient temperatures reach 110°F (43°C), the wire's baseline resistance will be higher before it even carries a load, and its ampacity must be derated according to NEC Table 310.15(B)(1).

Can I use aluminum wire to save money on long runs?

Aluminum has roughly 61% the conductivity of copper, meaning an aluminum wire will have about 1.6 times the resistance of a copper wire of the exact same AWG. If you switch to aluminum (like SER cable for a subpanel feeder) to save money on long runs, you must step up two AWG sizes to match the resistance and ampacity of the copper equivalent. Always use antioxidant paste (like Noalox) on aluminum terminations to prevent oxidation, which drastically increases connection resistance over time.

Stop guessing on long runs. Measure your distance, calculate the resistance, and step up your wire gauge before you pull it through the conduit. Your default pick for any problematic 20A residential run over 75 feet should always be 10 AWG THHN copper—it provides the ultimate insurance policy against voltage drop and heat buildup.