Wire resistance is the measurable opposition a specific length and gauge of conductor presents to the flow of electrical current, converting some electrical energy into heat. To find the resistance of a wire, you multiply the conductor's resistance-per-foot value (sourced from an AWG chart based on material and temperature) by the total round-trip length of the circuit. This single calculation dictates whether your installation will suffer from crippling voltage drop or operate efficiently under load.
While ampacity tells you if a wire will melt, resistance tells you if the device at the end of the wire will actually receive enough voltage to function. In a real circuit, wire resistance changes the voltage available at the load, alters the heat generated inside the conduit, and can even delay breaker trip times during a fault by limiting the available short-circuit current.
The Core Data: Wire Resistance by AWG
The most common way to find wire resistance without using a multimeter is to reference standard conductor properties. The National Electrical Code (NEC) Chapter 9, Table 8 provides the baseline DC resistance for uncoated copper and aluminum at 20°C (68°F). Below is a data-dense extraction of the most common residential and commercial wire sizes.
| AWG Size | Diameter (in) | Area (cmil) | Copper (Ω/1000 ft @ 20°C) | Aluminum (Ω/1000 ft @ 20°C) |
|---|---|---|---|---|
| 14 | 0.0641 | 4,110 | 2.525 | 4.220 |
| 12 | 0.0808 | 6,530 | 1.588 | 2.654 |
| 10 | 0.1019 | 10,380 | 0.9989 | 1.670 |
| 8 | 0.1285 | 16,510 | 0.6282 | 1.050 |
| 6 | 0.1620 | 26,240 | 0.3951 | 0.6603 |
| 4 | 0.2043 | 41,740 | 0.2485 | 0.4155 |
| 2 | 0.2576 | 66,360 | 0.1563 | 0.2613 |
Worked Example: Calculating Resistance and Voltage Drop
Let's apply this data to a real-world scenario. You are wiring a 120V branch circuit to a detached workshop using 12 AWG copper THHN wire. The one-way distance from the panel to the outlet is 150 feet, and you plan to run a continuous 16A load (like a portable heater or table saw).
Step 1: Determine Total Wire Length
Current must travel to the load and return to the panel.
Total Length = 150 ft × 2 = 300 feet.
Step 2: Find the Baseline Resistance
From the table above, 12 AWG copper is 1.588 Ω per 1,000 ft.
Base R = (300 / 1000) × 1.588 = 0.4764 Ω.
Step 3: Calculate Voltage Drop
Using Ohm's Law (V = I × R):
Voltage Drop = 16A × 0.4764 Ω = 7.62V.
Step 4: Evaluate the Result
Percentage Drop = (7.62V / 120V) × 100 = 6.35%.
Where You Meet Wire Resistance in Practice
Understanding wire sizing and resistance is not just an academic exercise; it solves specific jobsite and workbench problems.
- Long Feeder Runs to Subpanels: When running 240V feeders to a detached garage 200 feet away, the resistance of 4 AWG aluminum might cause a noticeable voltage sag when a heavy load like an EV charger or welder kicks on. Calculating resistance upfront prevents undersized feeders.
- Low-Voltage DC Systems: In 12V or 24V solar arrays, LED strip runs, or automotive wiring, resistance is the enemy. A 1V drop on a 120V circuit is negligible (0.8%), but a 1V drop on a 12V circuit is massive (8.3%), often causing LED strips to flicker or shift color temperature at the far end of the run.
- Troubleshooting 'Ghost' Voltage Drops: If lights dim when the AC compressor starts, the inrush current (often 3x to 5x the running current) is multiplying against the wire's resistance, causing a temporary but severe voltage sag. Measuring the voltage at the panel versus the outlet under load isolates the wire resistance as the culprit.
Common Confusions: Resistance vs. Resistivity vs. Ampacity
When discussing wire properties, three terms are frequently conflated. Knowing the difference prevents catastrophic design errors.
Resistance vs. Resistivity
Resistivity (ρ) is an intrinsic material property, measured in ohm-meters (Ω·m). Copper has a fixed resistivity of roughly 1.68 × 10⁻⁸ Ω·m regardless of its shape. Resistance (R) is the property of a specific physical object. A 5-foot spool of 12 AWG copper and a 500-foot spool of 12 AWG copper share the exact same resistivity, but vastly different resistances.
Resistance vs. Ampacity
Ampacity is the maximum current a wire can carry before its insulation degrades or melts, governed by NEC Article 310 and based on thermal limits (e.g., 90°C for THHN). Resistance is the electrical friction. A 14 AWG wire has an ampacity of 15A, but its high resistance means it cannot be used for a 15A load at 150 feet without violating voltage drop guidelines. Ampacity keeps the wire from catching fire; resistance keeps the equipment running.
Resistance vs. Impedance
In DC circuits, resistance and impedance are identical. In AC circuits, impedance (Z) is the vector sum of resistance (R), inductive reactance (Xl), and capacitive reactance (Xc). However, for standard 60Hz residential wiring under 1/0 AWG, the skin effect and proximity effect are negligible. For practical home wiring calculations, AC impedance is treated as functionally identical to DC resistance.
Quick Reference FAQ
Does stranded wire have more resistance than solid wire?
Technically, yes, but practically, no. Stranded wire has slightly higher resistance (about 1% to 2% more) than a solid wire of the same AWG. This is due to the spiral 'lay' of the strands (which makes the actual wire path slightly longer than the jacket length) and microscopic air gaps between the strands. For standard power wiring and voltage drop calculations, this difference is negligible and ignored.
How do I calculate wire resistance at higher temperatures?
Use the temperature coefficient formula for copper: R_T = R_20 × [1 + 0.00393 × (T - 20)].
For example, if your 12 AWG wire (1.588 Ω/kft at 20°C) is running through a 75°C attic environment:
R_75 = 1.588 × [1 + 0.00393 × (75 - 20)]
R_75 = 1.588 × [1 + 0.216] = 1.93 Ω/kft.
Always use the hot resistance value when calculating voltage drop for heavily loaded, continuous-duty circuits.
