Ideally, a wire should have as little resistance as possible, but practically, it will have a specific resistance determined by its AWG size, material, and temperature—typically ranging from 0.015 ohms per 1,000 feet for thick 4/0 AWG copper up to 10.4 ohms per 1,000 feet for thin 20 AWG copper. Wire resistance is the inherent opposition a conductor offers to electrical current flow, measured in ohms per unit length, dictated by its cross-sectional area and material resistivity. In a real installation, this resistance directly dictates voltage drop, determines how much heat the cable generates under load (I²R losses), and decides whether a distant motor will start or stall.
The Core Data: Copper Wire Resistance by AWG
To answer the question of how much resistance a specific wire has, you need to look at its American Wire Gauge (AWG) and whether it is solid or stranded. Stranded wire has slightly higher resistance than solid wire of the same AWG due to the air gaps between the strands and the spiraling of the conductors. The table below provides the DC resistance for stranded, uncoated copper wire at 20°C (68°F), alongside the maximum allowable ampacity.
| AWG Size | Area (Circular Mils) | Resistance (Ω / 1,000 ft) | Max Ampacity (Copper) | Common Application |
|---|---|---|---|---|
| 14 AWG | 4,110 | 3.14 Ω | 15A (60°C col) | Standard 120V lighting/outlet circuits |
| 12 AWG | 6,530 | 1.98 Ω | 20A (60°C col) | Kitchen/bathroom small appliance circuits |
| 10 AWG | 10,380 | 1.24 Ω | 30A (60°C col) | Dryers, water heaters, AC disconnects |
| 8 AWG | 16,510 | 0.778 Ω | 50A (75°C col) | EV chargers, subpanel feeders |
| 6 AWG | 26,240 | 0.491 Ω | 65A (75°C col) | Heavy subpanels, tankless water heaters |
| 4 AWG | 41,740 | 0.308 Ω | 85A (75°C col) | Main service entrance, large subpanels |
Note on Ampacity: Per NEC 240.4(D), conductors sized 14, 12, and 10 AWG are strictly limited to the 60°C ampacity column (15A, 20A, and 30A respectively) for overcurrent protection, even if you use 90°C THHN wire. Larger conductors (8 AWG and up) can utilize the 75°C column if the terminals are rated for it.
Worked Example: Calculating Voltage Drop in a Real Circuit
Let’s look at a real-world scenario to see how wire resistance changes a circuit's behavior. Suppose you are wiring a dedicated 120V outlet in a detached garage for a heavy-duty space heater. The run from the main panel to the garage is 50 feet. You decide to use 12 AWG stranded copper wire, and the heater pulls a continuous 16 amps.
First, we must calculate the total loop length. Electricity travels to the load and back, so a 50-foot one-way run equals a 100-foot total circuit loop.
- Base Resistance: From the table, 12 AWG stranded copper is 1.98 Ω per 1,000 ft.
- Loop Resistance: 1.98 Ω × (100 ft / 1,000 ft) = 0.198 Ω total resistance.
Now, we apply Ohm’s Law (V = I × R) to find the voltage drop:
- Voltage Drop: 16A × 0.198 Ω = 3.168 volts dropped.
- Percentage Drop: (3.168V / 120V) × 100 = 2.64%.
If this were a 240V circuit, the percentage drop would be cut in half (1.32%), demonstrating why higher voltages are preferred for long-distance power transmission and heavy loads.
Where You Meet Wire Resistance in Practice
Understanding how much resistance a wire should have isn't just academic; it dictates how you troubleshoot and design systems on the jobsite or at the workbench.
1. Sizing Long-Run Feeders and EV Chargers
When installing a Level 2 EV charger (e.g., a 48A continuous load on a 60A breaker) 100 feet away from the panel, standard 6 AWG wire will result in unacceptable voltage drop and excessive heat. You must intentionally upsize the wire to 4 AWG or even 3 AWG to lower the resistance, ensuring the charger receives adequate voltage and the wire stays cool inside the conduit.
2. Diagnosing Melted Terminals and Loose Connections
A wire itself should have very low resistance, but a connection should have virtually zero. If a terminal lug is loose, or if wire strands are corroded, you introduce a high-resistance fault point. Under a 20A load, even an extra 0.5 ohms of resistance at a loose neutral bar will generate 200 watts of localized heat (P = I²R), which is more than enough to melt the insulation and start a fire. When troubleshooting a hot breaker panel, an IR thermometer or thermal camera will quickly reveal these high-resistance joints.
3. Power over Ethernet (PoE) Limits
In low-voltage networking, wire resistance is the primary limiting factor for PoE cameras and access points. Standard 24 AWG Cat6 solid copper has a resistance of about 25.2 Ω per 1,000 feet. If you run 250 feet of cable to a security camera, the loop resistance is over 12 ohms. This massive resistance drops the 48V PoE voltage down below the 44V minimum required by the camera, causing it to reboot endlessly. This is why long PoE runs require 23 AWG wire or midspan injectors.
Common Confusions: What Wire Resistance Is Not
When testing circuits with a multimeter or megohmmeter, hobbyists and junior technicians frequently confuse conductor resistance with other electrical properties.
Continuity vs. Insulation Resistance
When you put your multimeter in continuity mode and touch the probes to both ends of a 10-foot piece of 12 AWG wire, the meter beeps and reads near 0.0 Ω. This is continuity (conductor resistance), confirming the wire is unbroken. However, insulation resistance is entirely different. It measures how well the plastic or rubber jacket prevents current from leaking to the ground or adjacent wires. According to testing standards outlined by Fluke's electrical testing guidelines, insulation resistance should be measured in megohms (MΩ) using a high-voltage insulation tester (Megger). A healthy 600V THHN wire should show >100 MΩ between the copper conductor and the bare ground wire. If you measure low resistance between a hot wire and a ground wire, you have a dead short or destroyed insulation, not a "good" wire.
DC Resistance vs. AC Impedance
The table at the top of this article lists DC resistance. In alternating current (AC) circuits, especially those involving large conductors or magnetic fields, the total opposition to current flow is called impedance. Impedance includes the wire's DC resistance plus its AC reactance (caused by the skin effect and the inductance of the wire interacting with the conduit or other phases). For standard residential branch circuits under 100 feet, DC resistance is perfectly adequate for voltage drop calculations. For massive industrial feeders or utility transmission lines, engineers must calculate full AC impedance to get accurate results.
Frequently Asked Questions
Does wire resistance change with temperature?
Yes. Copper has a positive temperature coefficient. As the wire heats up under load, its resistance increases. A wire that measures 1.98 Ω/1,000 ft at room temperature (20°C) will have roughly 20% more resistance when operating at its maximum 75°C rating, which slightly worsens voltage drop under heavy, sustained loads.
Is aluminum wire more resistive than copper?
Yes. Aluminum has about 61% higher resistance than copper for the exact same physical volume. To achieve the same resistance and ampacity, you must use an aluminum wire that is two AWG sizes larger than the equivalent copper wire (e.g., 2 AWG aluminum instead of 4 AWG copper for a 100A subpanel feeder).
For further reading on wire properties and standard sizing tables, reference the comprehensive wire resistance tables published by Electrical Technology, which break down metric and imperial measurements across various conductor materials.
