The resistance of a wire is the measurable opposition it presents to the flow of electrical current, determined by its material, length, cross-sectional area, and temperature. When electrons move through a conductor like copper, they collide with the metal's atomic lattice, converting some electrical energy into heat. In a real circuit, this resistance changes the available voltage at the load and dictates how much power is wasted as thermal loss along the run. Think of it like pushing water through a long, narrow garden hose; the longer and narrower the hose, the more pressure (voltage) you lose by the time the water reaches the nozzle.
The Physics Behind the Numbers
Every conductor has a baseline resistivity ($\rho$). For copper, this is roughly $1.68 \times 10^{-8} \, \Omega\cdot m$ at 20°C. However, on the jobsite or at the workbench, we rarely calculate raw resistivity. Instead, we rely on standardized tables, specifically NEC Chapter 9, Table 8, which provides the exact DC resistance per 1,000 feet for every AWG size and material.
Three physical factors dictate the final resistance of any wire you pull:
- Length: Resistance scales linearly. Double the wire length, and you double the resistance. Remember that for single-phase AC circuits, current must travel out to the load and back to the panel, meaning your effective wire length is twice the physical distance.
- Cross-Sectional Area (AWG): Resistance is inversely proportional to area. Dropping from 12 AWG to 10 AWG increases the cross-sectional area by about 59%, dropping the resistance by the same proportion.
- Temperature: Copper is a positive temperature coefficient (PTC) material. As the wire heats up under load, its resistance increases by approximately 0.4% for every 1°C rise. This is why the NEC provides separate columns for 60°C, 75°C, and 90°C ratings.
Worked Example: Calculating Voltage Drop on a 12 AWG Branch Circuit
Let's look at a common residential scenario: running a 120V, 15A branch circuit to a detached workshop receptacle located 100 feet from the main panel. You have a spool of 12 AWG solid copper THHN. Will it work?
- Load: 15 Amps (continuous)
- Voltage: 120V nominal
- Physical Distance: 100 feet
- Total Wire Length (Out + Back): 200 feet
- Wire: 12 AWG Solid Copper (75°C column)
According to NEC Chapter 9, Table 8, the resistance of 12 AWG solid copper at 75°C is 1.98 ohms per 1,000 feet.
Step 1: Calculate Total Resistance ($R$)
$R = 1.98 \, \Omega \times (200 \text{ ft} / 1000 \text{ ft}) = 0.396 \, \Omega$
Step 2: Calculate Voltage Drop ($V_{drop}$)
Using Ohm's Law ($V = I \times R$):
$V_{drop} = 15\text{A} \times 0.396 \, \Omega = 5.94\text{V}$
Step 3: Calculate Percentage Drop
$\text{Drop \%} = (5.94\text{V} / 120\text{V}) \times 100 = \mathbf{4.95\%}$
Where You Meet Wire Resistance in Practice
You might not measure wire resistance with a multimeter every day, but its effects show up constantly in modern electrical installations, especially as 2026 home loads like Level 2 EV chargers and whole-home heat pumps become standard.
Long Feeder Runs to Subpanels
When feeding a 100A subpanel in a detached garage 150 feet away, standard ampacity tables tell you that 3 AWG copper is sufficient. However, 300 feet of total wire length (out and back) introduces massive resistance. If you don't upsize to 1 AWG or 1/0 AWG, the voltage at the subpanel will sag heavily when the EV charger and air compressor kick on simultaneously, potentially tripping the main breaker due to brownout-induced overcurrent.
Low-Voltage DC Systems (Solar and LEDs)
In 12V or 24V DC systems, wire resistance is the ultimate bottleneck. Because voltage is so low, even a tiny 1V drop represents a massive 8% loss. A 16 AWG wire might easily handle the ampacity of a 10A LED strip, but over a 20-foot run, the resistance will cause the LEDs at the far end to visibly dim. In low-voltage DC, you almost always size wire for voltage drop (resistance), not ampacity.
High-Current Appliance Startups
Motors draw 5 to 7 times their running current when starting (Locked Rotor Amps, or LRA). If the wire resistance is too high, the voltage at the motor terminals drops below the threshold required to generate starting torque. The motor stalls, continues to draw massive current, and the thermal overload trips.
Common Confusions: Resistance vs. Ampacity vs. Impedance
Even experienced DIYers mix up these three terms. Here is how to separate them on the bench:
| Concept | What It Means | What Limits It |
|---|---|---|
| Resistance | Opposition to current flow causing voltage loss. | Wire length, AWG size, and material. |
| Ampacity | The maximum current a wire can carry safely. | Insulation temperature rating and heat dissipation (NEC 310.16). |
| Impedance ($Z$) | Total AC opposition, including resistance and reactance. | Frequency, inductance, and capacitance (matters mostly in data cables or massive industrial feeders). |
The Golden Rule: Ampacity keeps the wire from melting; resistance keeps the load from starving. A 14 AWG wire has the ampacity to run a 15A space heater, but if the heater is 200 feet away, the wire's resistance will drop the voltage so low that the heater's fan motor might burn out.
Decision Tree: Picking the Right Wire Size for Your Run
Stop guessing and use this decision matrix to select your wire gauge. This path prioritizes both NEC safety minimums and practical voltage drop limits.
| If Your Installation Is... | Then Your Priority Is... | Concrete Wire Pick (Copper) |
|---|---|---|
| Under 50 ft, standard 15A/20A 120V load | Standard Ampacity (NEC 310.16) | 14 AWG (15A) or 12 AWG (20A) |
| 50 to 100 ft, 20A 120V load | Voltage Drop (Target < 3%) | Upsize one step: 10 AWG THHN |
| Over 100 ft, 20A 120V load | Voltage Drop (Target < 3%) | Upsize two steps: 8 AWG THHN |
| 12V/24V DC Solar or LED, > 10A | Extreme Voltage Drop Limit (< 2%) | 8 AWG or 6 AWG Stranded Copper |
| 50A EV Charger, under 50 ft | Standard Ampacity (125% continuous rule) | 6 AWG THHN (rated for 65A at 75°C) |
Frequently Asked Questions
Does stranded wire have more resistance than solid wire?
Technically, yes. Stranded wire has tiny air gaps between the individual copper strands, and the spiraling "lay" of the strands makes the actual copper path slightly longer than the jacket length. However, for standard AWG sizes used in home wiring, this difference is less than 2% and is safely ignored in standard voltage drop calculations.
Can I use aluminum wire to save money on long runs?
Aluminum has about 61% the conductivity of copper, meaning an aluminum wire will have roughly 1.6 times the resistance of the same AWG copper wire. To achieve the same resistance and ampacity, you must upsize aluminum by two AWG sizes compared to copper (e.g., use 2 AWG aluminum where you would use 4 AWG copper). Always use anti-oxidant paste (like Noalox) on aluminum terminations to prevent high-resistance oxidation at the lugs over time.
