Table of Contents
Wire Resistance Basics
Every wire has resistance causing voltage drop and power loss (P = I²R). Resistance depends on material resistivity, length, and area. Longer and thinner wires have more resistance. Copper's low resistivity makes it the standard conductor for most applications.
Understanding wire resistance is critical for delivering adequate voltage to loads, especially over long distances. Excessive drop causes motors to run slowly, lights to dim, and electronics to malfunction.
Formula
Where R is resistance (Ω), ρ is resistivity (Ω·m), L is length (m), A is area (m²). For round-trip, use 2× one-way length.
Conductor Resistivity at 20°C
| Material | Resistivity (Ω·m) | vs Copper |
|---|---|---|
| Silver | 1.59×10&supmin;&sup8; | 0.95× |
| Copper | 1.68×10&supmin;&sup8; | 1.00× |
| Aluminum | 2.65×10&supmin;&sup8; | 1.58× |
| Steel | 1.0×10&supmin;&sup7; | 5.95× |
Frequently Asked Questions
How does temperature affect resistance?
Resistance increases ~0.4% per °C for copper. At 75°C (NEC rating), resistance is ~22% higher than at 20°C.
What voltage drop is acceptable?
NEC recommends ≤3% for branch circuits, ≤5% total. On 120V, 3% is only 3.6V.
Why copper over aluminum?
Copper has 1.6× lower resistivity, more reliable connections. Aluminum is cheaper/lighter but needs special connectors and 2 sizes larger wire.