Calcolatore Resistenza Filo
// CALCOLARE LA RESISTENZA ELETTRICA PER QUALSIASI SEZIONE E LUNGHEZZA //
Reference: 20°C (68°F)
This energy is dissipated as heat in the wire
Wire resistance increases with temperature. Metallic conductors have a positive temperature coefficient, meaning their resistance rises as they heat up. This effect is significant in high-current applications where wire heating occurs.
0°C (32°F)
1.463
Ω/1000ft
20°C (68°F)
1.588
Ω/1000ft (Reference)
60°C (140°F)
1.838
Ω/1000ft
90°C (194°F)
2.025
Ω/1000ft
What is Electrical Resistance?
Electrical resistance is a measure of how much a material opposes the flow of electric current. In wire conductors, resistance causes electrical energy to be converted into heat as current flows through the wire. This property is measured in Ohms (Ω) and is fundamental to understanding circuit behavior, voltage drop, and power efficiency in electrical systems.
The Resistance Formula
Resistance Calculation
R = ρ × L / A
| Symbol | Name | Unit | Description |
|---|---|---|---|
| R | Resistance | Ohms (Ω) | Opposition to current flow |
| ρ | Resistivity | Ω·m | Material property (copper: 1.68×10⁻⁸ Ω·m) |
| L | Length | meters | Total conductor length |
| A | Cross-sectional Area | m² | Wire cross-section (larger = lower R) |
Factors That Affect Wire Resistance
Wire Length
Resistance is directly proportional to length. Doubling the wire length doubles the resistance. This is why long wire runs require larger gauge wire to maintain acceptable voltage drop.
Wire Gauge (Diameter)
Resistance is inversely proportional to cross-sectional area. Larger wire (lower AWG number) has less resistance. Each 3 AWG decrease roughly doubles the cross-sectional area.
Conductor Material
Copper has lower resistivity than aluminum (1.68 vs 2.82 × 10⁻⁸ Ω·m). For equivalent resistance, aluminum wire must be approximately 1.6× larger in cross-section.
Temperature
Resistance increases with temperature in metals. Copper resistance increases about 0.393% per degree Celsius. At 90°C, resistance is ~27% higher than at 20°C.
Complete Wire Resistance Reference (at 20°C)
This table provides DC resistance values for solid copper and aluminum conductors at the standard reference temperature of 20°C (68°F). For stranded wire, resistance is typically 1-3% higher due to the air gaps between strands.
| AWG | Diameter (mm) | Area (mm²) | Copper Ω/1000ft | Copper Ω/km | Aluminum Ω/1000ft | Aluminum Ω/km |
|---|---|---|---|---|---|---|
| 1 | 7.348 | 42.41 | 0.124 | 0.407 | 0.204 | 0.669 |
| 2 | 6.544 | 33.63 | 0.156 | 0.512 | 0.257 | 0.843 |
| 3 | 5.827 | 26.67 | 0.197 | 0.646 | 0.324 | 1.063 |
| 4 | 5.189 | 21.15 | 0.249 | 0.817 | 0.408 | 1.339 |
| 6 | 4.115 | 13.30 | 0.395 | 1.296 | 0.649 | 2.129 |
| 8 | 3.264 | 8.37 | 0.628 | 2.060 | 1.032 | 3.386 |
| 10 | 2.588 | 5.26 | 0.999 | 3.278 | 1.640 | 5.381 |
| 12 | 2.053 | 3.31 | 1.588 | 5.210 | 2.609 | 8.560 |
| 14 | 1.628 | 2.08 | 2.525 | 8.284 | 4.148 | 13.609 |
| 1/0 | 8.252 | 53.48 | 0.098 | 0.322 | 0.162 | 0.531 |
| 2/0 | 9.266 | 67.43 | 0.078 | 0.256 | 0.128 | 0.420 |
| 3/0 | 10.404 | 85.01 | 0.062 | 0.203 | 0.102 | 0.335 |
| 4/0 | 11.684 | 107.22 | 0.049 | 0.161 | 0.081 | 0.266 |
Values shown are for solid conductors at 20°C (68°F). Add 1-3% for stranded conductors.
Temperature Correction for Resistance
Wire resistance changes with temperature according to a well-defined relationship. Understanding this is critical for accurate calculations in real-world applications where conductors operate above ambient temperature.
Temperature Correction Formula
R(T) = R₂₀ × [1 + α × (T - 20)]
Copper Temperature Coefficient
α = 0.00393 /°C
Resistance increases 0.393% per degree Celsius above 20°C
Aluminum Temperature Coefficient
α = 0.00403 /°C
Resistance increases 0.403% per degree Celsius above 20°C
Practical Temperature Multipliers
| Temperature | Copper Multiplier | Aluminum Multiplier | Application |
|---|---|---|---|
| 20°C (68°F) | 1.000 | 1.000 | Reference temperature |
| 30°C (86°F) | 1.039 | 1.040 | Warm ambient conditions |
| 60°C (140°F) | 1.157 | 1.161 | Typical operating temperature |
| 75°C (167°F) | 1.216 | 1.222 | Standard insulation rating |
| 90°C (194°F) | 1.275 | 1.282 | High-temp insulation rating |
Practical Wire Resistance Applications
Example 1: Voltage Drop Calculation
Scenario: 10 AWG copper wire, 200 ft round-trip, carrying 30A at 75°C operating temperature.
Base R @ 20°C
0.999 Ω/1000ft
Temp Multiplier
1.216
Total Resistance
0.243 Ω
Voltage Drop
7.29 V
Calculation: R = (0.999/1000) × 200 × 1.216 = 0.243Ω → V = I×R = 30 × 0.243 = 7.29V (3.0% on 240V circuit)
Example 2: Power Loss in Feeder
Scenario: 4/0 AWG copper feeder, 150 ft one-way (300 ft total), carrying 200A.
Resistance @ 20°C
0.0147 Ω
Current
200 A
Power Loss (P=I²R)
588 W
Annual Cost @$0.12/kWh
$618
Insight: Wire losses are permanent efficiency costs. Upsizing to 250 kcmil reduces losses by 37%, potentially paying for the larger wire over time.
Common Resistance Calculation Mistakes
Forgetting Round-Trip Distance
Current flows out AND back. For a 100-foot circuit, resistance calculation uses 200 feet of wire. This is the most common mistake in voltage drop calculations.
Ignoring Operating Temperature
Reference tables show resistance at 20°C, but conductors carrying current operate at higher temperatures. At 75°C, resistance is ~22% higher than the table value.
Using DC Resistance for AC Circuits
AC circuits experience skin effect, increasing effective resistance. For conductors larger than 1/0 AWG at 60Hz, AC resistance can be 5-10% higher than DC resistance.
Neglecting Connection Resistance
Wire terminals, splices, and connections add resistance. Poorly made connections can add significant resistance and become heat sources, causing fires.
Confusing Solid vs Stranded Values
Stranded wire has 1-3% higher resistance than solid wire of the same gauge due to air gaps between strands. Most building wire is stranded.