Wire Resistance Calculator
Calculate electrical resistance of wire conductors.
Precise Calculations
Resistance of copper and aluminum conductors at the temperature you choose.
Multiple Parameters
Calculate resistance and power loss for any conductor.
Engineering Data
Access comprehensive resistance tables and technical specifications.
Wire Resistance Calculator
Calculate wire resistance and power loss for electrical conductors
Wire Resistance Parameters
20 °C
- • R(T) = R20 × (1 + α × (T − 20 °C)). Copper: R20 from NIST Handbook 100 (kcmil sizes calculated), α = 0.00393. Aluminum EC-H19: R20 calculated from NIST Handbook 109 constants, α = 0.00403, 12 AWG and larger.
- • Power loss: 2 × I² × R × L ÷ 1000 for DC or single-phase, 3 × I² × R × L ÷ 1000 for three-phase (R in Ω per 1000 ft, L one way in ft).
Calculation Results
Resistance from Wire Size
- Resistance per 1000 ft:
- 1.590 Ω
- One conductor:
- 0.1590 Ω
- Loop (out and back):
- 0.3180 Ω
Understanding Wire Resistance and Impedance
Wire resistance is the opposition to electrical current flow in conductors, causing voltage drop and power loss. Understanding resistance characteristics is essential for proper circuit design, voltage drop calculations, and ensuring efficient power transmission in electrical systems.
Fundamental Principles of Wire Resistance
Wire resistance depends on four primary factors governed by the fundamental equation:
Resistance Formula
Material Properties
Geometric Factors
AC vs DC Resistance Characteristics
DC Resistance
- Current distributed evenly across conductor
- Only material resistivity affects resistance
- Temperature is primary variable factor
- Used for low-frequency and DC applications
- Base reference for all calculations
AC Impedance
- Includes resistance plus reactance
- Skin effect concentrates current at surface
- Proximity effect from nearby conductors
- Frequency-dependent characteristics
- Important for high-frequency applications
Real-World Wire Resistance Calculations
Temperature Effect on Conductor Resistance
Analysis:
4/0 AWG copper conductor resistance at various operating temperatures
Base Resistance (20 °C): 0.0490 Ω/1000 ft
Temperature Coefficient: 0.00393/°C
Resistance vs Temperature:
Impact on System:
- • Higher operating temperatures increase losses
- • Voltage drop increases with temperature
- • Critical for hot climates and loaded conductors
Design Considerations:
- • Use conductor temperature ratings for calculations
- • Consider ambient temperature and I²R heating
- • Size conductors for worst-case conditions
Wire Resistance Reference Tables
| Size | Copper, 20 °C (NIST Handbook 100; kcmil calculated), Ω/1000 ft | Copper, 75 °C (calculated), Ω/1000 ft | Aluminum EC-H19, 20 °C (calculated), Ω/1000 ft | Aluminum EC-H19, 75 °C (calculated), Ω/1000 ft |
|---|---|---|---|---|
| 18 AWG | 6.39 | 7.771 | — | — |
| 16 AWG | 4.02 | 4.889 | — | — |
| 14 AWG | 2.52 | 3.065 | — | — |
| 12 AWG | 1.59 | 1.934 | 2.604 | 3.181 |
| 10 AWG | 0.9988 | 1.215 | 1.638 | 2.001 |
| 8 AWG | 0.6281 | 0.7639 | 1.03 | 1.258 |
| 6 AWG | 0.3952 | 0.4806 | 0.6479 | 0.7916 |
| 4 AWG | 0.2485 | 0.3022 | 0.4073 | 0.4976 |
| 3 AWG | 0.1971 | 0.2397 | 0.3231 | 0.3947 |
| 2 AWG | 0.1563 | 0.1901 | 0.2562 | 0.313 |
| 1 AWG | 0.1239 | 0.1507 | 0.2032 | 0.2482 |
| 1/0 AWG | 0.09825 | 0.1195 | 0.161 | 0.1967 |
| 2/0 AWG | 0.07793 | 0.09477 | 0.1277 | 0.1561 |
| 3/0 AWG | 0.06182 | 0.07518 | 0.1013 | 0.1238 |
| 4/0 AWG | 0.04901 | 0.0596 | 0.08035 | 0.09816 |
| 250 kcmil | 0.04148 | 0.05045 | 0.06801 | 0.08308 |
| 300 kcmil | 0.03457 | 0.04204 | 0.05667 | 0.06923 |
| 350 kcmil | 0.02963 | 0.03604 | 0.04858 | 0.05934 |
| 400 kcmil | 0.02593 | 0.03153 | 0.04251 | 0.05193 |
| 500 kcmil | 0.02074 | 0.02523 | 0.034 | 0.04154 |
R(T) = R20 × (1 + α × (T − 20 °C)). Copper: R20 from NIST Handbook 100 (kcmil sizes calculated), α = 0.00393. Aluminum EC-H19: R20 calculated from NIST Handbook 109 constants, α = 0.00403, 12 AWG and larger.
Temperature Correction Factors
Copper Temperature Coefficient
R₂ = R₁ × [1 + 0.00393(T₂ - T₁)]
Aluminum Temperature Coefficient
R₂ = R₁ × [1 + 0.00403(T₂ - T₁)]
Technical Applications and Considerations
Power System Design
Loss Calculations:
- • Power loss = I² × R (watts per conductor)
- • Critical for high-current, long-distance applications
Design Considerations:
- • Use worst-case temperature for resistance calculations
- • Consider load growth and future expansion
- • Account for parallel conductor skin effect
Measurement and Testing
Test Methods:
- • DC resistance: Wheatstone bridge or digital ohmmeter
- • AC impedance: LCR meter at specific frequency
- • Four-wire (Kelvin) measurement for accuracy
- • Temperature correction for field measurements
Quality Control:
- • Verify conductor cross-sectional area
- • Check for strand breakage or defects
- • Validate resistance within specification limits
- • Document test conditions and results
Frequency Effects
Skin Effect:
- • Current concentrates at conductor surface at higher frequencies
- • Skin depth δ = √(2/(ωμσ))
- • Resistance increases with √frequency
Proximity Effect:
- • Magnetic field from adjacent conductors
- • Increases AC resistance beyond skin effect
- • Critical in parallel conductor installations
- • Consider conductor spacing and arrangement
Standards and Safety Considerations
Testing Requirements
Design Safety Factors
- •Use conservative temperature assumptions for resistance calculations
- •Account for aging effects and corrosion over system life
- •Consider connection resistance in addition to conductor resistance
- •Verify adequate short-circuit current capability
- •Design for thermal cycling and mechanical stress
Disclaimer
This calculator provides theoretical resistance values. Actual measurements may vary due to manufacturing tolerances, temperature, aging, and installation conditions. Always verify calculations with actual measurements and consult applicable standards and specifications.
Frequently Asked Questions
How do I calculate the resistance of a wire?
Wire resistance is calculated using R = ρL/A, where R is resistance, ρ (rho) is resistivity, L is length, and A is cross-sectional area. For practical calculations, use: R = (Resistance per 1000 feet × Length in feet) ÷ 1000. For example, 12 AWG copper has 1.93 ohms per 1000 feet at 75 °C, so 100 feet = 0.193 ohms.
What's the difference between resistance and impedance?
Resistance is the opposition to DC current flow, measured in ohms. Impedance is the total opposition to AC current flow, including both resistance and reactance (inductive and capacitive).
How does temperature affect wire resistance?
Wire resistance increases with temperature for copper and aluminum. The formula is: R₂ = R₁[1 + α(T₂ - T₁)], where α is the temperature coefficient (0.00393/°C for copper, 0.00403/°C for aluminum). For every 1 °C increase, copper resistance increases by about 0.39 %. At 75 °C vs 20 °C, resistance increases by approximately 22 %.
Why is copper wire resistance lower than aluminum?
Aluminum EC-H19 has about 64 % higher resistance than copper of the same size: 17.002 against 10.371 Ω·cmil/ft at 20 °C (NIST Handbooks 109 and 100).
How do I calculate voltage drop from wire resistance?
Voltage drop equals current times resistance: VD = I × R. For single-phase: VD = I × R × 2 (accounting for both conductors). For three-phase: VD = I × R × 1.732.
Related Electrical Calculators
Voltage Drop Calculator
Calculate voltage drop using wire resistance values
Ohm's Law Calculator
Apply Ohm's law with resistance calculations
Wire Size Calculator
Size conductors based on resistance requirements
Ampacity Calculator
Determine current capacity with heating effects
Three Phase Calculator
Three-phase power and resistance calculations
Low Voltage Calculator
DC resistance calculations for low voltage systems