Conductor resistance, voltage loss, and heating
The main resistance includes the selected path length at the entered conductor temperature. A loop doubles the one-way length because it includes equal outward and return conductors. Voltage drop and power loss use the current in advanced options. The reference bar and temperature table keep the conductor geometry fixed. Electrical quantities in the table use up to eight significant digits, with scientific notation for very small or large values.
To use a measured resistance with voltage or current, try the Ohm’s Law Calculator .
For an LED circuit’s separate current-limiting component, use the LED Resistor Calculator .
The formula
Length is converted to meters and area from mm² to m². Copper uses ρ20 = 1.72×10⁻⁸ Ω·m; aluminum uses 2.65×10⁻⁸ Ω·m. Both use the representative coefficient α = 0.0039/°C. These stated approximations support comparisons; actual conductor specifications can differ. AWG entries 1/0 through 4/0 map to gauge indices 0 through −3.
A 10-meter copper conductor
For copper area 2.5 mm² and one-way length 10 m at 20 °C, one conductor has resistance 0.0688 Ω. At 5 A it loses 0.344 V and dissipates 1.72 W. Selecting the equal return pair doubles resistance to 0.1376 Ω, voltage loss to 0.688 V, and power loss to 3.44 W. At 60 °C, the single-conductor model gives 0.079533 Ω.
How to use this calculator
- Select the conductor material and enter its cross-sectional area in mm² or select AWG. Use bare conductor area, not insulation diameter.
- Enter one-way length in meters or feet, choose the path, and set the conductor temperature. Changing the length unit does not convert the entered value.
- Set circuit current in advanced options if needed. Calculate and inspect the resistance comparison and temperature table.
Keep one-way distance separate from the return path
A cable route of 10 meters can contain 20 meters of current-carrying conductor when the circuit returns through an equal second wire. The loop option applies that factor once. Do not enter an already doubled distance and then choose the loop again. Unequal return sizes or materials require separate conductor calculations and addition of their resistances.
Choose conductor size without including insulation
A marked cable size normally refers to conducting area. The outside jacket does not contribute to that area. AWG mode uses the nominal round-wire gauge geometry; area mode accepts a stated mm² cross-section directly. For stranded, plated, alloyed, or installed cable, compare the estimate with the manufacturer’s specified DC resistance per unit length.
The temperature table is a comparison, not a thermal prediction
Each table row assumes the same conductor has been held at that temperature. The calculator does not determine the equilibrium temperature from the power loss. Cooling, insulation, bundling, installation conditions, joints, and duty cycle influence the actual operating state. This tool estimates DC resistance and its electrical consequences; it does not select a safe wire size or reproduce AC impedance.
Assumptions & limitations
What this calculation assumes
- Uniform copper or aluminum conductor with constant cross-section and a linear temperature approximation between −20 and 120 °C.
- The loop consists of two equal conductors; all current flows through the selected path.
What to keep in mind
- No ampacity recommendation, building-code sizing, contacts, connectors, AC skin effect, inductance, or calculated equilibrium temperature.
- Representative material constants are approximations; manufacturer cable data takes precedence for a specific product.
Common questions
Does the loop option mean two wires in parallel?
No. It adds the outward and return resistances in series along the complete current path. Two parallel conductors would be a different calculation.
Does this tell me how much current the wire can carry?
No. The entered current is used only for voltage and power loss. Safe current depends on installation, insulation, temperature limits, and applicable requirements.
Why does my measured resistance differ?
Material grade, stranding, conductor dimensions, temperature, joints, and meter accuracy can differ from the simplified assumptions. Use product resistance data when available.