Cable Resistance Calculator

The conductor resistance at 20°C is computed using: R₂₀ = ρ₂₀ × L / A

• ρ₂₀ is resistivity at 20°C (Ω·m).
• L is one-way conductor length (m).
• A is cross-sectional area (m²).

Temperature correction is applied as: R_T = R₂₀ × (1 + α × (T − 20))

• α is the temperature coefficient (per °C).
• T is the conductor temperature (°C).
• Loop resistance uses a multiplier (typically 2 for two-wire circuits).

1. Select the conductor material used on your project.
2. Enter the one-way cable length between endpoints.
3. Choose size input method: mm² or AWG selection.
4. Set expected conductor temperature for operating conditions.
5. Select circuit type to apply the appropriate loop factor.
6. Optionally enter current to estimate I²R power loss.
7. Press Calculate and review results above the form.

Purpose in Site Power Planning

Cable resistance is a core input for estimating electrical performance on active projects. Higher resistance increases voltage drop, reduces equipment starting torque, and raises heat in conductors. This calculator supports quick checks during temporary power layouts, plant commissioning, and permanent feeder planning. It improves early decisions on cable size and routing.

Key Inputs and Assumptions

Enter one-way length between endpoints, then select material and conductor size. The tool uses standard resistivity at 20°C and applies a linear temperature coefficient for copper or aluminum. Results represent DC resistance, which aligns with many planning workflows and manufacturer tables. Conductor area can be entered directly or selected from common AWG sizes. When standards require it, validate results with manufacturer resistance tables and document any deviations for approval formal.

Interpreting Results

Single-conductor resistance shows the base property of the run at the chosen temperature. Loop resistance applies a multiplier for two-wire circuits, which is useful for return-path calculations. The ohms-per-kilometer value lets teams compare cable sizes independently of length. If current is provided, the tool estimates I²R loss to highlight heating and efficiency impacts. Use the loss estimate to support derating and containment choices.

Typical Construction Use Cases

Use the outputs to screen generator feeders, tower crane supplies, site lighting circuits, and pump connections. Resistance checks help prioritize larger conductors on long runs and prevent nuisance trips from undervoltage. For three-phase systems, pair the single-conductor resistance with your preferred voltage-drop method and phase configuration. During troubleshooting, compare measured resistance to the estimate for quick checks. Keep notes on splice locations, connector types, and installed route length.

Quality Control and Documentation

Maintain consistent units and measure length along the installed route, not straight lines. Estimate conductor temperature based on loading, ambient conditions, bundling, and enclosure type. Compare calculated ohms-per-kilometer against supplier data to detect input errors early. Export CSV for spreadsheets and PDF for submittals, inspections, and handover packages. Store exports with drawing revisions so calculations stay traceable over time.

1) What length should I enter?

Enter the one-way run length between endpoints. Two-wire circuits use loop resistance, which accounts for the return path automatically.

2) Why does temperature change resistance?

Metal resistivity rises with temperature. The calculator applies a standard temperature coefficient to adjust resistance from 20°C to your expected operating temperature.

3) Is this AC impedance or DC resistance?

This is a DC resistance estimate with temperature adjustment. For AC impedance, skin effect, and reactance, use manufacturer impedance data for the installed frequency.

4) When should I use loop resistance?

Use loop resistance for two-wire circuits such as single-phase or DC runs. For three-phase, use single-conductor resistance and your project voltage-drop approach.

5) How is I²R loss calculated?

If current is entered, power loss is computed as current squared times loop resistance. This highlights heating and efficiency impacts on long or heavily loaded runs.

6) Can I use AWG instead of mm²?

Yes. Select the AWG option and choose a size. The tool converts the selection to an approximate cross-sectional area to run the same resistance equations.