Eejit's Guide

Cable Size & Voltage Drop Guidance Tool

Quickly check an existing cable or estimate the minimum standard cable size needed to stay within a selected voltage-drop limit.

Designed for quick site checks: Select the circuit type, enter the voltage, current and one-way cable length, then get an immediate result.
Important disclaimer: This tool provides voltage-drop guidance only. It is not a complete cable design and does not check current-carrying capacity, installation method, grouping, ambient correction factors, protective-device selection, fault protection, earth fault loop impedance, thermal withstand, motor starting, harmonics or every applicable standard requirement. Results must be reviewed by a competent person against the actual cable data, manufacturer information, project requirements and applicable electrical regulations before equipment is selected or installed.

Quick Cable Check

Five main inputs. No account required.

What do you want to do?
Supply type
V
A
Enter the expected operating current, not only the protective-device rating.
m
Enter the distance from the supply to the load. The return path is included automatically where applicable.
%
Advanced options and assumptions
°C
Used to estimate conductor resistance. The default is 70°C.
Calculation assumption: The tool estimates voltage drop using conductor resistance. It does not model cable reactance, power factor, motor starting current or transient conditions. For AC circuits, manufacturer voltage-drop data should be used for final selection.

Cable size —
Voltage drop —
Percentage drop —
Estimated voltage at load —
Selected limit —
Maximum length at limit —

What This Tool Checks

This calculator is designed for a quick voltage-drop check when you are standing beside a panel, checking a field device or reviewing a cable route.

It can be used to:

  • Check the likely voltage drop of an existing cable.
  • Estimate the minimum standard cable size needed to meet a selected voltage-drop limit.
  • Compare DC, single-phase and three-phase circuits.
  • Estimate the voltage available at the load.
  • Estimate the maximum cable length for the selected size and limit.

What This Tool Does Not Check

Voltage drop is only one part of cable selection.

Before using a cable in a real installation, checks may also be required for:

  • Current-carrying capacity
  • Installation method
  • Ambient temperature
  • Cable grouping
  • Thermal insulation
  • Protective-device rating and characteristics
  • Earth fault loop impedance
  • Fault current and disconnection time
  • Short-circuit thermal withstand
  • Motor starting current
  • Power factor and cable reactance
  • Manufacturer and client requirements
Do not select a cable based only on the suggested size shown by this tool. A cable may pass the voltage-drop check but still be unsuitable for the load, protective device, installation method or fault conditions.

How the Estimate Is Calculated

The tool estimates conductor resistance using the selected conductor material, cable cross-sectional area and conductor temperature.

For DC and single-phase circuits, it includes the outgoing and return conductor length. For balanced three-phase circuits, it uses the square-root-of-three relationship.

The result should be treated as a quick engineering guide. Final checks should use the actual cable manufacturer data and the requirements that apply to the project.

My simple rule: Use this tool to identify whether a cable looks sensible or needs further investigation. Do not use a green result as automatic design approval.

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