EngineerHub.orgDrawing No. EH–EE–004 // Electrical Engineering
Cable Sizing Calculator
Reviewed August 2026
Estimate a low-voltage conductor size by checking load current, derated ampacity, voltage drop and short-circuit withstand.
What problem does this solve?
Screen a low-voltage cable size against design current, derated ampacity, voltage drop and adiabatic short-circuit withstand.
Inputs
Screening results
| Size | Derated ampacity | Voltage drop | Short-circuit | Overall |
|---|
Background
Engineering basis, assumptions and interpretation for preliminary cable sizing.
A cable must carry the load without overheating, keep voltage drop acceptable, and withstand fault current until protection clears. The largest required cross-section controls.
The page uses a resistive conductor model with temperature-adjusted resistivity. Long AC circuits may also require reactance and detailed impedance data.
The screening relation is S ≥ I√t/k. The correct k value depends on conductor, insulation and permitted initial/final temperatures.
Frequently Asked Questions
Practical questions about inputs, assumptions, standards and interpretation.
No. Use the governing national wiring rules, installation method tables, correction factors and manufacturer data.
Long circuits can have acceptable current capacity but excessive resistance and voltage loss.
It is the physical distance from source to load. The formula applies the correct return-path factor for single-phase circuits.
No. Medium-voltage design requires insulation coordination, charging current, screen losses, fault duties and dedicated standards.