Cable Sizing by Voltage Drop: Pick the Right Conductor Every Time
Sizing cable is not just about current rating — voltage drop often dictates a bigger conductor than ampacity alone would suggest, especially on long feeders.
This guide shows the voltage-drop sizing method and how to apply derating.
The formula
A = (2 × L × I × ρ) ÷ Vd_max — solve for the minimum cross-section that keeps drop within budget, then round up.
Worked example
For 50 m at 30 A with a 10 V budget: A = (2 × 50 × 30 × 0.0175)/10 = 5.25 mm² → choose the next standard size, 6 mm².
Don't forget derating
Apply temperature and grouping factors from IEC 60364 or the NEC after the voltage-drop check.
Related calculators
Frequently asked questions
Ampacity or voltage drop first?
Check both — pick whichever demands the larger conductor, then derate.
Why round up?
Standard cable sizes are discrete; rounding up guarantees you stay within the drop budget.
This guide is for educational purposes. Always verify against the relevant standard before final design.