Cable Size Calculator (Voltage Drop Method)
Estimate the minimum conductor cross-sectional area (mm²) required for your circuit.
Voltage Drop Calculator (Known Cable Size)
Check how much voltage drop occurs with an existing or proposed cable.
Why cable calculators matter
Cable sizing is one of those design decisions that looks simple at first—and then gets expensive when it is done wrong. If a cable is undersized, you can end up with poor equipment performance, nuisance trips, dim lighting, motor starting issues, and unnecessary heat. If it is oversized, your project still works, but you may be spending more copper or aluminum than you need.
Good cable calculations help you balance safety, efficiency, and cost. In practical terms, you are trying to answer three questions:
- Will the conductor safely carry the current?
- Will the voltage drop stay within acceptable limits?
- Will the chosen cable support reliable operation over the run length?
What these calculators estimate
This page gives you two quick engineering estimators:
- Cable Size Calculator: Calculates required cross-section based on current, length, material, system type, and allowable voltage drop.
- Voltage Drop Calculator: Calculates voltage drop for a known cable size and tells you whether it meets your target limit.
These tools are excellent for planning, budgeting, and early-stage design review.
Core inputs you should understand
1) Load current (A)
Higher current means more resistive loss and more drop. As current doubles, voltage drop doubles (all else equal).
2) Cable length (m)
Longer runs increase total conductor resistance. For single-phase and DC two-wire circuits, the return path matters, which is why the equations use a factor of 2.
3) Material (copper vs aluminum)
Copper has lower resistivity than aluminum, so for the same current and length, copper usually needs less cross-sectional area to achieve the same drop target.
4) System voltage and allowable drop (%)
The same drop in volts has a different impact at different system voltages. For example, a 5 V drop is much more significant on a 24 V system than on a 400 V system.
5) Conductor temperature
Resistance rises with temperature. The calculator corrects resistance from 20°C reference to the value at your entered operating temperature.
Formulas used
These calculators use a resistance-based voltage drop model:
- Single-phase AC / DC 2-wire: ΔV = 2 × I × L × R′
- Three-phase AC: ΔV = √3 × I × L × R′
- Conductor resistance per meter: R′ = ρ / A
- Temperature correction: RT = R20 × [1 + α(T − 20)]
Where I is current, L is one-way length, ρ is resistivity, A is area, and α is temperature coefficient.
Common mistakes in cable sizing
- Using one-way length when your equation expects loop length (or vice versa).
- Ignoring conductor temperature rise.
- Selecting a size by ampacity only and forgetting voltage drop.
- Forgetting that motor starting current can be much higher than running current.
- Assuming field conditions match ideal tables (grouping, ambient temperature, installation method).
Important design note
These calculators focus on voltage drop sizing, not full compliance checking. Final cable selection should be verified against your applicable electrical code or standard (for example NEC, IEC, BS, AS/NZS, or local utility rules), including ampacity derating, insulation type, protective devices, short-circuit withstand, and installation environment.
Bottom line
Use these tools to get fast, practical estimates and shortlist cable sizes confidently. Then perform a formal standards-based check before procurement and installation. A few minutes of calculation now can save a lot of rework later.