Forward and reverse voltage-drop relationships
Voltage drop is a circuit-resistance calculation, not a complete cable-selection rule. This workspace handles the forward calculation and the three useful reverse questions on one canonical page: how far can this conductor run, how much current can it carry before a chosen drop target is exceeded, and what cross-sectional area would meet that target under the simplified resistivity model.
How to use this calculator
Choose forward voltage drop or one of the reverse solve modes.
Enter source voltage, one-way run, current, conductor area, material and the planning drop target; the selected unknown is solved from the other values.
Review volts lost, percent drop, load voltage and conductor-loss estimate. For minimum area, the next listed metric size is a voltage-drop-only reference—not a final cable recommendation.
Where people use it
- •Checking voltage drop on an existing run
- •Finding the maximum one-way run for a chosen conductor
- •Reverse-solving current or cross-sectional area from a design drop target
- •Comparing copper and aluminum resistance effects
Example: 230 V, 20 A, 30 m, 4 mm² copper
Using the simplified 20 °C resistivity model for a single-phase run gives about 5.17 V drop, roughly 2.25% of 230 V. Changing the solve mode can work the same relationship backward.
Voltage drop is only one cable-design constraint
- •The resistivity model uses approximate 20 °C values: copper 0.017241 and aluminum 0.028264 Ω·mm²/m. Actual conductor resistance rises with temperature and depends on construction.
- •Voltage-drop arithmetic does not determine ampacity, breaker/fuse selection, insulation rating, grouping/derating, short-circuit withstand or regulatory compliance.
- •Balanced three-phase mode uses the √3 relationship and does not model unbalance, reactance, harmonics or power factor. For larger AC conductors/runs, impedance rather than DC resistivity alone may be required.
Frequently asked questions
Why is one-way cable length entered?+
For DC and single phase the formula applies a factor of 2 for the outgoing and return conductors. Balanced three phase uses √3 with one-way length.
Can this choose my cable size?+
It can reverse-solve the cross-sectional area required by this simplified voltage-drop equation and show the next listed metric area, but final cable selection must also satisfy ampacity and installation rules.
Why does conductor temperature matter in real installations?+
Metal resistance increases as temperature rises, so a 20 °C resistivity estimate can understate voltage drop at operating temperature.
Can I calculate maximum cable length?+
Yes. Choose Maximum one-way length and the tool solves the entered allowed-drop limit backward from voltage, current, material and conductor area.
Semantic next steps
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