Underground Cable Calculator
Long buried cable runs lose voltage to conductor resistance, and too much drop means dim lights, weak motors, and wasted energy. This calculator works out the voltage drop, the drop as a percentage of supply, and the power lost as heat for a single-phase or DC run. Enter the current, the one-way length, the conductor resistance per unit length from a standards table, and the supply voltage. The maths is Ohm's law applied to the round-trip path, so the result is exact for your inputs. Resistance is an editable input because it depends on your conductor size and material.
Voltage drop formula
Total conductor length = 2 * one-way length
Total resistance = total length * resistance per metre
Voltage drop = current * total resistance
Drop percentage = voltage drop / supply voltage * 100
Voltage at load = supply voltage - voltage drop
Power loss = current^2 * total resistance
This is Ohm's law on the round-trip path of a single-phase or DC circuit. The factor of two covers both conductors. No empirical constant is assumed; resistance per metre comes from your standards table.
Keeping drop in check
- The National Electrical Code suggests about 3 percent on a branch and 5 percent total as guidance.
- Voltage drop grows directly with run length, so long buried runs need attention.
- A larger conductor has lower resistance per metre and less drop.
- Copper has lower resistance than aluminium for the same size.
- Power lost in the cable is dissipated as heat and wastes energy.
Underground cable: frequently asked questions
How is cable voltage drop calculated?
For a single-phase run, voltage drop equals two times the one-way length times the current times the conductor resistance per unit length. The factor of two accounts for the current flowing out and back through both conductors. The drop percentage is the voltage drop divided by the supply voltage, times 100.
Where do I get the resistance per unit length?
Use the resistance value for your conductor size and material from a standards table such as the National Electrical Code chapter 9 or an IEC conductor table. Copper and aluminium differ, as do gauges. Because this depends on your specific cable, it is a user-editable input.
What is an acceptable voltage drop?
The National Electrical Code recommends keeping voltage drop to about 3 percent on a branch circuit and 5 percent total including the feeder, as informational guidance. Check the rules that apply to your installation; this calculator reports the percentage so you can compare against your limit.
Why does length matter so much for buried runs?
Underground runs are often long, and resistance grows in direct proportion to length. Doubling the run doubles the drop. For long distances you may need a larger conductor to keep the drop within limits, which this calculator helps you check.
Does this handle three-phase circuits?
This model is for single-phase or DC, using the factor of two for the round trip. Three-phase voltage drop uses a different factor (the square root of three) and line-to-line voltage. Use a three-phase formula or tool for those circuits.
Official sources
- National Institute of Standards and Technology: SI electrical units (volt, ampere, ohm).
- U.S. Department of Energy: Electricity Transmission (conductor losses context).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.