Voltage drop calculator
A cable can be thick enough to carry the current and still be the wrong size, because over a long run the volt drop leaves the equipment at the far end undervolted. This checks that half of the decision. Enter the load, the distance and the size.
Result
Enter a current and a run length.
How this is calculated
From the resistance of the conductor, not from a lookup table, so you can check the arithmetic yourself. Resistance is resistivity multiplied by length and divided by cross-sectional area.
R = rho x L / A, and V = I x R
The resistivity used is 0.0225 ohm sq mm per metre, for copper at a 70 degree C operating temperature. That is deliberately a little above the textbook figure for pure copper, because a conductor sold as 2.5 sq mm is stranded rather than a solid bar, and IS 8130 sets a maximum resistance per kilometre that sits above what the nominal area alone would give. Cable makers work to that maximum.
Calculating from the textbook figure instead under-predicts the drop by five to eight percent, which is the wrong direction to be wrong in. The value here is calibrated against published resistance-based tables. In the tested range from 1.5 to 25 sq mm, the estimate reads between zero and 3.4 percent high. Larger sizes need a check that includes reactance.
A single phase circuit doubles the result, because the current flows out along the line and back along the neutral. A three phase circuit uses the root three relationship between line and phase quantities.
What this tool will not do
It will not tell you what size cable to use. Sizing needs the current-carrying capacity of the cable after derating for ambient temperature, for grouping with other cables, and for how the cable is installed. Six cables in one conduit in a hot roof void carry far less than the same cable in free air, sometimes by half.
Those conditions cannot be seen from a web page, and an authoritative wrong answer about cable capacity is the kind of mistake that causes a fire. So the tool does the half that is pure arithmetic and leaves the half that needs judgement to a person. Send us the load, the distance and how it is installed, and we will size it properly.
Common questions
- What is the maximum permitted voltage drop?
- The usual convention is 3% for lighting circuits and 5% for power, measured from the origin of the installation to the furthest point of use. Project specifications are sometimes tighter, so check the one you are working to.
- Why does voltage drop matter if the cable is thick enough to carry the current?
- They are independent constraints. A cable can be thermally adequate and still be the wrong size, because over a long run the volt drop leaves equipment at the far end undervolted. Motors are particularly unforgiving: low voltage means higher current, more heat and shorter life.
- Does this tell me what size cable to use?
- Only for voltage drop. The other constraint is current-carrying capacity after derating for ambient temperature, grouping and installation method, and that depends on conditions we cannot see from a web page. Take whichever of the two demands the larger cable. Send us the details and we will confirm it.
- How is the calculation done?
- From conductor resistance rather than a lookup table. Resistance is resistivity times length divided by cross-sectional area, using 0.0225 ohm sq mm per metre for copper at a 70 degree C operating temperature. Single phase doubles it for the return conductor; three phase uses the root three relationship. The figure is calibrated to the maximum conductor resistance IS 8130 permits, so it reads slightly high rather than low.