Installation design
Voltage drop as a percentage
- where
- voltage drop as a percentage (%)
- voltage drop (V)
- nominal voltage (V)
Explain this equation
Expresses a voltage drop as a percentage of the nominal voltage. Use it to compare a calculated drop with the design allowance.
ΔU is the voltage drop in volts. U_n is the nominal voltage of the circuit: 230 V for single-phase, or 400 V line-to-line for three-phase. ΔU% is the drop as a percentage.
Dividing by the nominal voltage gives a fraction, and the 100 turns it into a percentage. Percentages let you compare circuits at different voltages on the same footing.
4.6 V on 230 V is 2%. For installations fed from a public low-voltage supply, the usual guidance is around 3% for lighting and 5% for other uses, measured from the origin, but work to the allowance set for the project.
Watch out: match the voltage basis, so a three-phase line-to-line drop goes over 400 V, not 230 V. Include upstream drop when comparing with the total allowance.
Percentage drop expresses lost voltage relative to nominal circuit voltage. A 4.6 V drop on 230 V is 2%.
A design may allocate part of its total voltage-drop allowance to a submain and the remainder to a final circuit. Use the same voltage basis throughout.
Enter your values to see the result.
Common mistake
Using 230 V for a line-to-line 400 V drop.
Limits
Compare the result with the applicable project and regulatory allowance, including upstream drop. Equipment starting and operating requirements also matter.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.