Voltage drop equation
You can estimate cable voltage drop from load current, route length, conductor material, and conductor cross-section.
where:
- Vd
- Voltage drop, measured in volts[V]
- Vd%
- Voltage drop as a percentage of nominal voltage[%]
- I
- Load current, measured in amperes[A]
- ρ
- Conductor resistivity reference, measured in ohm square millimeters per meter[ohm mm²/m]
- L
- One-way cable length, measured in meters[m]
- S
- Conductor cross-sectional area, measured in square millimeters[mm²]
- n
- Number of parallel conductors per polarity or phase
- Vn
- Nominal system voltage, measured in volts[V]
The result excludes reactance, harmonics, operating temperature, and installation correction factors, so long feeder runs need a standards-based cable check.
How to calculate voltage drop
Start with the load current, nominal system voltage, one-way cable length, conductor material, and conductor cross-sectional area. The calculator converts the selected units, applies the appropriate single-phase or three-phase path factor, and divides the result by nominal voltage to find the percentage drop.
- Select DC, single-phase AC, or three-phase AC.
- Enter load current and nominal system voltage.
- Enter the physical one-way cable length.
- Select copper or aluminum and enter the conductor size.
- Add the number of parallel conductors per polarity or phase.
Voltage drop calculation example
Consider a balanced three-phase 400 V circuit carrying 32 A through a 50 m copper cable with a 10 mm² conductor and one conductor per phase. Using the simplified resistive equation:
Vd = √3 × 32 × 0.0175 × 50 / 10 = 4.85 V
Vd% = 4.85 / 400 × 100 = 1.21%
| Input or result | Value |
|---|---|
| System | Three-phase AC |
| Load current | 32 A |
| One-way length | 50 m |
| Copper conductor | 10 mm² |
| Voltage drop | 4.85 V |
| Voltage drop percentage | 1.21% |
Single-phase vs. three-phase voltage drop
DC and single-phase calculations use twice the one-way length because current travels to the load and returns through another conductor. A balanced three-phase calculation uses the √3 factor and the one-way phase-conductor length.
| Circuit | Simplified formula | Length entered |
|---|---|---|
| DC / single-phase | Vd = 2 × I × ρ × L / (S × n) | One-way length |
| Balanced three-phase | Vd = √3 × I × ρ × L / (S × n) | One-way length |
What is an acceptable voltage drop?
There is no single percentage that applies to every country, circuit, or load. The allowable voltage drop must come from the locally adopted wiring rules, project specification, and equipment voltage tolerance. This tool marks results above 3% for review and above 5% as a warning, but those thresholds are design references rather than a declaration of code compliance.
Motors, electronic controls, low-voltage DC equipment, EV charging circuits, and long feeders may need tighter limits. Always check the voltage required at the equipment terminals under normal operating load.
How to reduce voltage drop
Voltage drop can be reduced by changing the conductor, route, current, or system design.
- Increase conductor cross-sectional area.
- Shorten the cable route where practical.
- Use parallel conductors when the installation and code permit.
- Reduce circuit current or distribute the load across additional feeders.
- Use a higher distribution voltage where the equipment and system design allow it.
- Check terminals and joints for unwanted resistance.
Voltage drop and cable size
Voltage drop is only one cable-selection check. A conductor must also pass ampacity, installation derating, terminal-temperature, short-circuit withstand, and protective-device coordination requirements. Continue with the related sizing tools before finalizing a conductor.
Assumptions
- Approximate conductor resistivity at normal temperature
- Reactance and installation temperature effects are not included
- One-way length is used
Important Warnings
- Use project voltage-drop limits and local electrical code for final design.
- Long runs, high temperature, and grouped cables need additional derating.