Voltage drop is assessed across a stated circuit under stated load conditions. Use the relevant length, current and electrical characteristics with consistent units. The result should identify the endpoints and supply arrangement rather than appearing as an unexplained percentage detached from the circuit.
Consider more than normal running demand
The operating condition used for the calculation may differ from motor starting or another transient event. Identify whether separate checks are required for those conditions. Voltage drop along a cable also differs from voltage response at the generating set; both may matter to the connected equipment.
Application details
when R and X are consistent conductor impedance units and L is the one-way length.
Transient drop
Steady-state cable voltage drop is not the same as generator terminal voltage dip.
Calculate steady-state cable voltage drop
Use manufacturer-supplied conductor resistance and reactance at the design conditions. Enter one-way length in kilometres, not metres. Use line-to-line voltage for balanced three-phase circuits and line-to-neutral voltage for single-phase circuits. This tool assumes a lagging power factor and does not calculate generator transient dip, select a cable size or assess thermal capacity or protective-device suitability.
Match length and impedance units
For voltage-drop calculations using resistance and reactance in ohms per kilometre, express the route length in kilometres. Distinguish phase-to-phase from phase-to-neutral reference voltage, and use the correct single-phase or balanced three-phase expression.
Steady-state voltage-drop relationships
For a balanced three-phase circuit, ΔV = √3 × I × L × (R cosφ + X sinφ). For a single-phase two-wire circuit, ΔV = 2 × I × L × (R cosφ + X sinφ). L is the one-way route length; resistance and reactance per unit length must use matching length units. Current, power factor and impedance must represent the stated condition. These screening expressions do not establish transient performance or complete cable suitability.
Use the result within the cable review
An acceptable screening result does not establish every aspect of cable suitability. Installation conditions, protection and fault duties still require assessment. Retain the inputs and applicable criteria with the calculation so that the proposed circuit can be reviewed and updated when the route or load changes.
Use the planning tool
[rgcalculator type=”voltdrop”]Check the inputs and units before using the result. This tool supports preliminary planning; it does not replace equipment-specific selection, technical review or site verification.
Start Engineering Review
Start Engineering Review. Share the application, location and information available so the next discussion can address your actual requirement.
Related guidance
Engineering intent
Voltage Drop should be evaluated as part of the complete power system rather than as an isolated catalogue item. Operating duty, connected load, interfaces, protection, controls, installation conditions, testing requirements and lifecycle consequences determine whether the arrangement is appropriate.
Engineering considerations
Motor-starting warning
| if voltage drop during start may matter | Temporary cable warning |
|---|---|
| if rental or public-access cable is involved | Protection warning |
| if breaker/fault review is needed | Earthing warning |
| if PE/neutral data is missing | Next action |
| cable review / site survey / generator current calculator | Voltage drop means the load receives lower voltage than the source provides. But the motor terminal voltage may be too low after a long cable run. For awareness only, the tool may show simplified formulas. Voltage Drop depends on current, cable impedance, route length, This tool should not output final cable size from simplified formulas. Final voltage-drop calculation requires cable resistance/reactance data, installation method, conductor temperature, power factor, route length, |
Selection and design considerations
Correct voltage at the source does not guarantee correct voltage at the equipment terminals. As current flows through the cable, part of the supply voltage is lost along the circuit. For a balanced three-phase circuit, voltage drop may be estimated using: Vd = √3 × I × (R cosφ + X sinφ) × L ÷ 1000 Consider a 415 V three-phase load drawing 120 A through a 150 m copper cable. If the effective cable impedance factor results in a voltage drop of approximately 8.57 V: Such a value may be acceptable for many steady-state industrial circuits, subject to applicable standards, equipment requirements and the complete upstream-downstream voltage-drop allocation. However, design must not rely only on steady-state voltage drop.
Integration and protection considerations
VFDs, rectifiers, LED drivers, battery chargers, UPS systems and switched-mode power supplies can introduce harmonic currents. Modern facilities contain nonlinear loads that can introduce harmonics, causing overheating, neutral loading, voltage distortion, increased losses, relay maloperation and reduced equipment life. AVR diagnostics should therefore consider power quality when voltage complaints repeat. The generator terminal voltage is not always the load terminal voltage. If cable impedance is high, the generator may regulate voltage correctly at its terminals while the load receives poor voltage. Cable-engineering material explains that correct source voltage does not guarantee correct load voltage, because every cable has resistance/reactance and voltage drop depends on current, cable length, conductor resistance, reactance, power factor, conductor temperature and arrangement.
Testing, commissioning and verification
Copper and aluminium cables should not be treated as interchangeable just because they can both carry current. The conductor material must be part of the specification. A permanent generator feeder and a rental cable may have different mechanical lives. But the remote load may receive less because current flowing through cable impedance causes voltage drop. For a balanced three-phase circuit, a commonly used engineering relationship is: ΔV = √3 × I × L × (R cosφ + X sinφ) Do not assume one universal allowable voltage-drop percentage for every generator project. For single-phase two-conductor circuits, the corresponding concept normally includes the round-trip conductor path. The calculation must follow the actual circuit configuration and conductor data.
Direct answer
Voltage drop is assessed across a stated circuit under stated load conditions. Use the relevant length, current and electrical characteristics with consistent units.
The practical decision for Voltage Drop should be based on verified load, equipment, interface and site information. Where equipment-specific data, regulatory applicability or a project study is required, this guidance does not replace that verification.