What is voltage drop in an electrical circuit?

Voltage drop is the reduction in voltage between the source and the load, caused by the resistance and reactance of the conductor carrying the current.

Why does voltage drop matter in design?

Excessive drop reduces equipment performance and can cause nuisance operation. Design limits are set by the applicable code and project specification.

Which circuit types does this calculator handle?

DC, single-phase AC and three-phase AC circuits are each calculated with their own formula, using the system voltage, load current, one-way run length, conductor size and conductor material you enter.

What results does it give?

The voltage drop in volts, the drop as a percentage of the system voltage, and the estimated receiving-end voltage at the load.

How can I reduce an excessive voltage drop?

Typical options, in rough order of cost, are increasing the conductor size, adding conductors in parallel per phase, shortening the run or relocating the distribution board closer to the load, raising the distribution voltage, or improving the load power factor. You can test each change by re-running the calculation.

Cables & Circuits

Voltage Drop Calculator

Check permissible voltage drop across feeders and final circuits.

Last updated: 11 September 2026Calculation basis: Transparent formulas & worked examples

Voltage drop calculation

Applicable references

BNBC 2020
Bangladesh National Building Code
National building code covering electrical installations, safety and building services requirements in Bangladesh.
NFPA 70 / NEC
National Electrical Code
Widely adopted code for safe electrical design, installation and inspection practice.
Source

Method, formulas and assumptions

Formulas used

Direct current (DC): V_drop = 2 × I × R. Both the outgoing and returning conductor are counted, and reactance is ignored because it has no effect on a DC circuit.
AC single phase: V_drop = 2 × I × (R·cos φ + X·sin φ). The factor of 2 accounts for the phase and neutral conductor of the loop.
AC three phase: V_drop = √3 × I × (R·cos φ + X·sin φ), expressed as a line-to-line drop against the line-to-line system voltage you enter.
Derived outputs: Drop % = V_drop ÷ V_system × 100. Receiving-end voltage = V_system − V_drop.

Assumptions and data source

Conductor resistance: R = ρ × L ÷ (A × n), where A is the nominal cross-sectional area in mm², n the number of conductors per phase and L the one-way run length. ρ is taken as 0.0214 Ω·mm²/m for copper and 0.0344 Ω·mm²/m for aluminium.
Conductor temperature: Both resistivity values correspond to a conductor operating temperature of 75 °C, the usual assumption for insulated building wire at full rated load. A cooler, lightly loaded circuit will drop slightly less than this tool reports.
Reactance: AC circuits use a single representative reactance of 0.000164 Ω per metre per conductor (≈0.05 Ω per 300 m), typical of conductors installed in conduit. Reactance varies with conductor spacing, raceway material and cable construction, so for long or heavily loaded feeders use the reactance published by your cable manufacturer instead.
Power factor: cos φ is the value you enter and sin φ is derived as √(1 − cos²φ), assuming a lagging (inductive) load. DC calculations force cos φ = 1 and sin φ = 0.
Conductor material and size: Only uncoated copper and aluminium are modelled. AWG and kcmil sizes are converted to their nominal metric area so both size systems use one consistent model; distances entered in feet are converted at 0.3048 m per foot.
Not modelled: Harmonic distortion, conductor bundling and ambient-temperature derating, motor starting inrush, parallel-run circulating currents, skin and proximity effect, and neutral loading on unbalanced three-phase circuits are outside the scope of this tool.

About this voltage drop calculation

Voltage drop is the loss of voltage along a conductor run caused by conductor resistance and, in AC circuits, reactance. Enter the system voltage, load current, one-way run length, conductor size and material, and the tool returns the voltage drop in volts, the drop as a percentage of system voltage, and the estimated receiving-end voltage. DC, single-phase and three-phase circuits are each handled with their own formula.

Excessive voltage drop is rarely a safety failure on its own, but it degrades everything connected downstream: motors draw more current and run hotter as terminal voltage falls, contactor coils chatter or fail to pull in, lighting output and driver life decline, and electronic loads reset during starting transients. Because the drop is proportional to both current and run length, the circuits worth checking first are the long ones, the heavily loaded ones, and any feeder serving motor loads at the far end of a site.

When a result exceeds your design limit, the usual remedies in order of cost are: increase the conductor size, add conductors in parallel per phase, shorten the run or relocate the distribution board closer to the load, raise the distribution voltage, or improve the load power factor so less current flows for the same real power. This tool lets you test each of those changes directly by re-running the calculation with the revised input.

Formula notes

Voltage drop depends on conductor resistance and reactance, circuit length, load current and power factor.
Permissible limits are defined by the applicable code and project specification, not by this tool.

Worked example — sewing-floor feeder in an RMG factory

Scenario

Three-phase 400 V feeder supplying a sewing floor sub-board
Load current 100 A at a power factor of 0.9
One-way run 150 ft (about 46 m) of 50 mm² copper, one conductor per phase
Design voltage drop limit taken from the project specification: 3 %

Steps, using the formulas on this page

1. Resistance of the runR = ρ × L ÷ (A × n), with ρ for copper at 75 °C and L the one-way length
2. Three-phase dropVdrop = √3 × I × (R·cos φ + X·sin φ)
3. Drop as a percentageDrop % = Vdrop ÷ Vsystem × 100
4. Voltage at the sub-boardVreceiving = Vsystem − Vdrop

Run these values in the calculator above and compare the drop percentage against the limit in your own specification. If it exceeds the limit, increase the conductor size, add a conductor per phase, or shorten the run and re-run the calculation.

Bangladesh practice — common questions

Does the Bangladesh National Building Code set the voltage drop limit for my feeder?: The permissible voltage drop for a project in Bangladesh is set by the standard and specification your design is being assessed against — commonly the Bangladesh National Building Code (BNBC) electrical provisions together with the client or buyer specification. OHMICLY does not substitute a figure for you: the limit in this calculator is the value you enter, so confirm it against the edition of BNBC and the project documents applicable to your work.
Why does voltage drop matter so much in RMG factory feeders?: Garment factory floors typically combine long horizontal cable routes with many motor loads on one feeder. As terminal voltage falls, motors draw more current and run hotter, contactor coils drop out, and lighting and electronic controls become unreliable. Because drop is proportional to both current and run length, the long, heavily loaded floor feeders are the ones worth checking first.

RSC & Nirapon audit resourcesElectrical audit checklists and templates used on RSC and Nirapon assessments in Bangladesh factories.Browse resources

Frequently asked questions

What is voltage drop in an electrical circuit?: Voltage drop is the reduction in voltage between the source and the load, caused by the resistance and reactance of the conductor carrying the current.
Why does voltage drop matter in design?: Excessive drop reduces equipment performance and can cause nuisance operation. Design limits are set by the applicable code and project specification.
Which circuit types does this calculator handle?: DC, single-phase AC and three-phase AC circuits are each calculated with their own formula, using the system voltage, load current, one-way run length, conductor size and conductor material you enter.
What results does it give?: The voltage drop in volts, the drop as a percentage of the system voltage, and the estimated receiving-end voltage at the load.
How can I reduce an excessive voltage drop?: Typical options, in rough order of cost, are increasing the conductor size, adding conductors in parallel per phase, shortening the run or relocating the distribution board closer to the load, raising the distribution voltage, or improving the load power factor. You can test each change by re-running the calculation.