How the Voltage Drop Calculator works.
single phase: 2 × I × L × R/1000; three phase: √3 × I × L × R/1000I is amperes, L is one-way feet, and R is ohms per 1,000 feet.
Worked example
At 10 A, 100 ft, and 1.24 Ω per 1,000 ft, single-phase drop is 2.48 V.
What the result means
Voltage drop is an estimate of conductor loss under the stated current and resistance.
Edge cases
Resistance changes with conductor material, temperature, connections, and actual route length.
Input units and circuit basis
Enter load current in amperes, one-way conductor length, conductor resistance in ohms per 1,000 feet, and the applicable circuit type. The calculator does not infer conductor size, material, route length, or supply configuration. If length is entered in metres through the page control, FigureNest converts that one-way length to feet before this formula runs; the resistance input still uses ohms per 1,000 feet.
Single-phase worked calculation
For a single-phase two-wire circuit, estimated drop is 2 × current × one-way length × resistance ÷ 1,000. With 10 A, 100 ft, and 1.24 Ω per 1,000 ft, the calculation is 2 × 10 × 100 × 1.24 ÷ 1,000 = 2.48 V. The factor 2 represents the outgoing and return conductors, so do not double the one-way length yourself.
Balanced three-phase calculation
For a balanced three-phase line-to-line estimate, the formula is √3 × current × one-way length × resistance ÷ 1,000. Using 10 A, 100 ft, and 1.24 Ω per 1,000 ft gives about 2.1477 V. This simplified result does not model phase imbalance, neutral current, harmonics, unequal conductor temperatures, or phase-specific loading.
Conductor resistance is an assumption
Resistance varies with conductor material, cross-sectional area or gauge, temperature, stranded construction, joints, terminals, and actual installed route. Use a resistance value from a suitable conductor table or project source and make sure its unit matches ohms per 1,000 feet. A correct formula cannot compensate for a resistance value taken from the wrong conductor or temperature basis.
From volts to percentage
Voltage-drop percentage is estimated drop ÷ selected supply voltage × 100. A 2.48 V drop on a 230 V supply is about 1.08%. Changing the page’s supply-voltage preference changes that percentage comparison but not the calculated drop in volts. Confirm whether the project basis is line-to-line or line-to-neutral before interpreting the percentage.
Safety, compliance and design scope
This is a preliminary voltage-loss estimate, not a conductor selection or code-compliance decision. It does not check ampacity, overcurrent protection, fault current, derating, installation method, termination rating, load diversity, local voltage-drop guidance, or equipment instructions. Have a qualified professional—an electrical designer or electrician—verify safety-critical or regulated work against current project data and the applicable code.
What this calculator does not include.
Electrical design must follow the applicable code and a qualified designer.
Reference guidance and methodology.
Voltage Drop Calculator questions.
What values and units should I enter?
Enter the load current in amperes, the one-way conductor length in feet or metres, and conductor resistance in ohms per 1,000 feet. Then choose the circuit type. Use resistance for the actual conductor material, size and operating condition rather than a generic value from a different cable.
Why does the single-phase formula multiply one-way length by two?
A single-phase two-wire circuit has an outgoing conductor and a return conductor. The factor 2 represents that complete current path while the length field remains the one-way route length, so you should not double the entered length yourself.
Why does the three-phase formula use √3?
For the balanced three-phase line-to-line estimate, the calculator applies √3 × current × one-way length × resistance per 1,000 feet. It does not use the single-phase return-path factor of 2. Unbalanced loads, neutral current, harmonics and phase-specific conditions require a fuller design calculation.
What happens when I select metres for conductor length?
FigureNest converts the entered one-way metres to feet before applying the formula. The resistance field remains ohms per 1,000 feet in either length mode, so convert a resistance value stated per kilometre or per metre before entering it.
How is voltage-drop percentage calculated?
The page divides the estimated drop in volts by the selected supply-voltage preference and multiplies by 100. Changing the supply voltage changes that percentage basis, not the calculated drop in volts. Confirm the displayed voltage and phase match the circuit you are assessing.
Does this result select a cable or prove electrical-code compliance?
No. The estimate does not select conductor size or account for every installation factor, including conductor temperature, termination resistance, route details, load variation, harmonics, derating, local voltage-drop guidance or applicable code rules. Use current project data and a qualified electrical designer or electrician where safety or compliance matters.