Voltage Drop Calculator
Calculate voltage drop step by step: Single-phase / DC: Vd = 2 × I × R.
In short
Formula: Single-phase / DC: Vd = 2 × I × R. Three-phase (line-to-line): Vd = √3 × I × R, where R = ρ × L ÷ A for one conductor.
Voltage drop
6.4512 V
- Percentage of supply
- 2.8%
- Voltage at load
- 223.5488 V
- Resistance of one conductor
- 0.2016 Ω
- Power lost in cable
- 103.2192 W
What this calculator does
Calculate voltage drop step by step: Single-phase / DC: Vd = 2 × I × R. Worked example, questions and limitations included.
Use it to turn System, Supply voltage, Load current, One-way cable length, and the other shown inputs into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this electrical calculation.
Inputs and what they mean
- System
- — choice value.
- Supply voltage
- — number value.
- Load current
- — number value.
- One-way cable length
- — number value.
- Conductor size
- — number value.
- Conductor resistivity
- — Copper ≈ 1.68 × 10⁻⁸, aluminium ≈ 2.65 × 10⁻⁸ Ω·m at 20 °C.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
The factor 2 counts the outgoing and return conductors; √3 applies to balanced three-phase line voltage.
Resistive method only — cable reactance and power factor are ignored, which suits small conductors.
Compare the percentage with the limit in the wiring code you follow.
Single-phase / DC: Vd = 2 × I × R. Three-phase (line-to-line): Vd = √3 × I × R, where R = ρ × L ÷ A for one conductor.
Inputs used: System, Supply voltage, Load current, One-way cable length, Conductor size, Conductor resistivity.
Worked example
16 A over 30 m of 2.5 mm² copper, 230 V
- R = 1.68 × 10⁻⁸ × 30 ÷ 2.5 × 10⁻⁶ = 0.2016 Ω.
- Vd = 2 × 16 × 0.2016 = 6.45 V.
- 6.45 ÷ 230 = 2.80%.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Ignores reactance, so large cables at low power factor are underestimated.
- Balanced three-phase load assumed.
- Does not size the cable for current-carrying capacity.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this voltage drop calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Common questions
What voltage drop is acceptable?
It depends on the code you work under; many guides recommend a few percent for branch circuits. Check the rule that applies to your installation.
Why enter one-way length?
The calculator multiplies by 2 (single-phase) or √3 (three-phase) itself, so entering round-trip length would double-count. Check the formula, example, and limitations on this page before using the result for a real electrical decision.
Does cable temperature matter?
Yes. Hot conductors have higher resistance and more drop. Enter a resistivity for the operating temperature for a closer answer.
How do I use the Voltage Drop Calculator?
Enter the required values for System, Supply voltage, Load current, One-way cable length, Conductor size, and the other fields shown. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Voltage Drop Calculator use?
Single-phase / DC: Vd = 2 × I × R. Three-phase (line-to-line): Vd = √3 × I × R, where R = ρ × L ÷ A for one conductor. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Voltage Drop Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
Related tools
Calculate electrical power step by step: P = V × I = I² × R = V² ÷ R (DC or resistive AC using RMS values).
Calculate voltage divider step by step: Vout = Vin × R2 ÷ (R1 + R2).
Calculate resistance step by step: R = V ÷ I, or for a conductor R = ρ × L ÷ A (A in m²).