Circuit inputs
Use one-way route length. Choose the circuit configuration carefully.
Example data
| System | Voltage | Current | Length | Conductor | Power factor | Example purpose |
|---|---|---|---|---|---|---|
| AC single-phase | 230 V | 20 A | 30 m | 4 mm² copper | 0.90 | Small feeder circuit |
| AC three-phase | 400 V | 65 A | 85 m | 25 mm² copper | 0.85 | Motor supply |
| DC / two-wire | 48 V | 12 A | 18 m | 6 mm² copper | 1.00 | Battery equipment |
Formula used
For DC two-wire circuits, this calculator uses Vd = 2 × I × R. The two represents outgoing and return conductors. Resistance comes from R = ρL / A. In that expression, ρ is resistivity, L is one-way length, and A is conductor area.
For AC single-phase circuits, it uses Vd = 2 × I × (R cosφ + X sinφ). For balanced three-phase circuits, it uses Vd = √3 × I × (R cosφ + X sinφ). Power factor gives cosφ. The calculator derives sinφ from the selected power factor.
Conductor resistance changes with temperature. The calculator corrects resistivity with ρT = ρ20 × [1 + α(T − 20)]. This produces a more realistic result for warm, loaded cables.
How to use this calculator
Choose the circuit type. Enter source voltage, expected current, and the one-way route length. Select metres or feet. Add the actual conductor area. Select copper, aluminum, or custom material data. Then enter conductor temperature.
AC circuits need power factor. Enter reactance when data is available. Resistance dominates many short runs. Reactance can matter on long AC routes, large cables, and inductive loads. Set the allowable percentage drop for the design.
Press Calculate voltage drop. The result appears above the form. It shows voltage drop, percentage drop, load voltage, estimated power loss, and target status. Download CSV for records. Download PDF for a compact calculation summary.
Why voltage drop matters
Voltage drop is the lost voltage between source and load. Conductors resist current flow. AC conductors also have reactance. Longer routes increase impedance. Smaller conductors increase it too. Higher current causes a greater reduction.
Large voltage drop can reduce equipment performance. Motors may run hotter. Lamps may dim. Electronics may reset or report faults. Heaters may provide less output. Low-voltage circuits are especially sensitive because every lost volt is a larger percentage of supply voltage.
Use the percentage result as a design check. A lower value is usually better. An above-target result needs review. Increase conductor area, shorten the route, reduce current, or raise distribution voltage where suitable.
Use realistic inputs
Use the longest expected route, not a straight-line guess. Include bends and equipment paths. Enter the actual conductor area. Check material, terminations, installation conditions, and cable temperature. This calculator estimates voltage drop only. It does not replace local code checks, ampacity checks, fault-loop calculations, protective-device coordination, or professional engineering review carefully.
Frequently asked questions
1. What is voltage drop?
Voltage drop is the reduction in voltage between the supply and the load. It occurs because conductors have resistance. AC circuits can also have reactance. Higher current, longer routes, and smaller conductors increase the drop.
2. Why does DC use a factor of two?
A standard two-wire DC circuit has an outgoing conductor and a return conductor. Current travels through both. The factor of two accounts for the full loop resistance when the entered cable length is one-way.
3. Why is √3 used for three-phase circuits?
Balanced three-phase voltage drop uses a line-to-line relationship. The √3 factor represents that relationship. It applies to the common three-phase formula when using line current and one-way conductor resistance and reactance.
4. Which length should I enter?
Enter the one-way route length from source to load. Do not double it for DC or single-phase circuits. The calculator applies the return-path factor automatically where that factor is required.
5. Does conductor temperature affect voltage drop?
Yes. Most conductor resistance rises as temperature rises. A warmer cable therefore produces a larger voltage drop. Use a realistic operating temperature rather than room temperature when the circuit will carry sustained load.
6. What power factor should I enter?
Use the expected power factor of the load for AC circuits. Resistive heating loads are often near one. Motors and inductive loads are lower. Check equipment data when accuracy matters.
7. Is reactance important on every cable?
No. Reactance is often small on short runs and small conductors. It can become important on long AC routes, large conductors, grouped conductors, and circuits with lower power factor.
8. How can I reduce voltage drop?
Increase conductor area, shorten the route, reduce load current, or use a higher distribution voltage. Correctly balancing three-phase loads can also help. Select methods that suit the electrical system and local rules.
9. Does the calculator check cable ampacity?
No. Voltage drop and ampacity are separate checks. A conductor may meet a voltage-drop target but still be unsuitable for current capacity, insulation temperature, installation method, or protective-device coordination.
10. Can I use aluminum conductors?
Yes. Select aluminum from the material list. Aluminum has higher resistivity than copper. It often needs a larger cross-sectional area to produce a similar voltage drop under the same load conditions.
11. Is this result suitable for final construction approval?
Use it as a design estimate and documentation aid. Verify every input. Confirm local code requirements, installation conditions, protective devices, cable ratings, and professional review before construction or energizing equipment.