Inputs
Enter job values, then calculate. Results appear above this form.
Formula used
This calculator estimates conductor voltage loss using resistance, optional reactance, and temperature correction.
Percent drop = (Vdrop / Vnom) x 100. Receiving voltage = Vnom - Vdrop.
How to use
- Choose system type and conductor material.
- Select conductor size or enter custom resistance.
- Enter one-way length, current, and nominal voltage.
- For AC, set power factor and reactance if known.
- Set estimated conductor temperature for better accuracy.
- Press calculate; review drop percent and receiving voltage.
- Download CSV or PDF for project documentation.
Example data table
Sample scenarios for quick reference. Replace with your site values.
| System | Material | Size | Length (m) | Current (A) | Voltage (V) | PF | Temp (C) |
|---|---|---|---|---|---|---|---|
| AC single-phase | Copper | 10 mm2 | 30 | 25 | 230 | 0.95 | 30 |
| AC three-phase | Aluminum | 35 mm2 | 80 | 60 | 400 | 0.90 | 40 |
| DC | Copper | 16 mm2 | 50 | 40 | 48 | N/A | 35 |
Accurate voltage drop estimates help protect critical site loads.
Professional guide to voltage drop on construction sites
1. Why voltage drop matters
Long temporary feeders, tower cranes, welders, and pumps can suffer noticeable loss between the panel and the load. Extra drop reduces motor torque, raises current draw, and increases heat at terminations. Planning the run prevents nuisance trips and premature equipment failure.
2. Inputs that dominate results
Current and distance are the biggest drivers. Doubling the one-way length doubles the drop, and doubling current doubles the drop again. Conductor size changes resistance strongly; for example, copper 10 mm2 is about 1.83 ohm/km at 20 C, while 35 mm2 is about 0.524 ohm/km.
3. Single-phase, three-phase, and DC
For DC and single-phase, the loop factor is 2 because current goes out and returns. For three-phase, the factor becomes sqrt(3) because the line-to-line relationship changes the drop calculation. Choosing the correct system type keeps estimates realistic for site distribution boards.
4. Power factor and reactance
On AC circuits, resistance is not the only contributor. Cable reactance adds drop, especially on inductive loads. This tool uses Vdrop = k × I × (Rcosφ + Xsinφ) × L. If you do not know X, a practical starting range is 0.06 to 0.10 ohm/km.
5. Temperature correction in the field
Resistance rises with conductor temperature. Copper uses a ≈ 0.00393 per degree C, so a 50 C conductor can be roughly 12% higher in resistance than at 20 C. When cables are bundled, in sun, or in trays, using a higher temperature improves planning accuracy.
6. Typical project targets
A common planning target is about 3% drop on branch circuits and about 5% from service to the farthest load. For example, a 230 V single-phase run with a 3% drop allows about 6.9 V loss. Keeping within targets helps lighting stay bright and motors start reliably for safe startup.
7. Interpreting the outputs
The calculator reports drop in volts, percent drop, and receiving voltage. Use percent to compare alternatives quickly, and use receiving voltage to check minimum equipment requirements. If the percent is high, increase conductor size, shorten the run, split loads, or move the source closer.
8. Documentation and verification
Export the CSV or PDF for submittals, commissioning notes, and daily reports. Record conductor size, run length, and expected current so crews can validate installation. Final values should be verified against local electrical requirements, manufacturer limits, and measured site conditions before energizing.
FAQs
1) Should I enter one-way length or total loop length?
Enter one-way length. The calculator applies the correct loop factor internally: 2 for DC and single-phase, and sqrt(3) for three-phase. This keeps input consistent across system types.
2) What power factor should I use for mixed construction loads?
If you do not have measured data, 0.90 to 0.95 is a reasonable planning range for motor-driven tools. Lower power factor increases the reactance component and can raise voltage drop on long runs.
3) Why does temperature change the result?
Conductor resistance increases as it warms. Higher resistance produces higher voltage drop at the same current and length. Use a realistic temperature when cables are bundled, in trays, or exposed to sun.
4) When should I use custom resistance?
Use custom resistance when you have manufacturer data, special cable types, or nonstandard materials. Enter the resistance in ohm per kilometer at 20 C to match the calculator’s temperature correction model.
5) What does receiving voltage tell me?
Receiving voltage is the estimated voltage at the load terminals after subtracting drop. Compare it against equipment nameplate limits, especially for motors, welders, and electronic supplies that can misbehave under low voltage.
6) What if my percent drop is too high?
Increase conductor size, reduce run length, redistribute loads, or add a closer temporary panel. On AC, improving power factor or using parallel conductors can also reduce losses, but verify feasibility and code requirements.
7) Are the built-in resistance values exact?
No. They are typical reference values for planning. Real installations vary by cable construction, connections, and temperature. Treat results as estimates and confirm with project specifications, qualified supervision, and site measurements.