Input Parameters
Enter motor and conductor details. The calculator estimates running and starting voltage drop and checks your target limits.
Example Data Table
Use this example to understand typical input ranges and output format.
| Voltage | Phase | Motor | PF | Eff | Material | Size | Length | Run Drop | Start Drop (6x) |
|---|---|---|---|---|---|---|---|---|---|
| 415 V | 3φ | 15 kW | 0.85 | 0.90 | Copper | 25 mm2 | 60 m | ≈ 1.0–2.0% | ≈ 6.0–12.0% |
| 480 V | 3φ | 25 HP | 0.88 | 0.92 | Aluminum | 70 mm2 | 120 m | ≈ 2.0–3.5% | ≈ 12.0–21.0% |
Example ranges vary with reactance model and temperature factor.
Formula Used
This calculator uses the impedance method commonly applied in electrical design.
- I is line current (A). Starting uses the multiplier.
- R and X are resistance and reactance (ohm/km).
- L is one-way length in km (converted from m/ft).
- cosφ is power factor; sinφ = √(1 − cos²φ).
- Percent drop: %Vd = (Vd / V) × 100.
How to Use This Calculator
- Enter system voltage and select phase type.
- Choose motor power or enter known full-load current.
- Set efficiency and power factor for accurate current.
- Pick conductor material, size, and one-way length.
- Enable reactance for long runs and three-phase feeders.
- Set running and starting drop limits to match design criteria.
- Press calculate to show results and download reports.
Always confirm final sizing with drawings, local codes, and motor manufacturer requirements.
Motor Voltage Drop in Construction Projects
Motor circuits often run farther than lighting or receptacle branches, especially in plants, pumping stations, rooftop equipment, and large commercial fit-outs. As distance increases, conductor impedance causes a measurable voltage drop. Excessive drop can raise motor current, reduce torque, increase heating, and shorten equipment life. This calculator helps you estimate both steady running drop and the higher, short-duration drop during starting.
The calculation uses resistance (R) and reactance (X) to represent the conductor’s impedance. Resistance increases with temperature and installation conditions, so a temperature/AC factor is provided to fine-tune results. Reactance becomes more noticeable on longer feeders and in three-phase runs, so you can enable or disable it depending on your design approach. The output includes volts dropped, percent drop, receiving voltage, and pass/fail checks against your limits.
In practical design, running limits around 3% are commonly used for branch circuits, while feeders may be designed around 5%, depending on local requirements and coordination with upstream voltage drop. Motor starting is often permitted a higher temporary drop because it lasts only a few seconds, but sensitive loads and weak utility sources may require tighter control. If your starting check fails, consider increasing cable size, shortening the route, using parallel conductors, or applying soft-start or variable-frequency drives to reduce inrush.
If you select the power-based method, the tool estimates current from motor power using the selected phase type. Power factor and efficiency matter because they change the line current for the same mechanical output. A lower power factor or lower efficiency increases current, which increases voltage drop and conductor heating. When reviewing the results, focus on receiving voltage at the motor terminals. Compare that voltage to the motor nameplate tolerance and the expected utility or generator voltage under load. If the receiving voltage is marginal, your protective devices may nuisance-trip during start, contactors may chatter, and acceleration time can increase. For troubleshooting, run two or three scenarios: current from nameplate, current from design load, and a conservative case with reduced power factor. This approach produces a clear design margin and supports field coordination.
Example scenario
- System: 415 V, three-phase, 50 Hz
- Motor: 15 kW, PF 0.85, efficiency 0.90
- Feeder: copper 25 mm2, one-way length 60 m
- Targets: 3% running, 15% starting, 6× inrush
With these inputs, the running drop is typically within acceptable limits, while the starting drop depends heavily on inrush and route length. Use the report downloads to attach calculation summaries to submittals and QA packages.
FAQs
1) What length should I enter: one-way or round-trip?
Enter the one-way distance from the source to the motor. The formula applies the correct multiplier internally for single-phase return paths and three-phase systems.
2) Should I include reactance (X) for all jobs?
Include reactance for long feeders, especially three-phase runs, or when you want a closer impedance-based estimate. For short runs, resistance-only results are often very close.
3) Where do I get motor current if I do not know it?
Use the power-based method with kW or HP, then enter realistic efficiency and power factor. If nameplate full-load amps are available, the full-load current method is preferred.
4) Why is starting voltage drop much higher than running drop?
Starting current can be several times the running current. Voltage drop is proportional to current, so inrush conditions create a larger temporary drop and lower receiving voltage.
5) What does the temperature/AC factor change?
It scales conductor resistance to reflect higher operating temperature, grouping, or AC effects. A higher factor increases resistance, which increases calculated voltage drop.
6) The suggested size shows “no match.” What should I do?
Tight limits, long distances, or high starting multipliers may require sizes beyond the list. Try relaxing limits, reducing length, using parallel conductors, or applying a soft-start/VFD strategy.
7) Are these results a substitute for final design checks?
No. Treat this as an estimating and comparison tool. Always verify conductor sizing, protection, and voltage tolerances with applicable codes, project specifications, and manufacturer requirements.