Medium Voltage Cable Voltage Drop Calculator

Calculator

Circuit type

Line voltage (kV)

Load input type

Load value

Running power factor

Power factor mode

One way cable length

Length unit

Parallel cable sets

Cable preset

Conductor material

Resistance R (ohm/km)

Reactance X (ohm/km)

R base temperature (°C)

Operating temperature (°C)

Demand factor (%)

Motor starting multiplier

Starting power factor

Maximum drop limit (%)

Cable ampacity per set (A)

Project notes

Example Data Table

Case	Voltage	Load	Length	PF	R ohm/km	X ohm/km	Expected use
Plant feeder	11 kV	250 A	750 m	0.90 lag	0.0991	0.095	Running voltage check
Pump motor	6.6 kV	1.5 MVA	1.2 km	0.86 lag	0.1640	0.098	Starting screen
Utility tie	33 kV	8 MW	4 km	0.95 lag	0.0601	0.090	Long feeder review

Formula Used

For three phase circuits:

ΔV = √3 × I × L × (R cosφ ± X sinφ) / N

For single phase circuits:

ΔV = 2 × I × L × (R cosφ ± X sinφ) / N

Use plus for lagging power factor. Use minus for leading power factor.

Rt = Rbase × [1 + α × (Top - Tbase)]

Loss = 3 × I² × Rt × L / N for three phase. Use 2 instead of 3 for single phase.

Here, I is current in amps. L is one way length in kilometers. R and X are ohms per kilometer. N is parallel cable sets.

How to Use This Calculator

Select the circuit type and enter the line voltage.
Choose a load type, then enter current or power.
Enter the running power factor and lagging or leading mode.
Add the one way cable length and its unit.
Select a cable preset or enter project impedance values.
Enter temperature, demand factor, starting data, and ampacity.
Press Calculate to view results above the form.
Use CSV or PDF buttons to save the report.

Medium Voltage Cable Planning Notes

Medium voltage feeders need careful voltage drop checks. A small error can affect motors, transformers, and protection settings. Long cable runs create resistance and reactance. Both parts change the receiving voltage. This calculator combines those values with current and power factor. It also shows loss and voltage regulation.

Design Use

Use the tool during early design. It helps compare cable sizes and routes. It is also useful for checking existing feeders. Always confirm final values with the cable maker. Real installations may include duct banks, soil temperature, armor, screens, and harmonic currents. Those factors can change ampacity and impedance.

Inputs and Assumptions

The calculator supports three phase and single phase circuits. Most medium voltage systems are three phase. Enter the line voltage, load, length, and power factor. You may enter current directly. You may also use kVA, MVA, kW, or MW. The tool converts the load into design current. A demand factor can reduce the connected load when allowed by the design basis.

Cable impedance is entered as ohms per kilometer. Preset examples are only starting points. Use project data for final work. Resistance is adjusted by temperature. Copper and aluminum use different coefficients. Reactance is kept constant for this practical estimate. Parallel cable sets divide impedance and loss.

Results and Review

The result includes voltage drop in volts. It also gives drop percent, load end voltage, and copper loss. The starting drop estimate helps with motor feeders. It multiplies running current by the selected starting factor. This is only a screening value. Motor starting studies may need detailed source impedance.

A low voltage drop improves equipment performance. It also reduces heating and wasted energy. Many projects set a maximum drop limit. The limit depends on the standard, owner rule, and equipment type. Use the warning result as a guide. Then adjust cable size, length, or parallel runs.

Good records matter. Save the CSV file for spreadsheets. Use the PDF report for design notes. Keep assumptions with each result. Include the cable data source, operating temperature, load basis, and selected power factor. This makes later review easier and safer.

For underground feeders, installation conditions matter. Cable spacing affects reactance. Thermal limits affect conductor temperature. These details should match drawings, specifications, and field practice before procurement and energizing work.

FAQs

What is voltage drop in a medium voltage cable?

It is the voltage difference between the source and load end of a cable. It is caused by conductor resistance, reactance, current, length, and power factor.

Which voltage should I enter?

Enter the line voltage of the circuit. For most three phase medium voltage feeders, this is the line to line voltage, such as 6.6 kV, 11 kV, or 33 kV.

Should I use actual cable impedance?

Yes. Preset values are only examples. Final calculations should use manufacturer data, project specifications, and installation details for the selected cable construction.

How does power factor affect voltage drop?

Lagging power factor increases the reactance part of the drop. Leading power factor can reduce the calculated drop and may create a voltage rise in some cases.

What do parallel cable sets mean?

Parallel sets divide current across more than one cable per phase. This reduces effective resistance, reactance, voltage drop, and losses when cables are installed correctly.

Is the starting voltage drop exact?

No. It is a screening estimate using a starting current multiplier and starting power factor. Detailed motor studies should include source impedance and motor data.

What voltage drop limit should I use?

Use the project specification, utility rule, or applicable design standard. Many designers use strict limits for feeders serving motors or sensitive equipment.

Why is temperature included?

Conductor resistance rises as operating temperature increases. The calculator adjusts resistance from the entered base temperature to the selected operating temperature.