Three Phase Voltage Drop Calculation Example

Advanced Calculator

Load name

Line voltage, V

Load current, A

Load factor, %

One way length

Length unit

Conductor material

Conductor size

Custom resistance

Use ohm per 1000 ft, or ohm per km.

Conductor temperature, °C

Power factor

Power factor type

Reactance

Use ohm per 1000 ft, or ohm per km.

Parallel runs per phase

Installation multiplier

Allowable voltage drop, %

Formula Used

Three phase voltage drop: VD = √3 × I × L × (R × cosθ + X × sinθ).

Percentage drop: VD% = (VD ÷ line voltage) × 100.

Temperature correction: R₂ = R₁ × [1 + α × (T₂ − 20)].

For leading power factor, the reactance term is subtracted. Parallel runs divide resistance and reactance.

How to Use This Calculator

Enter the line to line voltage for the three phase system.
Enter the full load current and any design load factor.
Add the one way feeder length and choose the unit.
Select conductor material and size, or enter custom resistance.
Enter power factor, reactance, parallel runs, and allowed drop.
Press the calculate button and review the result above the form.
Use CSV or PDF export for project notes.

Example Data Table

Case	Voltage	Current	Length	Conductor	Power Factor	Allowed Drop	Use
Motor feeder	480 V	80 A	250 ft	1/0 AWG copper	0.85 lagging	3%	Check start and running voltage.
Panel feeder	400 V	125 A	90 m	3/0 AWG aluminum	0.92 lagging	4%	Compare cable sizes.
Pump station	415 V	60 A	180 m	2 runs custom	0.80 lagging	5%	Review long route loss.

Three Phase Voltage Drop Guide

Three phase voltage drop matters on long feeders. It shows how much line voltage is lost before the load. Large drop can reduce motor torque. It can also heat cables and waste energy. A good estimate helps before wire is purchased.

Why It Happens

Current flows through conductor resistance and reactance. Resistance creates heat. Reactance appears because alternating current makes a magnetic field. The effect grows with current, distance, and poor power factor. Three phase systems use the square root of three in the line voltage formula.

Important Design Inputs

The calculator uses line voltage, design current, one way length, conductor size, material, temperature, power factor, and reactance. Parallel runs are also included. They reduce the effective impedance. A temperature setting adjusts resistance because hot conductors resist current more. The installation multiplier can add design allowance for crowded raceways, old conductors, or conservative planning.

How To Read Results

The voltage drop value shows the estimated lost volts. The percentage compares that loss with the source line voltage. The load voltage shows the expected voltage at equipment terminals. The pass or review message compares the result with the allowed limit. Many designers aim for a low branch circuit drop. Long feeders may need a larger conductor.

Practical Example

Assume a 480 volt motor feeder. The current is 80 amps. The one way run is 250 feet. The power factor is 0.85 lagging. A copper conductor has resistance and reactance along the run. The formula estimates line loss. If the percentage is high, try a larger conductor. You can also shorten the route, use parallel runs, or improve power factor.

When Larger Wire Helps

Larger wire lowers resistance. That reduces heat and line loss. It may improve starting voltage for motors. It can also support future load growth. The tradeoff is higher material cost and larger raceway needs.

Design Notes

This calculator is for planning and comparison. Final installations should follow local electrical code. Use the actual cable table required by your standard. Check conductor ampacity, temperature rating, terminals, conduit fill, fault current, and protection settings. Voltage drop is only one part of safe design. Always document assumptions before ordering material and future maintenance checks later.

FAQs

What is three phase voltage drop?

It is the voltage lost between the source and the load in a three phase feeder. It happens because conductors have resistance and reactance. Long runs and high current increase the drop.

Why is the square root of three used?

Three phase line voltage is related to phase values by √3. The common voltage drop formula uses line voltage, so the multiplier adjusts the impedance drop for a balanced three phase circuit.

Should I use one way or round trip length?

Use one way length for this calculator. The three phase formula already represents the circuit relationship. Do not double the route length unless your reference formula requires it.

What power factor should I enter?

Use the expected operating power factor of the load. Motors often use lagging power factor. If you are unsure, check equipment data or use a conservative value for planning.

How does temperature affect voltage drop?

Higher conductor temperature increases resistance. That increases voltage drop and heat loss. This calculator adjusts resistance from a 20°C reference using a material temperature coefficient.

What does the installation multiplier do?

It adds a planning allowance to conductor impedance. Use 1.00 for normal calculation. Use a higher value when you want conservative estimates for difficult routes or uncertain field conditions.

Can parallel runs reduce voltage drop?

Yes. Parallel conductors share current. This lowers effective resistance and reactance per phase. Each run must still meet code rules for length, termination, material, and installation method.

Is this calculator enough for final design?

No. It supports planning and comparison. Final design should check local code, ampacity, protection, short circuit rating, equipment terminals, raceway fill, and project specifications.