Example Data Table
| Input | Example Value | Meaning |
|---|---|---|
| System type | Three phase | Industrial motor feeder |
| Voltage | 480 V | Line to line voltage |
| Motor power | 50 hp | Rated shaft output |
| Locked rotor multiplier | 6 | Starting current factor |
| Conductor | 250 ft, copper 1/0 AWG | One way cable run |
| Source data | 500 kVA, 5.75 percent | Transformer contribution |
| Expected result | About 7.86 percent total drop | Based on default assumptions |
Formula Used
Three phase full load current: I = W / (1.732 × V × Eff × PF)
Single phase full load current: I = W / (V × Eff × PF)
Starting current: Istart = IFLA × locked rotor multiplier × starting method factor
Three phase cable drop: Vdrop = 1.732 × Istart × (R × cosθ + X × sinθ) × L / 1000
Single phase cable drop: Vdrop = 2 × Istart × (R × cosθ + X × sinθ) × L / 1000
Transformer drop percent: Drop = transformer impedance percent × starting kVA / transformer kVA
Total drop percent: Total drop = cable drop percent + transformer drop percent + extra source sag percent
How to Use This Calculator
Enter the system type, voltage, motor rating, efficiency, and running power factor.
Add the locked rotor multiplier from the motor data sheet.
Select the starting method. Use custom factor when project data is known.
Enter conductor length, material, size, reactance, and parallel sets.
Add transformer and source information when available.
Press Calculate. The result appears above the form.
Use CSV or PDF buttons to save the current calculation.
Motor Starting Voltage Drop Guide
Why Starting Drop Matters
Motor starting is one of the hardest moments for a feeder. A motor can draw several times its running current when it starts. That surge moves through conductors, starters, transformers, and utility service gear. Each part has impedance. Impedance creates voltage loss. If the loss is high, the motor may start slowly. Contactors may chatter. Lights may dip. Drives and controls may trip.
What the Calculator Checks
This calculator helps review that short starting event. It estimates full load current from motor power, voltage, efficiency, and power factor. It then applies a locked rotor multiplier and a starting method factor. Direct online starting usually gives the highest current. Star delta, autotransformer, soft starter, and drive starts reduce current. The actual value still depends on motor design and load torque.
Cable and Source Effects
Cable drop is estimated from conductor resistance, reactance, route length, phase type, and parallel sets. A temperature multiplier is included because hot conductors have more resistance. You can use the built in conductor table, or enter a custom resistance value. This is useful when a cable schedule, manufacturer sheet, or site measurement gives better data.
Source sag is also important. A strong cable cannot fix a weak transformer or long upstream service. The tool estimates transformer contribution from starting kVA, transformer kVA, and impedance. It also accepts an extra source sag percent. That field covers utility drop, generator reactance, or upstream feeder effects.
Design Review Notes
Use the result as an engineering check, not as a final code decision. Confirm conductor ampacity, short circuit ratings, protection settings, starter limits, and local rules. Review voltage at the motor terminals under the worst expected start. Compare it with motor, starter, and driven equipment requirements. A lower drop target is better for pumps, compressors, and high inertia loads. Record assumptions in the exported file. That makes design review easier and keeps future troubleshooting clear.
Good starting studies also consider repeated starts. Several starts close together can heat cables and windings. Long acceleration time increases heating further. Check the mechanical load curve when possible. Fans often start easier than loaded crushers. Generators may need larger margins than utility supplies. When readings are available, compare measured voltage with calculated voltage onsite. Differences often reveal loose joints, incorrect cable data, or weak source capacity.
FAQs
What is motor starting voltage drop?
It is the voltage loss that occurs during motor acceleration. Starting current is high, so cable and source impedance create a temporary voltage dip at the motor terminals.
Why is starting current higher than running current?
A stopped motor has little counter electromotive force. It draws locked rotor current until speed rises. The current falls as the motor accelerates.
What locked rotor multiplier should I use?
Use the motor nameplate, data sheet, or manufacturer information. If data is missing, many preliminary studies use a broad estimate from five to seven times full load current.
Does a soft starter reduce voltage drop?
Yes, it can reduce line current during acceleration. The reduction depends on torque demand, ramp settings, and starter limits. Heavy loads may still need high current.
Why include transformer impedance?
The transformer voltage also sags during motor starting. Higher starting kVA and higher transformer impedance usually increase the supply side voltage drop.
Should I use one way or round trip length?
Enter one way length. The calculator applies the correct phase factor. Single phase calculations use a factor of two for outgoing and return conductors.
Can I use custom conductor resistance?
Yes. Enter a custom resistance when a cable schedule, manufacturer table, or measured value is more accurate than the built in reference table.
Is this a final design approval?
No. It is a calculation aid. Final design should confirm code rules, conductor ampacity, protection, motor requirements, and project specifications.