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Motor Starting Voltage Drop Calculator

LRA from Code Letter, Transformer Impedance, Cable Impedance, and Risk-Tiered Results for Industrial Power Systems

Free motor starting voltage drop calculator for electrical engineers, plant electricians, and power system designers who need to verify that a motor start will not cause excessive voltage dip. Enter the motor horsepower, NEMA code letter, transformer kVA and impedance, and cable length and size, and the calculator returns the expected voltage drop at the motor terminals during locked-rotor inrush. Results are classified into risk tiers: OK (under 5%), CAUTION (5-10%), HIGH (10-15%), and FAIL (over 15%) to help you assess whether the start is acceptable.

Large motor starts are one of the most common causes of nuisance trips, PLC faults, and flickering lights in industrial facilities. A 200 HP motor on a 1000 kVA transformer can pull 6-7 times its full load current during starting, and if the transformer impedance and cable impedance are high enough, the voltage can sag below the threshold where contactors drop out or VFDs fault. This calculator models the transformer impedance plus cable impedance against the motor locked-rotor inrush, giving you a conservative screening estimate before you energize the motor for the first time.

The calculator shows the voltage at the motor terminals after both transformer and cable drop, which is critical for motors with high-inertia loads that need sufficient torque during acceleration. If the terminal voltage drops below about 80% of rated, the motor may stall because torque drops as the square of voltage.

Pro Tip: If you are borderline on voltage drop, check whether the motor can be started with a reduced-voltage starter (autotransformer, wye-delta, or soft starter) before upsizing the transformer or cable. A soft starter that limits inrush to 3x FLA instead of 6x FLA cuts the voltage drop roughly in half and often gets you under the 10% threshold without any infrastructure changes.

Size transformers for motor loads and demand factors

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Check conduit fill with motor feeder conductors

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Understand motor starting voltage drop analysis methods

Motor Starting Voltage Drop Guide →

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Motor Starting Voltage Drop Calculator
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How It Works

  1. Enter Motor Data

    Input the motor horsepower, voltage rating (single-phase or three-phase), and NEMA code letter. The calculator uses the code letter to determine the locked-rotor kVA per HP and computes the locked-rotor amps (LRA) per NEC Table 430.7(B).

  2. Enter Transformer Data

    Input the transformer kVA rating and impedance percentage. The transformer impedance is the dominant factor in most motor starting voltage drop calculations. The calculator assumes an infinite bus on the primary side.

  3. Enter Cable Data

    Input the cable size (AWG or kcmil), length in feet, and conduit material (steel or PVC). The calculator uses NEC Chapter 9 Table 9 impedance values to compute the cable voltage drop contribution.

  4. Review Risk Tier and Recommendations

    Check the terminal voltage, voltage drop percentage, and the assigned risk tier. If the result is CAUTION or worse, the calculator suggests mitigation options such as reduced-voltage starting, larger cable, or a dedicated transformer.

Built For

  • Electrical engineers checking whether a new 150 HP compressor motor will cause bus voltage to sag below acceptable limits on an existing 750 kVA transformer
  • Plant electricians troubleshooting why a large motor start trips the MCC contactors or causes PLC faults on adjacent equipment
  • Power system designers selecting transformer size and cable gauge to support a new motor installation with acceptable voltage drop
  • Consulting engineers preparing a motor starting study for a permit application or equipment procurement specification

Features & Capabilities

Transformer + Cable Impedance Model

Models the voltage drop through the transformer impedance and cable impedance during locked-rotor inrush. Calculates the motor terminal voltage and percentage drop with the sqrt(3) factor for three-phase systems.

NEMA Code Letter LRA Lookup

Automatically determines locked-rotor amps from the motor horsepower and NEMA code letter per MG-1 Table 10-1. Covers code letters A through V for standard induction motors.

Risk-Tiered Results

Classifies the voltage drop into four tiers: OK (under 5%), CAUTION (5-10%), HIGH (10-15%), and FAIL (over 15%). Each tier includes practical guidance on whether the start is acceptable or what mitigation to consider.

Cable Impedance from NEC Tables

Uses NEC Chapter 9 Table 9 resistance and reactance values for common conductor sizes in steel and PVC conduit. Accounts for both resistive and reactive components of cable impedance.

Assumptions

  • Motor locked-rotor current is derived from the NEC code letter kVA/HP range, assuming a standard NEMA Design B induction motor.
  • Cable impedance values are based on NEC Chapter 9 Table 9 for 60 Hz, 75 C conductor temperature.
  • The utility source is assumed to be an infinite bus at the transformer primary. Utility source impedance is not modeled.

Limitations

  • Does not model the transient voltage recovery profile during motor acceleration, only the initial locked-rotor voltage dip.
  • Does not account for voltage support from power factor correction capacitors, other running motors, or generator excitation response.
  • Assumes a single motor starting on a radial system. Multiple motors starting simultaneously or motors on networked buses require a full power system study.

References

  • IEEE 141-1993 (Red Book) - Recommended Practice for Electric Power Distribution for Industrial Plants, Chapter 8: Motor Starting.
  • NEMA MG-1 - Motors and Generators, Table 10-1: Locked-Rotor kVA per Horsepower by Code Letter.
  • NEC Chapter 9 Table 9 - AC Resistance and Reactance for 600 V Cables in Conduit.

Frequently Asked Questions

IEEE 141 (Red Book) recommends keeping the bus voltage drop below 15% during motor starting, with 10% being the preferred target. NEMA MG-1 states that motors should be capable of starting with terminal voltage as low as 80% of rated. In practice, most facilities target less than 10% bus voltage dip because sensitive loads like PLCs, contactors, and VFDs can malfunction at larger dips. Lighting circuits are even more sensitive, with visible flicker starting at about 3-5% voltage dip.
Transformer impedance is usually the largest contributor to voltage drop during motor starting. A typical distribution transformer has 5-6% impedance, meaning the bus voltage will drop roughly by the ratio of motor starting kVA to transformer kVA times the impedance percentage. A 200 HP motor pulling 1200 kVA starting load on a 1500 kVA transformer with 5.75% impedance will cause about a 23% bus voltage dip if cable impedance and utility impedance are ignored.
The NEMA code letter is stamped on the motor nameplate and indicates the locked-rotor kVA per horsepower. Code letter A means 0-3.14 kVA/HP (low inrush), while code letters toward the end of the alphabet indicate progressively higher inrush. Most standard NEMA Design B motors are code letter G (5.6-6.29 kVA/HP) or H (6.3-7.09 kVA/HP). You multiply the code letter kVA/HP value by the motor horsepower to get the starting kVA, then divide by the rated voltage to get the locked-rotor amps.
A soft starter limits the inrush current to a user-set multiple of full-load amps, typically 3x to 4x FLA instead of the 6-7x FLA for across-the-line starting. This directly reduces the starting kVA and proportionally reduces the voltage drop. A VFD eliminates the inrush problem entirely during normal starts because it ramps the frequency and voltage gradually, drawing only about 1-1.5x FLA. However, if the VFD bypasses to across-the-line for any reason, the full inrush returns.
No. This calculator assumes an infinite bus at the transformer primary, modeling only the transformer and cable impedance. For most industrial facilities, the utility source impedance is small compared to the transformer, adding only 1-3% to the total voltage drop. However, on weak utility sources (low available fault current), the actual drop can be noticeably higher than the estimate. If you need to model utility impedance, a full power system study using ETAP or SKM is recommended.
Disclaimer: This calculator provides estimates based on the IEEE 141 simplified method. Actual motor starting voltage drop depends on system conditions, motor characteristics, and load inertia that may not be fully captured in a simplified analysis. Critical installations should be verified with a full power system study using software such as ETAP or SKM.

Learn More

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Motor Starting Voltage Drop Analysis

How to calculate voltage drop during motor starting per IEEE 141, including locked rotor current from NEC code letters, transformer impedance modeling, and cable sizing.

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