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Motor Inrush Drop

Calculate extreme instantaneous voltage drop at the motor terminals during electromagnetic locked-rotor inrush (LRA) to prevent contactor chatter and stalls.

Motor Electrical Characteristics

Amps
Feet

Marginal Inrush Envelope

A 10.8% drop during startup is generally the absolute limit for standard inductive loading. Motor requires strong utility feed rigidity. Ensure control transformers are tapped on separate circuits.

Startup Sag (Terminal Voltage)

428.0 V
Voltages measured at motor lugs during LRA blow

Instantaneous Drop

52.0 V
Lost to wire resistance

Percentage Loss

10.8 %
Against system nominal

Source Panel

480V

- 52.0 V LOSS
M

Terminal Lug

428.0V

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What is The Mechanics of Inrush Voltage Drop?

Electric motors can easily draw 5 to 10 times their full-load current (FLA) during the first few fractions of a second of startup. Because this 'Locked Rotor' current is so incredibly high, a wire gauge that is perfectly safe for running continuous loads might act like a severe resistor during startup. If the wiring is too thin or the run is too long, the voltage will drop massively at the terminal box.

Mathematical Foundation

Conductor Resistance (R)
R=L1000×Ω×KphaseR = \frac{L}{1000} \times \Omega \times K_{phase}
LL= One-way wire length (feet) from the panel to the motor
Ω\Omega= Ohms per 1,000ft (Standard NEC Chapter 9, Table 8 metric)
KphaseK_{phase}= Multiplier (2 for 1-Phase return paths, 1.732 for 3-Phase vectors)
Inrush Voltage Drop (V_d)
Vd=LRA×RV_d = LRA \times R
LRALRA= Locked Rotor Amps (Absolute peak startup current before RPM increases)

Laws & Principles

  • NEC 3% Recommendation: The National Electrical Code (NEC) suggests keeping branch circuit voltage drop under 3% for standard running loads to maintain efficiency.
  • Startup Limit: Because startup is a transient event, a 10% to 15% drop is generally the maximum acceptable limit for reliable motor starting. If it drops further, the magnetic field is too weak to spin the heavy rotor.
  • The Contactor Chatter Phenomenon: If the voltage drops too low (often below 85% nominal), the magnetic coil holding the motor contactor closed loses its grip. The contactor springs open. When it opens, the LRA stops, the voltage shoots back up to 100%, and the coil slams the contactor shut again. This rapid machine-gun 'chattering' will weld the contacts together.
  • VFD Mitigation: If your calculated drop is unsafe and up-sizing thousands of feet of copper wire is too expensive, installing a VFD or Soft-Starter eliminates the LRA spike entirely, curing the voltage drop immediately.

Step-by-Step Example Walkthrough

" An agricultural exhaust fan pulls a massive 100A LRA during startup. It is wired on a 100ft run of thin 10 AWG wire running 1-Phase 240V. "

  1. 1. Identify Ohms limit: NEC Table 8 shows 10 AWG solid copper at roughly 1.0Ω per 1000ft.
  2. 2. Calculate Total Wire Resistance: (100ft / 1000) * 1.0Ω * 2 (for both hot legs) = 0.2Ω total resistance.
  3. 3. Calculate Voltage Drop Blast: 100 Amps of LRA * 0.2Ω = 20 Volts instantly lost to heat in the wire.
  4. 4. Calculate Drop Percentage: (20V / 240V) * 100 = 8.3%.
Final Result: During the exact moment of startup, the motor only receives 220V (an 8.3% drop). Because this is under the 10% critical threshold, the contactors should remain latched and the motor will successfully start. As the motor spins up, the amperage will drop to 15A, and the voltage drop will shrink to a negligible 3V.

Quick Answer: Why do lights dim when a large motor starts?

Lights dim when an AC motor starts because the motor demands an extreme spike of current — called Locked Rotor Amps (LRA) — during the first fraction of a second. This massive amperage pulls so much power through the localized wires that the voltage 'sags' or drops across the entire branch circuit. You can calculate the exact percentage of this brownout using this Motor Startup Voltage Drop Calculator to ensure you are within safe equipment operating limits.

Underlying Formula

Voltage Drop = Locked Rotor Amps × Total Conductor Resistance

Resistance Variables:
  • Use an NEC Table 9 Ω/kFT metric for the specific AC wire gauge.
  • Multiply single-phase distances by 2.0 (outbound and return).
  • Multiply three-phase distances by 1.732.

Typical Motor Startup Voltage Sag Allowances

Voltage Drop Condition Percentage Loss Equipment Behavior
Normal Running Load < 3.0% 100% Efficient Operation
Acceptable LRA Startup 5.0% - 10.0% Slight Dimming, Safe Start
Deep Transient Sag 10.0% - 15.0% Heavy Labored Start, Risk of Stalling
Critical Failure Threshold > 15.0% Contactor Chatter, Melted Windings

Real-World Conductor Failures

The Far-Field Well Pump Trap

A farmer installs a heavily loaded 5 HP irrigation pump 800 feet away from the service panel. He calculates the normal Full Load Amps (FLA) at ~14A. He sizes the copper safely according to the 14A load, keeping running voltage drop under 3%. However, he forgot to calculate the LRA shock. When the pump attempts to start, it demands 95 Amps across 800 feet of wire. The voltage drops 24%. The contactor violently chatters and burns out the $1,500 pump block.

Electronic Control Board Reboots

A manufacturing plant wires an industrial laser cutter onto the same transformer branch as a pair of 20 HP air compressors. Every time the compressors automatically kick on, a massive 200 Amp LRA transient hits the line. The voltage sags by 12% for a fraction of a second. This is fine for the motor, but the sensitive 5V DC power supply inside the laser cutter assumes a blackout is occurring and immediately drops the cutting program.

Field Mitigation Strategies

Do This

  • ✓Isolate critical control circuits. Do not run the 24V control transformers off the exact same massive feed block as the motor itself. If the motor sags the local block voltage, the control transformer sags as well, unlatching the coils.
  • ✓Switch entirely to VFDs on runs over 500 feet. Copper is expensive. If you are running wires very long distances, up-sizing heavy gauge wire just to handle a half-second startup spike is financially terrible. A VFD limits the LRA strictly to regular running amps, allowing you to use dramatically thinner wire.

Avoid This

  • ✗Never assume aluminum wire acts like copper. If you switch to aluminum conductors for a long motor run to save money, you MUST recalculate the drop using Aluminum Ohms/kFT parameters. Aluminum has significantly higher resistance and will drop voltage drastically faster.

Frequently Asked Questions

What causes contactor chatter during motor startup?

Contactors use a magnetic coil to hold the heavy electrical contacts closed against spring pressure. If the motor startup pulls so much electricity (LRA) that the voltage plummets by 15-20%, the magnetic coil weakens. The spring overcomes the weak magnet and snaps the contactor open. Once open, the LRA stops, voltage returns to normal, the magnet gets strong again, and slams closed. This repeats rapidly, creating a loud 'chattering' sound that destroys the equipment.

How does the Motor Startup Voltage Drop Calculator interpret LRA?

The calculator allows you to explicitly input your Locked Rotor Amperage. It then pairs this absolute maximum current blast with the physical electrical resistance of your selected wire gauge (using NEC Chapter 9 resistance standards) over your specified distance. It vectors phase adjustments to determine the absolute lowest localized voltage trench you will experience.

Is 10% Voltage Drop safe for an electric motor?

A 10% voltage drop is fundamentally unsafe for continuous running operations, but it is generally considered the standardized acceptable margin for the fraction of a second taken by a startup transient. As long as the voltage rebounds to within 3-5% of nominal as soon as the rotor reaches operating speed, a brief 10% dip is survivable by the contactors.

Can upgrading the transformer fix startup voltage drops?

Upgrading the transformer only fixes supply-side drops (if the transformer's kVA is too low to support the inrush). If your transformer is huge, but your wire run between the transformer and the motor is thin and extremely long, the voltage will still drop inside the physical wire itself due to frictional heat loss. You must cure wire resistance with thicker copper or lower initial amperage.

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