Motor Inrush Drop

Estimate conductor voltage drop at the motor terminals while a motor draws locked-rotor current (LRA) at start, using a resistance-only model for single-phase or three-phase copper feeders.

Trades & Construction

Motor Electrical Characteristics

Locked-Rotor Amps (LRA)

Amps

System Voltage

One-Way Wire Distance

Feet

Phase

Copper Conductor

Above 10% Starting Drop

An estimated 13.3% drop during starting is above the 10% figure commonly used as a planning guideline for across-the-line starts. Review the source capacity and consider supplying control transformers from a circuit not affected by the motor start.

Scope: conductor resistance only (Ω/1,000 ft value used: 0.4910). Conductor reactance, the low starting power factor, and utility/transformer source impedance are not modeled, so actual starting drop is typically higher than shown.

Estimated Terminal Voltage During Start

416.2 V

At motor terminals while drawing LRA

Conductor Drop

63.8 V

Resistive drop in the feeder

Percentage Drop

13.3 %

Relative to source voltage

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Worked Example: 480 V Three-Phase Motor on a 300 ft Run

Calculator default inputs, resistance-only model

Design Scenario

A three-phase motor with a locked-rotor current of 250 A is fed at 480 V through 300 ft (one way) of 6 AWG copper.

Design Parameters

Locked-rotor current250 A

System voltage480 V, 3-phase

One-way length300 ft

Conductor6 AWG Cu (0.491 ohm/kft, NEC Table 8)

Mathematical Solution

1Loop resistance

Three-phase runs use the square root of 3 factor with one-way length.

R = (300 / 1000) x 0.491 x 1.732 = 0.2551 ohm

2Starting voltage drop

Vd = 250 A x 0.2551 ohm = 63.78 V

3Percent drop

Above the 10% screening flag, below the 15% severe flag.

63.78 / 480 x 100 = 13.29%

4Terminal voltage during start

480 - 63.78 = 416.2 V

Engineering Conclusion: The run drops about 63.8 V (13.3%) during the start, leaving roughly 416.2 V at the motor before any transformer or upstream sag. Because this exceeds the 10% screening guideline, conductor size and starter selection should be verified against motor torque and contactor hold-in ratings.

What is Motor Starting Voltage Drop?

Induction motors draw locked-rotor current, often several times full-load current, for the first part of a start. That current flows through the same conductors as running current, so a run sized for acceptable running drop can still sag noticeably while the motor accelerates. This page estimates only the conductor portion of that sag.

Mathematical Foundation

Conductor resistance of the run

R = \frac{L}{1000} \times R_{kft} \times k

L

= One-way conductor length from source to motor (ft)

R_{kft}

= Conductor resistance per 1,000 ft for the selected gauge (ohms), from the calculator table

k

= Circuit factor: 2 for single-phase (out and back), 1.732 (square root of 3) for three-phase

Starting voltage drop

V_d = I_{LRA} \times R \qquad \%V_d = \frac{V_d}{V_{nom}} \times 100

I_{LRA}

= Locked-rotor current from the motor nameplate code letter or manufacturer data (A)

V_{nom}

= Nominal system voltage (V)

Laws & Principles

The NEC informational notes in 210.19 and 215.2 suggest about 3% branch-circuit and 5% total running voltage drop for efficiency. They are not mandatory and do not address starting drop.
Starting tolerance is equipment-specific: motor accelerating torque falls roughly with the square of applied voltage, and contactor coils can drop out if control voltage sags too far.
The 10% and 15% flags in the calculator are screening thresholds chosen for planning, not code limits.
Soft starters and variable frequency drives limit starting current and are a common alternative to upsizing long runs.

Step-by-Step Example Walkthrough

" A single-phase 240 V motor with 100 A locked-rotor current is fed through 100 ft (one way) of 8 AWG copper. "

1. Loop resistance: (100 / 1000) x 0.778 ohm/kft x 2 = 0.1556 ohm.
2. Starting drop: 100 A x 0.1556 ohm = 15.56 V.
3. Percent drop: 15.56 / 240 x 100 = 6.48%.
4. Terminal voltage during start: 240 - 15.56 = 224.4 V.

Final Result: About 6.48% conductor drop during the start, well within the 10% screening flag. Transformer and source impedance will add some additional sag.

Quick Answer

Multiply the motor locked-rotor current by the loop resistance of the feeder. For a 250 A start on 300 ft of 6 AWG copper at 480 V three-phase, the calculator estimates about 63.8 V (13.3%) of conductor drop based on NEC Chapter 9 Table 8 stranded copper resistance. The result excludes transformer and utility sag, which add further voltage reduction at the terminals.

Conductor Resistance Reference (NEC Chapter 9 Table 8)

The calculator uses standard DC resistance values from NEC 2023 Chapter 9 Table 8 for uncoated stranded copper conductors at 75°C, covering branch-circuit sizes through heavy commercial feeder sizes up to 500 kcmil.

Conductor Size	NEC Table 8 DC Resistance (ohm/kft at 75°C)	Typical Application
8 AWG	0.778	Small fractional-HP motors
6 AWG	0.491	5–10 HP branch circuits
4 AWG	0.308	15–20 HP branch circuits
2 AWG	0.194	25–30 HP branch circuits
1 AWG	0.154	40 HP branch circuits
1/0 AWG	0.122	50 HP feeders
2/0 AWG	0.0967	60–75 HP feeders
3/0 AWG	0.0766	100 HP feeders
4/0 AWG	0.0608	125 HP feeders
250 kcmil	0.0515	150 HP feeders
350 kcmil	0.0367	200 HP heavy feeders
500 kcmil	0.0258	Major industrial motor feeders

Screening Bands Used by the Calculator

Starting drop	Calculator status	Suggested follow-up
10% or less	Within screening range	Confirm transformer sag and other loads on the feeder.
Above 10% to 15%	Warning	Check motor accelerating torque and contactor dropout voltage with manufacturer data.
Above 15%	Severe	Consider a larger conductor, shorter run, or reduced-voltage starting.

Frequently Asked Questions

What does the motor startup voltage drop result represent?

It is the voltage lost across the feeder conductors while the motor draws locked-rotor current, calculated as LRA multiplied by the loop resistance of the run. It does not include transformer or utility sag, so the voltage at the motor can be lower than shown.

What starting voltage drop is acceptable?

There is no single NEC limit for starting voltage drop. The calculator flags results above 10% and above 15% as screening thresholds. The actual tolerance depends on the motor torque requirement, the starter, contactor coil dropout voltage, and other loads on the same feeder, so check manufacturer data for the equipment involved.

How are the conductor resistance values determined?

The calculator uses standard DC resistance values from NEC Chapter 9 Table 8 for uncoated stranded copper conductors at 75°C (ranging from 0.778 ohms/kft for 8 AWG down to 0.0258 ohms/kft for 500 kcmil). Conductor reactance and power factor are not included in this screening model.

Where do I get the locked-rotor amps?

Use the manufacturer data sheet if available. Otherwise the nameplate code letter and NEC Table 430.7(B) give a kVA-per-horsepower range that can be converted to amps. Across-the-line starting current is commonly about 5 to 8 times full-load current for many induction motors.

How can starting voltage drop be reduced?

Common options are a larger conductor, a shorter run, a soft starter or variable frequency drive that limits starting current, or a reduced-voltage starting method. A larger supply transformer helps only when the transformer, not the conductors, is the main source of the sag.

Related Calculations

Calculation Provenance & Validation Record

Method

Resistance-only estimate: starting voltage drop equals locked-rotor current times the loop resistance of the copper run (2 for single-phase, 1.732 for three-phase).

Formula

V_d = I_{LRA} imes rac{L}{1000} imes R_{kft} imes k

Assumptions

Resistance-only model: conductor reactance and power factor during starting are ignored.
Source (utility or transformer) voltage is assumed stiff; transformer and upstream feeder sag are not included.
Copper conductors; resistance values come from the calculator's built-in table, which is lower than NEC Chapter 9 Table 8 for the same sizes (see disclosure below).
Locked-rotor current is treated as constant for the duration of the start.

References

NFPA 70, National Electrical Code (NEC 2023), Chapter 9 Table 8 - conductor DC resistance
NEC 2023 Article 430 - motor circuits; Table 430.7(B) locked-rotor indicating code letters
NEMA MG 1 - motor voltage tolerance guidance for running conditions

Last substantive review: 2026-10-03

Automated test status: 5 golden test vectors passing (MID-01, MID-02, MID-03, MID-04, MID-05)

Method, assumptions & governing standards

Standard Sourced

Calculation Methodology

Resistance-only estimate: starting voltage drop equals locked-rotor current times the loop resistance of the copper run (2 for single-phase, 1.732 for three-phase).

Governing Standard 2023 NEC / NEMA MG-1-2021

Standard:NFPA 70 (NEC) / NEMA MG-1

Statutory building, electrical, and mechanical codes vary by jurisdiction. Confirm local municipality amendments before installation.

Key Assumptions & Constraints

Resistance-only model: conductor reactance and power factor during starting are ignored.
Source (utility or transformer) voltage is assumed stiff; transformer and upstream feeder sag are not included.
Copper conductors; resistance values come from the calculator's built-in table, which is lower than NEC Chapter 9 Table 8 for the same sizes (see disclosure below).
Locked-rotor current is treated as constant for the duration of the start.

Regression Tests: 6 golden vectors

Last Verified: 2026-10-03

Primary References: 3 documented

Field Trade Notice: For trade planning and engineering estimates. Final installations must conform to project blueprints, authority having jurisdiction (AHJ) code approvals, and site-specific inspections.