Motor starting voltage drop is the temporary reduction in voltage at the motor terminals while the motor accelerates. It is calculated using the motor’s starting current—typically locked-rotor current (LRA) or manufacturer-provided starting data—not merely its normal running current.
A motor branch circuit can show an acceptable voltage drop at full-load current yet experience a substantial terminal-voltage reduction during starting. Evaluating this condition during conductor and feeder planning helps identify whether the branch circuit, feeder, transformer, generator, or other upstream source impedance may affect motor acceleration and connected equipment.
Use the Motor Starting Current Calculator to establish the starting-current basis. Then evaluate conductor-related voltage loss with the Motor Branch Voltage Drop Calculator and, where applicable, the Motor Feeder Voltage Drop Calculator.
Running Current vs. Starting Current
Motor circuits have at least two distinct voltage-drop conditions. One applies while the motor operates under load; the other applies during the short interval when the motor is accelerating.
| Condition | Current Used | What the Result Indicates |
|---|---|---|
| Running condition | Motor full-load current or measured operating current | Voltage loss during normal loaded operation |
| Starting condition | Locked-rotor current, nameplate LRA, or selected starting-current value | Temporary voltage reduction while the motor accelerates |
For a fixed conductor path and impedance, voltage drop rises approximately in direct proportion to current. If a motor draws seven times its running current at start, its conductor-only starting voltage drop will be approximately seven times the running voltage drop under the same impedance and conductor-temperature assumptions.
\(\displaystyle \text{Starting voltage drop} \approx \text{Running voltage drop} \times \frac{I_\text{start}}{I_\text{run}}\)
This relationship is useful as a quick reasonableness check. The final calculation should use the actual starting current, conductor material and size, circuit length, phase configuration, and the selected resistance or impedance basis.
A temporary starting voltage reduction can be tolerable even when the same percentage drop would be undesirable during continuous operation. However, motor terminal voltage affects available motor torque. Severe voltage reduction can extend acceleration time, increase heating, keep a loaded motor from reaching speed, or interfere with controls and contactors.
For terminology and starting-current selection, see Motor Current, Starting Current, and FLC.
What a Motor Starting Voltage Drop Calculator Determines
A motor starting voltage drop calculator estimates the conductor-related voltage loss during acceleration and the voltage remaining at the motor terminals.
The primary outputs are:
- Starting voltage drop in volts
- Starting voltage drop as a percentage of system voltage
- Estimated motor-terminal voltage during starting
- A comparison between normal running voltage drop and starting voltage drop
The terminal-voltage result is found by subtracting the calculated starting voltage drop from the nominal system voltage:
\(\displaystyle V_\text{motor,start} = V_\text{system} - V_{D,\text{start}}\)
For example, a 480 V motor circuit with a calculated 48.5 V starting drop would have an estimated branch-circuit-only terminal voltage of:
\(\displaystyle 480 - 48.5 = 431.5\text{ V}\)
That value represents the voltage at the motor after the modeled branch-conductor loss. It does not automatically include voltage reduction from the feeder, transformer, generator, busway, disconnect, starter, terminations, or utility supply.
Inputs That Affect Starting Voltage Drop
A useful calculation requires the same core circuit data used for a normal voltage-drop review, plus a reliable starting-current value.
| Input | Unit | Typical Source or Decision |
|---|---|---|
| System voltage | V | Motor nameplate voltage and supply configuration |
| Phase configuration | Single-phase or three-phase | Motor and distribution-system configuration |
| Starting current | A | Motor nameplate LRA, manufacturer data, or approved design input |
| Running current | A | Nameplate current, calculated FLC, or measured operating current |
| One-way circuit length | ft | Actual electrical route length |
| Conductor material | Copper or aluminum | Project conductor selection |
| Conductor size | AWG or kcmil | Proposed branch-circuit or feeder conductor |
| Conductor resistance or impedance basis | Resistance or AC impedance | Calculator method and conductor assumptions |
| Power factor, where used | Decimal or percent | Motor-starting or system-study assumption |
Use the electrical route length rather than a straight-line building measurement. Most voltage-drop methods use one-way length, although the formula must match the calculator’s input convention.
For a single-phase circuit, the calculation typically accounts for both outgoing and return conductors. For a three-phase circuit, the appropriate three-phase relationship is used. Do not manually double a one-way length unless the calculator specifically calls for total loop length.
For non-motor circuit evaluations, use a general Voltage Drop Calculator.
Starting Voltage-Drop Formulas
The simplest conductor voltage-drop relationship uses current and effective circuit resistance:
\(\displaystyle V_D = I \times R\)
Where (R) is the effective resistance of the conductor path.
For AC circuits, a more complete voltage-drop method may include conductor resistance, reactance, and power factor. With one-way length (L), and resistance and reactance expressed in matching per-length units, common forms are:
Single-Phase Circuit
\(\displaystyle V_D = 2IL(R\cos\phi + X\sin\phi)\)
Three-Phase Circuit
\(V_D = \sqrt{3}IL(R\cos\phi + X\sin\phi)\)
The percentage voltage drop is:
\(\displaystyle \%V_D = \frac{V_D}{V_\text{system}} \times 100\)
A resistance-based approach may be appropriate for a preliminary conductor comparison when the selected method and conductor data support that assumption. A system-level motor-starting review may require resistance, reactance, motor power factor, source characteristics, and other upstream impedance components.
Worked Example: 480 V Three-Phase Motor
Consider a 480 V, three-phase motor branch circuit with the following assumed values:
- Running current: 65 A
- Starting current: 455 A
- Starting-current ratio: (455 \div 65 = 7)
- One-way branch-circuit length: 200 ft
- Conductor impedance basis: \(0.308\ \Omega/\text{kft}\)
- Scope: branch-circuit-only review
Using the three-phase resistance-based approximation, the running voltage drop is:
\(\displaystyle V_{D,\text{run}} = \frac{\sqrt{3} \times 65 \times 200 \times 0.308}{1000} = 6.93\text{ V}\)
\(\displaystyle \%V_{D,\text{run}} = \frac{6.93}{480} \times 100 = 1.44\%\)
At the 455 A starting current, the branch-circuit voltage drop becomes:
\(\displaystyle V_{D,\text{start}} = \frac{\sqrt{3} \times 455 \times 200 \times 0.308}{1000} = 48.5\text{ V}\)
\(\displaystyle \%V_{D,\text{start}} = \frac{48.5}{480} \times 100 = 10.1\%\)
The calculated motor-terminal voltage during starting is:
\(\displaystyle V_\text{motor,start} = 480 - 48.5 = 431.5\text{ V}\)
The running drop is relatively modest, while the starting drop is substantially higher because the motor draws seven times the running current. This is why a normal full-load voltage-drop calculation cannot substitute for a motor starting voltage-drop calculation.
Run the selected branch conductors through the Motor Branch Voltage Drop Calculator. Where the feeder is separate from the motor branch circuit, calculate the feeder independently with the Motor Feeder Voltage Drop Calculator and evaluate the total source-to-motor path.
Where Branch-Circuit Results Stop
A branch-circuit calculation evaluates only one part of the voltage dip the motor may experience. The actual motor terminal voltage during start can be affected by all series impedance between the source and the motor, including:
- Service transformer impedance
- Generator capability and transient response
- Feeder conductors
- Busway and bus impedance
- Disconnects, starters, and terminations
- Branch-circuit conductors
- Concurrent loads and utility-service conditions
Increasing branch-circuit conductor size reduces the branch-circuit portion of voltage drop. It may not resolve the condition if feeder, transformer, generator, or utility-source impedance is the primary contributor.
Motor starting method also affects the current profile and acceleration behavior. Confirm whether the motor starts across the line, with a reduced-voltage starter, soft starter, or VFD before selecting the starting-current input.
The required starting torque must also be considered at the actual driven load. Fans, pumps, compressors, conveyors, and high-inertia loads can impose different acceleration requirements. Starting voltage reduction can also affect contactors, relays, controls, PLCs, drives, lighting, and other loads connected to the same electrical system.
Apply the Result in Design and Field Review
Motor starting voltage drop should be reviewed alongside—not substituted for—other electrical design and installation requirements. Voltage-drop arithmetic does not establish conductor ampacity, overload protection, short-circuit and ground-fault protection, raceway fill, terminal ratings, temperature correction, adjustment factors, current-carrying conductor requirements, equipment listing, or final code compliance.
A practical review sequence is:
1. Obtain motor nameplate LRA or manufacturer-supported starting data.
- Confirm the supply voltage, phase configuration, starting method, and actual one-way electrical route length.
3. Calculate branch-circuit voltage drop using the selected starting current.
- Calculate feeder voltage drop separately when the feeder contributes to the source-to-motor path.
- Review transformer, generator, bus, service, and utility conditions when a system-level voltage-dip assessment is required.
- Compare the estimated starting terminal voltage with the motor manufacturer’s starting requirements and the actual load-torque condition.
- Verify final conductor, equipment, protection, installation, permitting, and inspection requirements with the responsible electrical professional and AHJ.
Motor voltage drop during starting is best evaluated with locked-rotor or manufacturer-supported starting current, not normal running current alone. A motor starting voltage drop calculator provides the conductor-related loss and estimated terminal voltage during acceleration, helping identify whether the branch circuit is adequate or whether the review must extend upstream to the feeder and source equipment.