Starting an induction motor can be one of the highest short-duration demands on a power system. The actual current and voltage dip depend on the motor nameplate, manufacturer data, source impedance, transformer, conductors, starting method, load torque, acceleration time, simultaneous loads, utility limits, and protective devices. The motor code letter is a useful input, but it does not by itself approve a starter, breaker, conductor, VFD, soft starter, voltage-dip result, or safe energization.
This guide explains how code-letter starting-current estimates fit into a design-review workflow. Use the related calculator for preliminary locked-rotor arithmetic, then verify the adopted code edition, product data, power-system study, utility/AHJ requirements, and qualified electrical review before design or installation.
Code Letters: What They Mean and Why They Matter
The code letter is a nameplate cue for locked-rotor kVA per horsepower. "Locked rotor" means the rotor is stationary, which is the starting condition before the motor accelerates. The NEC code-letter table converts to starting kVA and current, but the adopted NEC edition and selected motor data control design use.
Code-letter rows are normally shown as kVA/HP ranges. For a preliminary estimate, multiply the kVA/HP value by motor HP, then divide by voltage and by sqrt(3) for three-phase motors. Use the actual range bounds and treat open-ended rows, such as code V, as minimum bounds unless manufacturer data gives a tighter value.
Higher code letters generally mean higher starting current, but acceptable starting behavior depends on more than current. Source impedance, voltage dip, load torque, acceleration time, motor design, protective-device timing, utility limits, and starter or drive manufacturer data still need review.
NEMA design letters describe torque-speed behavior, while code letters indicate locked-rotor kVA/HP. Neither field alone approves a replacement motor, starter, protection setting, voltage-dip result, or safe operation.
Locked-Rotor Current (3-phase) = (code-letter kVA/HP x HP x 1000) / (V x 1.732)
Use authorized code text, selected motor data, and manufacturer guidance before treating local fixture rows as design values.
Motor Starting Current / Code Letter Calculator
Convert NEC code letters (A through V) to locked-rotor starting amps. Starting-to-running ratio and starter recommendation for any motor.
Voltage Dip: The Real-World Impact of Starting Current
When a motor starts, inrush current flows through transformer, feeder, bus, and utility source impedance. That can produce a temporary voltage dip affecting other loads on the same system.
A voltage-dip estimate requires system impedance and source data, not code letter alone. Transformer impedance, conductor impedance, utility short-circuit capacity, generator transient response, simultaneous loads, motor acceleration time, and control-device ride-through behavior all change the result.
Published or site voltage-dip limits vary by utility, equipment class, process sensitivity, and engineering criteria. Use a real motor-starting study for critical loads, large motors, weak sources, generators, or sensitive controls.
Do not infer voltage dip from transformer kVA alone. Use source impedance, conductor data, selected motor data, starting method, utility criteria, and qualified power-system review.
Across-the-Line (DOL) Starting
Direct-on-line (DOL) starting connects the motor directly to line voltage through suitable control equipment. It can be simple, but it also applies the full locked-rotor inrush and full starting torque to the electrical source and driven equipment.
DOL suitability depends on the actual motor, source capacity, voltage dip, OCPD timing, starter ratings, load torque and inertia, mechanical shock, utility limits, and AHJ requirements. Horsepower alone is not a reliable approval threshold.
DOL equipment still needs selected-product ratings for contactor, overload, SCCR, enclosure, disconnect, branch-circuit and feeder protection, conductors, grounding, bonding, and installation conditions.
If full-voltage starting is not supported by source data, reduced-voltage or drive-based starting may be reviewed. Those methods reduce current but also affect available torque, acceleration time, heating, harmonics, bypass operation, and product-specific limits.
Treat DOL as one candidate method. Verify source capacity, voltage dip, load torque, OCPD timing, starter ratings, utility rules, and manufacturer data before selecting it.
Reduced-Voltage Starting Methods
Reduced-voltage starting methods reduce applied motor voltage during startup, which can reduce current but also reduces available torque. The selected method must still accelerate the actual load without overheating the motor or violating manufacturer limits.
Wye-delta starter: Uses different winding connections for starting and running on motors built for that method. Transition behavior, lead count, overload placement, timing, and torque suitability require manufacturer and engineering review.
Autotransformer starter: Uses transformer taps to reduce starting voltage. Tap selection, torque, transition, thermal rating, enclosure, protection, and control sequence are product- and application-specific.
Soft starter: Uses solid-state control to ramp voltage or limit current. Settings, bypass operation, thermal capacity, harmonic behavior, acceleration time, and short-circuit coordination depend on the selected product and load.
Variable frequency drive: Controls voltage and frequency during start and may also control running speed. VFD suitability depends on motor insulation, cable length, dv/dt, harmonics, grounding, EMC, enclosure, bypass, line/load reactors or filters, process needs, and manufacturer data.
Compare candidate methods with selected-product manuals, load torque, acceleration time, voltage-dip limits, protection coordination, utility rules, and qualified electrical review.
Reviewing Candidate Starting Methods
Starting-method review depends on load type, starting torque, acceleration time, allowable voltage dip, source impedance, utility limits, protection coordination, selected equipment ratings, enclosure and environmental conditions, process needs, and budget.
Constant-torque loads can need substantial torque before movement. Reduced-voltage methods may not provide enough torque unless manufacturer curves and load data support the setting.
Variable-torque loads may be easier to start, but fan and pump behavior still depends on inertia, control sequence, check valves, dampers, process conditions, and selected drive or starter data.
High-inertia loads can create long acceleration times and motor heating. Review motor thermal limits, starter or drive duty, protection timing, and process constraints before selecting any method.
Use a documented comparison instead of a shortcut: actual motor data, load torque/inertia, source and conductor impedance, protection coordination, utility limits, selected product manuals, installation conditions, and qualified review.
No shortcut selects the method. Use the code-letter estimate as one input, then document the motor, load, source, protection, utility, manufacturer, and AHJ basis.
Reading a Motor Nameplate
The motor nameplate is the starting point for source review, but it is not the only source needed for circuit design, starter selection, troubleshooting, or replacement decisions.
HP (Horsepower): The rated mechanical output power at the shaft at rated conditions. Electrical input depends on efficiency and power factor.
Voltage: The rated voltage(s) and connection basis for the motor. Verify the actual connection, supply, and installation instructions.
FLA (Full Load Amps): The nameplate current at rated load. NEC table current and nameplate current may be used for different code purposes, so verify the required basis.
RPM: The rated full-load speed. Slip, load, voltage, frequency, and motor condition affect actual running speed.
Service Factor: A nameplate rating that must be interpreted with ambient, voltage, enclosure, duty, altitude, and manufacturer limits.
Code Letter: A locked-rotor kVA/HP cue used as one input for starting-current estimates, not a complete starter or protection design.
Design Letter: NEMA torque-speed context that still requires selected-motor and driven-equipment review.
Frame Size: A physical-dimension cue. Manufacturer drawings, shaft, keyway, bolt pattern, enclosure, and fit still control replacement decisions.
Insulation Class and Efficiency: Nameplate context for thermal and energy review, not proof of suitability for a drive, overload setting, ambient, or duty cycle.
Photograph every motor nameplate during commissioning and file it with the equipment records. When the paint fades and the plate corrodes, that photo becomes the only reliable source of the motor's design data.