Motor Nameplate Explained: How to Read Every Field
How to read a motor nameplate — voltage, FLA, HP/kW, RPM, service factor, efficiency, insulation class, frame, and what each value means for VFD and protection setup.
Every electric motor leaves the factory with a metal plate riveted to its frame. That nameplate is the single source of truth for every electrical and mechanical decision you will make about that motor — from the wire gauge you pull to the overload relay setpoint you dial in to the parameters you key into a VFD. Miss one field and you risk nuisance trips at best, catastrophic motor failure at worst.
This guide walks through every field on a motor nameplate in the order you are most likely to encounter it, explains exactly what the value means, and — critically for controls engineers — tells you where that value goes when you are commissioning a drive or protection relay.
What a Motor Nameplate Is
A motor nameplate is the manufacturer's certified data sheet stamped permanently onto the motor housing. Standards bodies — NEMA (National Electrical Manufacturers Association) in North America and IEC (International Electrotechnical Commission) everywhere else — define exactly which fields must appear and how they are expressed. The nameplate supersedes any catalogue data, drawing, or verbal specification. When in doubt, trust the plate.
Nameplate data serves three audiences:
- Installers and electricians — sizing conductors, disconnects, and overcurrent protection
- Controls engineers — programming VFDs, soft starters, and overload relays
- Maintenance technicians — matching replacement motors and diagnosing faults
You will use the plate repeatedly throughout the equipment lifecycle. Photograph it during commissioning — dust, paint, and corrosion can obscure it within months.
The Key Fields Explained
Voltage (V)
The rated voltage is the line-to-line supply voltage at which the motor delivers its rated output. Common NEMA voltages include 208 V, 230 V, 460 V, and 575 V. IEC motors typically show 230/400 V or 400/690 V, reflecting star/delta connection options (more on this below).
A dual voltage listing such as 230/460 V means the motor can be internally reconnected for either supply. At 230 V the stator windings are connected in parallel; at 460 V they are connected in series. The wiring diagram on the terminal box cover shows which leads to join.
Tolerance: NEMA MG-1 allows ±10% of rated voltage. IEC EN 60034-1 allows ±5% voltage variation combined with ±2% frequency variation. Running a motor persistently outside these limits increases winding temperature and shortens insulation life.
Full Load Amps (FLA)
FLA — also written as F.L.A., I_N, or I_rated on IEC plates — is the current the motor draws from the supply when it is operating at rated voltage, rated frequency, and delivering its rated shaft power. This is the most important single number on the nameplate for protection and drive setup.
FLA is not the maximum current the motor can survive. It is the design operating point. Starting current (locked-rotor current) is typically 600–700% of FLA for a NEMA Design B motor and flows for the duration of acceleration.
Where FLA appears in your controls work:
- Overload relay: set the trip current to 100–115% of FLA (or to the motor's service factor current if SF > 1.0 — see below)
- VFD rated output current must be ≥ FLA
- Conductor ampacity per NEC 430.22: 125% of FLA as a minimum
Horsepower / kW (HP / kW)
Nameplate HP or kW is the rated shaft output power — the mechanical energy delivered at the motor coupling under full load at rated voltage and frequency. It is not the electrical input power.
The relationship between input and output:
P_input (kW) = (HP × 0.746) ÷ Efficiency
A 20 HP, 91% efficient motor draws approximately 16.4 kW from the supply at full load. That 16.4 kW is what your panel designer and utility bill care about; the 20 HP is what your mechanical engineer uses to size the driven load.
IEC motors express output in kW. When you see a nameplate with both (common on export motors), treat kW as primary and HP as the rounded equivalent.
RPM and Poles
RPM on the nameplate is the full-load speed — the shaft speed when the motor is delivering rated torque at rated voltage and frequency. It is always slightly less than synchronous speed due to slip.
Synchronous speed is determined by supply frequency and the number of magnetic poles:
n_s = (120 × f) ÷ P
Where f is frequency in Hz and P is the number of poles.
| Poles | Synchronous speed (60 Hz) | Synchronous speed (50 Hz) | Typical nameplate RPM (60 Hz) |
|---|---|---|---|
| 2 | 3600 RPM | 3000 RPM | ~3450–3480 RPM |
| 4 | 1800 RPM | 1500 RPM | ~1740–1775 RPM |
| 6 | 1200 RPM | 1000 RPM | ~1150–1175 RPM |
| 8 | 900 RPM | 750 RPM | ~855–875 RPM |
The difference between synchronous speed and nameplate RPM is slip. A 4-pole motor at 1750 RPM has a slip of 50 RPM, or about 2.8%. Higher slip motors tend to have higher starting torque (NEMA Design D) but lower efficiency under running conditions.
VFD note: When you program a VFD, you enter the nameplate RPM as the base speed — the speed the motor reaches at base frequency (60 Hz or 50 Hz) under full load. Above base frequency, the drive operates in field-weakening and torque capacity decreases.
Frequency (Hz)
The rated supply frequency — 60 Hz in North America, 50 Hz in most of the rest of the world, or a dual rating such as 50/60 Hz for global motors. Frequency and voltage are inseparable: a motor rated 460 V / 60 Hz has a volts-per-hertz ratio of 7.67 V/Hz. The VFD must maintain this ratio across the speed range to avoid magnetic saturation below base speed.
Running a 60 Hz motor on 50 Hz at the same voltage reduces available torque by ~28% and increases losses. Running a 50 Hz motor on 60 Hz at the same voltage increases flux and saturates the core. Neither is recommended without derating.
Service Factor (SF)
Service factor is a multiplier that indicates how much overload the motor can sustain for short periods at rated voltage and frequency without exceeding its temperature limits. A motor with SF 1.15 can operate continuously at 115% of its nameplate HP under ideal conditions (rated voltage, rated frequency, no altitude derating).
NEMA general-purpose motors commonly carry SF 1.15. Severe-duty and hazardous-location motors are typically rated SF 1.0. IEC motors are almost always SF 1.0 — the IEC standard does not recognize service factor in the same way.
Why this matters for protection:
When the motor has SF > 1.0, some engineers set the overload relay trip current to FLA × SF rather than FLA alone, permitting the motor to use its full thermal capacity. This is acceptable only when the power supply is clean and stable. If voltage is low or unbalanced, back the setpoint to FLA × 1.0.
Why this matters for a VFD:
A VFD feeding a motor effectively removes the service factor because the drive can only output its own rated current. Size the VFD so its continuous output current rating is ≥ FLA × SF if you want to preserve full motor capacity.
Efficiency (EFF or η)
Nameplate efficiency is the ratio of shaft output power to electrical input power at full load, expressed as a percentage. Efficiency data on NEMA Premium motors references the NEMA MG-1 Table 12-12 test procedure (IEEE 112 Method B). IEC motors reference IEC 60034-30-1 efficiency classes: IE1 (Standard), IE2 (High), IE3 (Premium), IE4 (Super Premium).
| IE Class | Approximate 4-pole 15 kW efficiency |
|---|---|
| IE1 | ~88.0% |
| IE2 | ~91.0% |
| IE3 | ~92.6% |
| IE4 | ~94.0% |
Efficiency is highest near full load and drops significantly at 25–50% load. This is why oversizing motors (a common field habit) hurts energy consumption disproportionately — the motor runs in its low-efficiency region all day.
Insulation Class and Temperature Rise
Insulation class defines the maximum allowable winding temperature under rated conditions. The nameplate may show the class letter, the temperature rise, or both.
| Class | Max winding temperature | Allowable temperature rise (40°C ambient) |
|---|---|---|
| A | 105°C | 60°C |
| B | 130°C | 80°C |
| F | 155°C | 105°C |
| H | 180°C | 125°C |
Most modern motors are Class F insulation with a Class B temperature rise — meaning they use Class F materials but are designed to operate within Class B limits. This gives a thermal safety margin of 25°C, extending insulation life significantly (insulation life roughly doubles for every 10°C reduction below the rated limit).
Altitude derating: Standard motors are rated for up to 3300 ft (1000 m) elevation. Above this, ambient air is less dense and cooling is reduced. Derate the motor approximately 3% per 1000 ft (300 m) above 3300 ft.
VFD-driven motors: VFD output waveforms contain harmonic content that increases motor losses and winding temperature compared to sinusoidal supply. If the motor is not rated for inverter duty (NEMA MG-1 Part 31 or IEC 60034-17), the insulation may degrade prematurely, especially from high dV/dt switching voltages. Always specify inverter-duty or VFD-rated motors for variable speed applications.
Frame Size
Frame size is a standardized set of dimensions — shaft height, shaft diameter, mounting bolt pattern, shaft length — that allows motors from different manufacturers to be swapped without modifying the driven equipment or mounting structure.
NEMA frames use a numbering system where the first two or three digits relate to shaft centerline height in quarter-inches. Frame 56 has a shaft centerline 56/4 = 14 inches from the mounting foot. Frame 182 has a shaft centerline of 18/4 = 4.5 inches. The additional letters (T, TC, TS, Y, etc.) indicate shaft and face details.
IEC frames use a two-letter prefix (B3, B5, B14, etc.) indicating mounting type, plus a frame number corresponding to shaft centerline height in millimetres. IEC 132 = 132 mm shaft centerline.
When specifying a replacement motor, the frame size must match exactly to ensure mechanical interchangeability. Do not assume that matching HP/kW and RPM guarantees physical fit.
Duty Cycle
Duty or duty cycle describes the pattern of on/off operation the motor is designed to sustain. NEMA and IEC both define standard duty types:
| NEMA | IEC | Description |
|---|---|---|
| Continuous | S1 | Constant load for an indefinitely long period |
| Intermittent | S3 | Repeated identical cycles of load then rest |
| — | S4 | Intermittent with starting |
| — | S5 | Intermittent with electric braking |
Most general-purpose industrial motors are S1 / Continuous duty. If a motor is duty-rated (S3, S4), its nameplate HP/kW cannot be compared directly with a continuous-duty motor — the intermittent-duty motor is thermally smaller and will overheat if run continuously at nameplate load.
Enclosure (IP Rating / NEMA Enclosure Type)
Enclosure designation tells you what the motor housing protects against — not what voltage it operates at, not how it is mounted.
NEMA enclosure types (common industrial types):
| Type | Description |
|---|---|
| ODP (Open Drip-Proof) | Ventilated; allows air circulation; suitable for clean, dry indoor locations |
| TEFC (Totally Enclosed Fan Cooled) | Sealed frame, external fan; suitable for dusty or wet outdoor locations |
| TENV (Totally Enclosed Non-Ventilated) | Sealed, no fan; for low-power or slow-speed applications |
| TEAO (Totally Enclosed Air Over) | Cooled by an external airstream from driven equipment; used on blowers |
| Explosion-proof (XP) | NEMA 7 (Class I gas) or NEMA 9 (Class II dust); for hazardous areas |
IEC IP codes use two digits: the first indicates solid particle protection (0–6), the second indicates liquid ingress protection (0–9K). A motor marked IP55 is dust-tight (5) and protected against water jets from any direction (5). IP65 is fully dust-tight. IP66 withstands powerful water jets.
Choose enclosure based on environment first, then verify the IP/NEMA rating matches your installation's area classification requirements.
Connection Diagram (Star / Delta / Wye)
For dual-voltage motors, the nameplate or the inside of the terminal box cover shows a wiring diagram specifying how to connect the nine (NEMA) or six (IEC) lead wires for each voltage.
NEMA nine-lead motor (230/460 V example):
- 230 V (parallel / low voltage): Connect leads 1-7, 2-8, 3-9 together; supply to T1, T2, T3
- 460 V (series / high voltage): Connect leads 4-7, 5-8, 6-9 together; supply to T1, T2, T3; cap ends of 7, 8, 9
IEC six-lead motor (230/400 V example — star/delta):
- 400 V star (Y): Join U2, V2, W2 together (neutral point); supply to U1, V1, W1
- 230 V delta (Δ): Connect U1-W2, V1-U2, W1-V2; supply to the three junction points
Incorrect connection is one of the most common commissioning mistakes. A 460 V motor connected for 230 V on a 460 V supply will draw approximately four times rated current and fail within seconds.
Power Factor (PF or cos φ)
Power factor is the ratio of real power (kW) to apparent power (kVA). A motor with a nameplate PF of 0.85 at full load draws 0.85 kW of real work for every 1 kVA of apparent power from the supply. The remaining 0.526 kVAR is reactive magnetizing current that does no useful work but flows through conductors and transformers.
Power factor is worst at light load (it can drop below 0.5 at 25% load) and highest near full load. Oversized motors therefore impose a larger reactive penalty than correctly sized ones.
For billing and power quality purposes, sites often install power factor correction capacitors. Never connect capacitors between a VFD and the motor — the capacitors will resonate with the drive's output inductance and damage both.
NEMA vs IEC Nameplates
The two standards cover the same physical motor but express data differently. The table below maps the most common fields:
| Parameter | NEMA label | IEC label |
|---|---|---|
| Output power | HP | kW |
| Full load current | F.L.A. or A | I_N (A) |
| Synchronous speed basis | RPM (nameplate is FL speed) | rpm (same) |
| Service factor | SF (e.g., 1.15) | Not listed (assumed 1.0) |
| Insulation class | INS.CL. | Thermal class (e.g., F) |
| Efficiency rating | NEMA MG-1 Table 12-12 | IE1/IE2/IE3/IE4 |
| Frame | NEMA frame number (e.g., 213T) | IEC frame (e.g., IEC 132M) |
| Enclosure | NEMA type (ODP, TEFC, XP) | IP code (e.g., IP55) |
| Frequency | Hz | Hz |
When sourcing a replacement from a different region, map each field carefully. Do not assume a 15 kW IE3 motor is a drop-in equivalent to a 20 HP NEMA Premium motor without checking FLA, frame, enclosure, and insulation class individually.
The Controls View: Which Nameplate Values You Enter and Where
This is where the nameplate becomes a commissioning document. The following sections address the two most common controls scenarios.
Programming a VFD from the Nameplate
A VFD needs motor data to run its V/Hz or vector control algorithm correctly. The exact parameter labels vary by manufacturer (Siemens, ABB, Allen-Bradley, Danfoss, Yaskawa), but the source values are always the same nameplate fields.
| VFD parameter | Nameplate field | Notes |
|---|---|---|
| Motor rated voltage | Voltage (V) | Match to the connection used (star or delta) |
| Motor rated frequency | Hz | 50 or 60 Hz |
| Motor rated speed | RPM | Nameplate full-load RPM, not synchronous speed |
| Motor rated current | FLA (A) | Used for output current limiting and overload |
| Motor rated power | HP or kW | Used by some drives for auto-tuning |
| Number of poles | Calculated from RPM and Hz, or read directly if listed | P = (120 × f) ÷ n_s |
| Motor cos φ | PF | Used in vector control calculations |
After entering motor data, run the drive's auto-tune (sometimes called motor ID or self-commissioning) routine with the motor uncoupled if possible. Auto-tune measures actual stator resistance, rotor resistance, and leakage inductance, which are more accurate than nameplate estimates and improve speed regulation, especially in sensorless vector mode.
For a full walkthrough of VFD parameter setup from PLC, see the VFD programming with PLC guide and the dedicated how to program a VFD tutorial.
Setting an Overload Relay from the Nameplate
Overload relays — whether bimetallic, electronic, or solid-state — are set by entering or dialing the motor's full-load current. The relay then monitors actual current and trips after a time-current curve if the motor exceeds its thermal limit.
| Relay parameter | Nameplate field | Setting guidance |
|---|---|---|
| Trip current (Ir or I_set) | FLA | Set to FLA; adjust up to FLA × SF if SF > 1.0 and supply is clean |
| Trip class | Not on nameplate | Select based on start time: Class 10 for light loads (<10 s start), Class 20 for moderate (<20 s start), Class 30 for heavy loads |
| Phase loss sensitivity | Not on nameplate | Enable always; single-phasing causes rapid overheating |
| Manual / auto reset | Not on nameplate | Manual reset for safety-critical loads; auto for pumps with supervision |
Service factor interaction: If the motor has SF 1.15 and the relay is set to FLA, the motor is being protected more conservatively than necessary — it cannot access its 15% overload capacity. Setting the relay to FLA × 1.15 allows full use of the service factor but increases the thermal risk if supply conditions are poor.
For a complete treatment of relay settings, motor protection covers thermal modelling, phase loss detection, and ground fault protection in depth. If you are deciding between a VFD and a soft starter for a new installation, the comparison in VFD vs soft starter addresses protection implications for both.
For ladder logic implementation of motor start/stop interlocking, the motor start-stop ladder logic tutorial shows the typical rung structure and how overload relay contacts are wired into the control circuit.
Frequently Asked Questions
How do you read a motor nameplate?
Start with the three values that define the power system interface: voltage, FLA, and frequency. These tell you what supply the motor needs and how much current it draws. Then read HP or kW and RPM to understand mechanical output. Finally, check insulation class, service factor, enclosure, and frame for application suitability. Photograph the nameplate at installation — it will be unreadable within a year on most industrial sites.
What is FLA on a motor nameplate?
FLA (Full Load Amps) is the current the motor draws from the supply when it is running at rated voltage, rated frequency, and delivering its rated shaft power. It is the primary value used to set overload relay trip current and to verify VFD output current capacity. FLA does not represent starting current (which is typically 600–700% of FLA) or the maximum current the motor can survive before damage.
What is service factor on a motor nameplate?
Service factor (SF) is a multiplier indicating how much beyond nameplate HP the motor can operate for short periods without overheating, provided supply voltage and frequency are at rated values and the ambient temperature does not exceed 40°C. An SF of 1.15 means the motor can sustain 115% of nameplate HP temporarily. IEC motors are almost always rated SF 1.0. Service factor is not a continuous overload rating — repeated or sustained operation at SF loading accelerates insulation degradation.
What nameplate data does a VFD need?
A VFD requires at minimum: rated voltage (V), rated frequency (Hz), rated speed (RPM), and rated current (FLA). Most modern drives also ask for rated power (HP/kW) and power factor (cos φ) to improve vector control accuracy. The drive uses these values to set its V/Hz ratio, output current limit, and motor model for sensorless vector algorithms. Always follow motor data entry with the drive's auto-tune routine for best accuracy.
Understanding every field on a motor nameplate is not academic — it directly affects how you wire, protect, and commission every motor-driven system you work on. The nameplate is the contract between the motor manufacturer and the installation. Honour it, and the motor will run reliably for decades.
