The motor nameplate is a compressed data sheet stamped into a small piece of stainless steel. Every piece of information needed to install, protect, and maintain the motor is encoded there — if you know how to read it. A nameplate that reads "10 HP, 460V, 3PH, 60Hz, 1755 RPM, 12.8 FLA, SF 1.15, NEMA B, INS F, FRAME 215T, ENCL TEFC" tells you the motor's power, voltage, phase, frequency, speed, current draw, overload capacity, torque design, thermal class, physical dimensions, and cooling method. That is a lot of engineering packed into one line.
Misreading or ignoring the nameplate leads to undersized wire, incorrectly set overload relays, mismatched VFD programming, and premature motor failure. In the best case, the motor trips nuisance overloads. In the worst case, it burns out because the protection was set for a different motor.
This guide decodes every field on a standard motor nameplate, explains the NEMA and IEC standards behind the ratings, and covers NEC requirements for wire sizing, overcurrent protection, and overload relay settings.
Horsepower, Voltage, Phase, and Frequency
Horsepower (HP) is the rated mechanical output at the shaft, not the electrical input. A 10 HP motor produces 10 HP (7.46 kW) of mechanical power at full load. The electrical input will be higher than this due to motor losses (typically 8 to 15 percent). Some motors show power in kW instead of HP, particularly IEC-rated motors. 1 HP = 0.746 kW.
Voltage is the supply voltage for which the motor is designed. Common US industrial voltages are 208V, 230V, 460V, and 575V for three-phase, and 115V and 230V for single-phase. Dual-voltage motors show both ratings: "230/460V" means the motor can be connected in low-voltage (delta or parallel) or high-voltage (wye or series) configuration. The slash notation is important — "230/460V" means both voltages are acceptable, while "460V" means only 460V is acceptable.
Phase (PH) indicates single-phase (1PH) or three-phase (3PH). Three-phase motors produce smoother torque, are more efficient, and cost less per horsepower than single-phase motors. Single-phase motors above 5 HP are uncommon in industrial applications.
Frequency (Hz) is the supply frequency — 60 Hz in North America and most of the western hemisphere, 50 Hz in Europe, Asia, and much of the rest of the world. Running a 60 Hz motor on 50 Hz without derating reduces the speed by 17 percent and requires a voltage reduction to maintain the volts-per-hertz ratio and prevent core saturation.
Full-Load Amps and Service Factor
Full-Load Amps (FLA), also labeled as Full-Load Current (FLC), is the current the motor draws when delivering rated HP at rated voltage. This is the number used for sizing wire, setting overload relays, and programming VFD parameters. For a 10 HP, 460V, 3-phase motor, the typical FLA is 12 to 14 amps. At 230V, the same motor draws twice the current — 24 to 28 amps.
Note that NEC Table 430.250 provides standard full-load current values for motor circuit design. These table values are used for sizing conductors and short-circuit protection, regardless of the nameplate FLA. The nameplate FLA is used only for overload protection sizing. If the nameplate FLA differs from the NEC table value, use the table value for wire sizing and the nameplate value for overload relay settings.
Service Factor (SF) indicates how much continuous overload the motor can handle above its rated HP without exceeding its thermal limits. An SF of 1.15 means the motor can operate continuously at 115 percent of rated load (11.5 HP for a 10 HP motor) under standard conditions. At the service factor load, the motor runs hotter and its insulation life is reduced, but it will not fail immediately.
Do not treat the service factor as free capacity. It is a safety margin intended to accommodate voltage variations, ambient temperature above 40°C, and altitude above 3,300 feet. Motors running continuously at service factor load should be flagged for upsizing at the next replacement opportunity. A motor that needs its service factor to carry the normal load is undersized for the application.
Speed, Frame Size, and Enclosure Type
The nameplate RPM is the full-load speed — the actual shaft speed when the motor delivers rated HP. As covered in the motor slip guide, this is always less than synchronous speed. The nameplate speed tells you the pole count: 1750 RPM = 4-pole, 1160 RPM = 6-pole, 3500 RPM = 2-pole. The exact number also indicates the slip: 1755 RPM has less slip (and likely higher efficiency) than 1740 RPM on the same 4-pole, 60 Hz motor.
Frame size defines the physical dimensions of the motor. NEMA frame sizes (e.g., 143T, 215T, 256T, 364T) specify the mounting bolt pattern, shaft height, shaft diameter, and shaft extension. The 'T' suffix indicates a T-frame motor, which is the current standard. Older U-frame motors have different dimensions for the same frame number. The frame number encodes the shaft height: divide the first two digits by 4 to get the shaft centerline height in inches (215T = 21/4 = 5.25 inches from the base to the shaft center).
Enclosure type describes how the motor is protected from the environment. The most common types are:
- ODP (Open Drip Proof) — allows air to circulate through the motor for cooling but prevents dripping liquids from entering at angles up to 15 degrees from vertical. Suitable for clean, dry, indoor environments.
- TEFC (Totally Enclosed Fan Cooled) — sealed housing with an external fan on the shaft end. No air exchange between inside and outside. Suitable for dusty, wet, or outdoor environments. The most common industrial enclosure.
- TENV (Totally Enclosed Non-Ventilated) — sealed housing with no fan. Relies on natural convection and radiation for cooling. Used on small motors or where the external fan would be damaged.
- TEBC (Totally Enclosed Blower Cooled) — sealed housing with a separately powered blower. Used on VFD applications where the motor runs at low speed and the shaft-mounted fan cannot provide adequate cooling.
Insulation Class and Efficiency Rating
Insulation class defines the maximum temperature the motor winding insulation can withstand continuously. The classes, from lowest to highest temperature rating, are: A (105°C), B (130°C), F (155°C), and H (180°C). Most modern industrial motors use Class F insulation with a Class B temperature rise, meaning the motor is designed to operate at Class B temperatures (80°C rise above 40°C ambient = 120°C hotspot) but has Class F insulation that provides a 35°C margin before the insulation temperature limit is reached.
This margin is important because every 10°C increase in winding temperature above the insulation class rating halves the insulation life. A motor rated for Class F insulation running 20°C above its limit (175°C winding temperature) will have its insulation life reduced to approximately one-quarter of normal. Conversely, running 10°C below the rated temperature doubles the expected life.
Efficiency is expressed as a percentage at full load. NEMA Premium efficiency motors (meeting NEMA MG 1 Table 12-12) are now the minimum legal standard for most new motors sold in the US. A 10 HP, 4-pole, NEMA Premium motor has a minimum nominal efficiency of 91.7 percent. Standard efficiency (meeting the older EPAct levels) would be about 89.5 percent. The 2.2 percent difference does not sound large, but over 8,760 operating hours per year it saves roughly 1,400 kWh annually — about $140 per year at $0.10/kWh for a 10 HP motor. Over a 20-year motor life, the premium efficiency motor saves $2,800 in electricity, which far exceeds the incremental purchase cost.
The nameplate may also show the NEMA nominal efficiency and the NEMA minimum (guaranteed) efficiency. The minimum is the value guaranteed by the manufacturer and is typically 1 to 2 points below the nominal. When calculating energy costs or comparing motors, use the nominal efficiency for planning and the minimum efficiency for worst-case analysis.
NEC Wire Sizing and Motor Protection
The National Electrical Code (NEC) Article 430 governs motor circuit design. It requires three levels of protection: branch-circuit short-circuit and ground-fault protection (fuses or circuit breakers), motor overload protection (overload relays or integral protection), and the branch circuit conductors sized to carry the motor current continuously.
Conductor sizing per NEC 430.22: branch circuit conductors must be rated at least 125 percent of the motor's full-load current from NEC Table 430.250 (not the nameplate FLA). For a 10 HP, 460V, 3-phase motor, Table 430.250 gives 14 amps. The minimum conductor ampacity is 14 x 1.25 = 17.5 amps. From NEC Table 310.16, 12 AWG THHN copper (rated 30 amps at 90°C, derated to 25 amps at 75°C column) is adequate for ampacity, but most installations use 10 AWG or larger for voltage drop and practical reasons.
Short-circuit protection per NEC 430.52: the maximum fuse or breaker size depends on the motor type and the protective device type. For a Design B motor with dual-element (time-delay) fuses, the maximum is 175 percent of FLA. For inverse-time circuit breakers, the maximum is 250 percent of FLA. These deliberately oversized protective devices allow the motor to start (drawing 5-7x FLA for a few seconds) without tripping. They protect against short circuits and ground faults, not overloads.
Overload protection per NEC 430.32: the overload relay trip current must not exceed 115 percent of the nameplate FLA for motors with a 1.15 or higher service factor, or 125 percent for motors with a 1.0 service factor. For a motor with 12.8 amps nameplate FLA and 1.15 SF, the maximum overload relay setting is 12.8 x 1.15 = 14.7 amps. The overload relay protects the motor from sustained overcurrent that would overheat the windings — it is the motor's last line of defense against thermal damage.
Special Markings and Code Letters
The NEMA code letter (not to be confused with the design letter) indicates the locked-rotor kVA per horsepower. It appears on the nameplate as "CODE" followed by a letter from A through V. Code A means 0-3.15 kVA/HP (very low inrush), while Code L means 9.0-10.0 kVA/HP (high inrush). Most general-purpose motors are Code F through H (4.5-6.3 kVA/HP). The code letter is used to calculate the locked-rotor current for sizing the branch circuit protection and determining if a reduced-voltage starter is needed.
To calculate locked-rotor amps from the code letter: LRA = (Code kVA/HP x HP x 1000) / (V x sqrt(3)) for three-phase. For a 10 HP, 460V motor with Code G (5.6-6.3 kVA/HP), using the midpoint of 5.95: LRA = (5.95 x 10 x 1000) / (460 x 1.732) = 59,500 / 797 = 74.7 amps. This is about 5.8 times the full-load current of 12.8 amps, which is typical for a NEMA Design B motor.
Other nameplate markings include: duty cycle (CONT for continuous, or S1-S10 for IEC duty ratings), ambient temperature rating (typically 40°C unless otherwise specified), altitude rating (standard is 3,300 feet / 1,000 meters above sea level), bearing type and lubrication requirements, and the manufacturer's model and serial numbers for ordering replacement parts or obtaining certified dimension drawings.
For motors rated for use with VFDs, the nameplate may include an inverter duty rating per NEMA MG 1 Part 31. This indicates that the winding insulation is rated for the voltage spikes produced by PWM inverters (typically 1,600V peak for 460V class) and that the motor has been designed to handle the additional heating from harmonic currents in the VFD output waveform.