How to Read an Electric Motor Nameplate

A motor nameplate packs a lot into a few lines of abbreviations. Read correctly, it tells you how much power the motor delivers, what it draws, how hard it can be pushed, how it behaves when it starts, and which replacement will fit. Here is a typical plate, then each field in turn.

An Example Nameplate

The values below describe an illustrative general-purpose three-phase motor; your motor's figures will differ, but the fields are the same on most North American (NEMA) nameplates.

Power, Voltage and Current

HP is the rated output at the shaft, not the electrical power drawn. Because no motor is perfectly efficient, the input is always higher. Many plates, especially IEC-standard motors, show kW instead; 1 hp = 0.746 kW.

Volts is the supply the motor is designed for. A plate reading 230/460 V is a dual-voltage motor: the leads are connected one way for the lower voltage and another for the higher one, following the diagram on the plate or inside the terminal box. Amps are listed in the same order — here 12.6 A at 230 V and 6.3 A at 460 V. Doubling the voltage halves the current for the same power.

The nameplate current (FLA) is what this motor draws at full load. For sizing conductors and short-circuit protection, the National Electrical Code requires the standard table current instead, which is usually a little higher; overload protection is set from the nameplate figure. Our motor FLA chart lists the table values.

Speed and Frequency

An induction motor's speed is set by the supply frequency and the number of magnetic poles in its winding:

The nameplate speed is a little lower than synchronous speed because an induction motor needs some slip to produce torque. A plate reading 1,750 rpm is a 4-pole motor slipping about 2.8% at full load. That small difference matters: a motor that runs well below its nameplate speed is overloaded, and running a 50 Hz motor on 60 Hz raises its speed by 20% (the reverse lowers it by about 17%), which also changes its torque, current and cooling.

Service Factor, Code Letter and Design Letter

Service factor (SF) is how far beyond its rated horsepower the motor can be loaded continuously at rated voltage and frequency. A 5 hp motor with SF 1.15 can deliver up to 5.75 hp, at the cost of higher temperature and shorter insulation life. Treat it as a safety margin for occasional overloads, not as extra capacity to plan around.

Code letter is the locked-rotor indicating code from NEC Table 430.7(B). It gives the range of kVA per horsepower the motor draws at the instant of starting — code J, for example, means 7.1–7.99 kVA per hp. It is the number you need when sizing a generator or checking whether a supply can handle the start; see what size generator you need for a motor.

Design letter (NEMA designs A, B, C and D) describes the shape of the torque and current curves. Design B is the general-purpose standard with normal starting torque and current. Design C has higher starting torque for hard-to-start loads such as loaded conveyors, and Design D has very high starting torque with high slip, used for loads like punch presses. When replacing a motor, match the design letter as well as the horsepower.

Insulation, Ambient, Duty, Frame and Enclosure

• Insulation class sets how hot the windings may run. A Class F motor that only reaches Class B temperatures in service has a margin that extends its life.
• Ambient (usually 40 °C) is the hottest surrounding air the rating assumes. In hotter locations the motor may need to be derated.
• Duty: CONT means the motor can run at full load indefinitely. Intermittent-duty motors show a time limit.
• Frame is a standard size code for mounting holes, shaft height and shaft diameter, so a replacement with the same frame bolts straight in. On NEMA T-frames, the first two digits divided by four give the shaft height in inches — 184T has a 4.5-inch shaft height.
• Enclosure: ODP (open drip-proof) motors are cooled by outside air flowing through them and suit clean, dry indoor spaces; TEFC (totally enclosed, fan-cooled) motors are sealed and cooled by an external fan, suiting dusty or damp locations.

What You Can Calculate From the Plate

The nameplate figures are consistent with one another, which lets you check a motor or fill in missing information. For the example motor:

If a measured current is well above the nameplate FLA, the motor is overloaded or the supply voltage is low; if the calculated power factor comes out implausibly high or low, one of the readings is probably wrong. The electric motor HP calculator runs these numbers for single-phase, three-phase and DC motors, and HP to amps works in the other direction.