Motor Nameplate Reading: HP, kW, Amps, PF & Service Factor
Understand standard parameters listed on motor tags. Sizing capacitors and switches.
How to Read an Electric Motor Nameplate
Every industrial and residential electric motor comes with a metal tag attached to its casing known as the nameplate. This tag is the motor's "ID card." It contains critical information needed for safe installation, maintenance, and power source sizing. Misinterpreting this data can result in damaged equipment or electrical fires.
Key Nameplate Parameters Explained
1. Horsepower (HP) or Kilowatts (kW)
This indicates the rated mechanical power output the motor can deliver at its shaft safely on a continuous basis. In the US, this is listed in HP. In Europe and globally, it is often listed in kW.
2. Voltage (V)
The voltage at which the motor is designed to operate. Common single-phase residential voltages are 115V and 230V. Motors listed as "115/230V" are dual-voltage and can be wired for either.
3. Full Load Amps (FLA) or Amperage (A)
This is arguably the most important number on the plate. The FLA tells you the current the motor will draw when operating at its full rated horsepower and voltage. You use this number to size your circuit breakers and wire gauges.
4. RPM (Revolutions Per Minute)
The rotational speed of the shaft at full load. Standard AC induction motor speeds in the US (60 Hz) are typically 1725 RPM or 3450 RPM.
5. Service Factor (SF)
The Service Factor is a multiplier that indicates how much continuous overload the motor can handle without damaging its insulation. A motor with 1.0 HP and a 1.15 SF can be run safely at 1.15 HP.
6. Phase (PH)
Indicates if the motor runs on single-phase (1PH) or three-phase (3PH) alternating current.
7. NEMA Frame Size (FR)
The frame size standardizes motor mounting dimensions, shaft height, and diameter, ensuring that motors from different manufacturers can be swapped directly.
Example Nameplate Reading
Let's decipher a sample motor nameplate:
| Field | Value | Meaning |
|---|---|---|
| HP | 1.5 | Outputs 1.5 Horsepower |
| Volts | 115 / 230 | Can run on 120V or 240V circuits |
| Amps | 14.0 / 7.0 | Draws 14 Amps at 115V, or 7 Amps at 230V |
| RPM | 3450 | High speed 2-pole motor |
| SF | 1.15 | Can output up to 1.725 HP safely |
Calculating True Watts for this motor: If wired for 115V, the motor draws 14 Amps.
Watts = Volts × Amps = 115V × 14A = 1,610 Watts.