Understanding NEC Rules: A Guide for Electrical Engineers

Motor Name Plate Terminology Motor Name Plate Terminology General Terminology 1. Service Factor The service factor is a multiplier that indicates the amount of overload a motor can handle. For example, a motor with a 1.15 service factor can safely manage intermittent loads up to 15% beyond its nameplate horsepower . Service Factor Amperage: This is the current the motor will draw under the service factor load condition. 2. Slip Slip is used in two forms: Slip RPM: The difference between the synchronous speed and the full-load speed. Percent Slip: When the slip RPM is expressed as a percentage of the synchronous speed. Most standard motors operate with a full-load slip of 2% to 5% . 3. Synchronous Speed This is the speed at which the motor's magnetic field rotates. It is also approximately the motor's speed under no-load conditions. Example: A 4-pole ...

Abstract of National Electrical Code for Size of Cable for Motors Abstract of National Electrical Code for Size of Cable for Motors National Electrical Code 430.22 (Size of Cable for Single Motor) The size of the cable for a branch circuit that has a single motor connection must be 125% of the motor's full load current capacity. Example: What is the minimum rating in amperes for cables supplying one number of 5 horsepower, 415-volt, 3-phase motor at 0.8 power factor? Full-load current for 5 horsepower = 7 amperes. Minimum capacity of cable = (7 × 125%) = 8.75 amperes. National Electrical Code 430.6(A) (Size of Cable for Group of Motors or Electrical Load) Cables or feeders supplying more than one motor or other loads must have an ampacity not less than 125% of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as det...

Induction Motor Starters Introduction to Induction Motor Starters Different starting methods are employed for starting induction motors because Induction Motors draw more starting current during starting . To prevent damage to the windings due to the high starting current flow, we employ different types of starters. The simplest form of motor starter for the induction motor is the Direct On Line (DOL) starter . The DOL starter consists of an MCCB or Circuit Breaker, Contactor, and an overload relay for protection. The electromagnetic contactor can be opened by the thermal overload relay under fault conditions. Principle of DOL Starter To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The moto...