Chapter 4: Contractor’s and Magnetic Motor Starters

Chapter Overview

Chapter 4 will cover contactors and magnetic motor starters. Contactors and motor starters are very similar except that motor starters control motors and include overload protection. It is important to keep in mind that motors consume more total power than any other type of load in most electrical systems. As such, the NEC requires overload protection for all motors (Part III of Article 430).

Contactors and Motor Starters

Contactors and motor starters are very similar, in the way that they work, and both are sized using the NEMA contact rating universally adopted in the United States. The difference being what they are designed to do. Contactors are devices that typically use an electromagnetically operated coil (solenoid) to accomplish the task of opening and closing sets of electrical contacts to control a multitude of electrical devices and equipment. Motor starters on the other hand are used exclusively to control and protect motors according to Part III of Article 430 in the NEC. Motor starters always contain an element of “overload” protection that is designed exclusively to protect motors from excessive heat.

This inherent overload protection is often referred to as thermal overload heaters or “heaters” because they protect the motor being controlled from potentially damaging heat. Motor specific overload protection is fundamentally different from “overcurrent” protection that is designed to protect the electrical supply conductors from excessive amounts of electrical current. Proper selection of overload protection starts with obtaining the FLA or full load amps that is listed on the motor nameplate along with the service factor (SF), which is the percentage of demand that that be placed on a motor beyond its rating for a short time. The ambient or average temperature that the motor will be exposed to is the final consideration when properly determining overload ratings.

Overload Protection Requirements

Full load ampacity (FLA) refers to the manufactures suggested load rating in amperes for a specific motor. FLA is only used to size overload protection, as the NEC requires the use of the FLC or “full load current” listed in tables 430.247 through 430.250 to size motor branch circuits. These tables should always be used to correctly and safely size branch circuit conductors as they take into account differences in motor efficiencies based on varying motor quality.

Service factor (SF) is the percentage of demand (in amperes) that can be placed on for a relatively short period of time above the motors suggested ampere rating (FLA). Service factor directly translates to the quality of the motor and should be accounted for any time you are installing motors at elevations higher than 3300’ due to the motors increasing inability to cool itself as altitude is increased. A motor installed on the top of Sandia Crest (10,679 ft) should have a SF rating of at least 1.35, or 135% of the FLA to account for the motor’s inability to cool itself effectively at such a high elevation.

Ambient or “average” temperature that motors will experience is the final factor in selecting motor overload protection. A relatively high average outside temperature might cause the heaters to trip prematurely causing “nuisance” tripping where the motor is not really overheated. In these cases, you would need to undersize the overload protection to keep the motor starter from tripping prematurely causing unnecessary and costly shutdowns. Conversely, where the average ambient temperature is relatively low, you would need to oversize your heaters as the colder temperatures might lead to the heaters not tripping when the motor really is overheating.