Motor starters and contactors are motor-starting devices. They combine a contactor with overload protection and are designed for starting AC or DC motors. Motor starter ratings include continuous current, thermal current, motor voltage, and power. Continuous current, a specification which is also known as continuous ampere rating, measures the ability of motor starters and contactors to handle current over a continuous period. Thermal current is directly proportional to the coefficient of thermal conductivity (k), the property of a material which indicates its ability to conduct heat. Power ratings for motor starters and contactors vary by motor type. For example, AC motor starters and contactors carry ratings for single-phase power or three-phase power. DC motor starters have a DC horsepower rating.