In the realms of power distribution and electric control, contactors are the central switching components that facilitate the reliable on-off control of diverse electrical loads. A closer look at a contactor’s nameplate often reveals cryptic codes like AC-3, AC-4, or DC-13, which play a critical role in correct selection and application. These codes define the utilization category, a standard-based classification system that outlines the electrical characteristics and switching behavior the contactor is designed to handle.

These utilization categories, standardized by IEC 60947-4-1 (formerly IEC947) and GB/T 14048.4, are not arbitrary—they provide essential information about the type of load, its electrical nature, and how the contactor is expected to operate under load switching conditions.

What Do the AC and DC Categories Mean?

• AC: For alternating current applications

• DC: For direct current applications

Each category is followed by a numeral or letter indicating the load type and switching condition. Different categories correspond to specific load behaviors such as resistive, inductive, or motor-driven loads, each with their own startup currents, inrush conditions, and arc-quenching demands.

Common AC Utilization Categories

AC-1 – Resistive and Slightly Inductive Loads

Used for non-inductive or slightly inductive loads such as resistive heaters, electric ovens, and heating coils. These loads do not generate significant inrush currents and are comparatively gentle on the contactor’s contacts.

AC-2 – Wound-Rotor Motor Starting

This category involves starting wound-rotor induction motors and interrupting the current during start-up. Since this switching interrupts high current with limited inductive load, the contactor must handle moderate arcing.

AC-3 – Squirrel Cage Motor Starting and Running

By far the most prevalent category in motor control. AC-3 contactors are designed for starting squirrel cage motors and disconnecting them while running. The breaking current is equal to the motor’s full-load current. Inrush current during start-up can reach 5 to 7 times the rated current.

AC-4 – Plugging, Jogging, and Reverse Braking

A harsher environment where contactors are required to switch motors during high inrush and high back-EMF conditions, such as reversing direction or dynamic braking. Contact erosion and thermal stress are considerable in AC-4 applications.

Specialized AC Categories:

• AC-5a: Switching discharge (gas) lamps

• AC-5b: Switching incandescent lamps with high inrush

• AC-6a: Switching transformer primaries, such as welding transformers

• AC-6b: Switching capacitor banks, which create high-frequency inrushes

• AC-7a: Low-inductive household appliance loads

• AC-7b: Household motor loads, such as compressors and fans

• AC-8a: Sealed compressor motor control with manual reset overloads

• AC-8b: Same as AC-8a, but with automatic reset

Common DC Utilization Categories

• DC-1: Resistive or slightly inductive loads (e.g., DC heaters)

• DC-2: Series motor starting and braking

• DC-5: Shunt motor starting, reversing, and braking

• DC-6: Incandescent lighting

• DC-12: Switching resistive loads and LEDs isolated by solid-state devices

• DC-13: Switching solenoids and electromagnetic coils

• DC-14: Economized electromagnetic loads with resistive damping

• DC-20 to DC-23: Cover increasing levels of inductive burden and arc stress

Why Utilization Categories Matter

Different electrical loads produce vastly different electrical stresses. Resistive heaters exhibit predictable, steady-state behavior. Motors, in contrast, generate surges during startup and regenerative voltage during stop cycles. Capacitors and transformers introduce transient phenomena such as oscillations and magnetizing inrush.

Hence, selecting a contactor solely based on rated current or voltage, without considering the utilization category, could lead to premature failure, arc welding, or erratic operation.

Typical Load Scenarios and Their Categories

1. Heating Element Loads – AC-1

Wire-wound resistive elements, such as in heating panels or furnaces, generate minimal inductance. However, they may draw up to 1.4 times the rated current at the moment of energization. When voltage fluctuations of ±10% are considered, contactors must have a margin above the calculated load.

2. Lighting Loads – AC-5a, AC-5b

Incandescent lamps draw substantial inrush current—up to 15 times their rated current. Fluorescent and discharge lighting introduce different characteristics including power factor distortion and variable inrush, categorized under AC-5a and AC-5b.

3. Low-Voltage Transformer Loads – AC-6a

Transformers, especially those used in welding applications, present a formidable burden. Initial energization can cause inrush currents 15–20 times the rated current. For applications like welding machines that repeatedly cause high current surges, proper understanding of the transformer’s magnetization characteristics and secondary short-circuit conditions is essential.

4. Capacitive Loads – AC-6b

Capacitor banks cause an oscillatory inrush at energization, with currents reaching thousands of amperes and frequencies in the kilohertz range. To mitigate this, capacitor switching contactors are often equipped with precharge resistors or inductive limiting components. This prevents arc welding of the contact tips and limits power quality disturbances.

5. Motor Loads – AC-2, AC-3, AC-4

Wound-Rotor Motors – AC-2

When starting these motors, external resistors are placed in the rotor circuit to limit inrush. Contactors for these applications must switch under moderate inductive stress and often disconnect during acceleration.

Squirrel Cage Motors – AC-3

These motors typically start directly across the line. The contactor must carry the full starting current (4–8 times full load) and break the full load current during normal operation. Most industrial motor loads fall under this category.

Frequent Start/Stop and Reversing – AC-4

For demanding applications involving jogging, plugging, or reversing, the contactor must endure repetitive cycles of energization and de-energization under motor starting current conditions. Applications include cranes, hoists, and reversing conveyor belts. The mechanical and electrical life of a contactor in this category is significantly reduced compared to AC-3.

Summary of Category Selection Guidelines

Conclusion

The symbols AC-1, AC-3, DC-13, and their counterparts are more than just alphanumeric codes—they are essential indicators of the load profile a contactor is designed to manage. Selecting a contactor based on its correct utilization category ensures operational longevity, thermal reliability, and safe switching performance. In industrial, commercial, or residential settings, aligning contactor specifications with actual load behavior is a non-negotiable prerequisite for electrical system integrity.