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Main Contacts vs Auxiliary Contacts: What Is the Difference in a Contactor?
The primary difference between main contacts and auxiliary contacts is their purpose within a contactor. Main contacts are responsible for switching electrical power to loads such as motors, HVAC equipment, and lighting systems, while auxiliary contacts are used for control logic, signaling, monitoring, and safety functions. Although both operate simultaneously when the contactor is energized, they serve very different roles inside an electrical system.
Understanding this distinction is important for electricians, HVAC technicians, panel builders, and maintenance professionals. Selecting the correct contact configuration helps improve reliability, simplify troubleshooting, and support safer system operation.
In this guide, we'll explain how main contacts and auxiliary contacts work, where they are used, and why both are essential components in modern electrical contactors.
What are main contacts in a contactor?
Main contacts are the power-carrying contacts inside a contactor.
Their job is to connect or disconnect the primary electrical circuit that supplies power to the load.
When the contactor coil is energized, the main contacts close and allow current to flow to equipment such as:
- Electric motors
- Compressors
- Pumps
- HVAC systems
- Industrial machinery
- Large lighting circuits
Because they handle significant current levels, main contacts are built using durable contact materials capable of withstanding:
- High current loads
- Motor starting currents
- Electrical arcing
- Frequent switching cycles
Depending on the contactor design, these terminals are commonly identified as:
- L1 → T1
- L2 → T2
- L3 → T3
for three-phase applications.
In simple terms, if power is flowing to the equipment, it is passing through the main contacts.
What are auxiliary contacts in a contactor?
Auxiliary contacts are secondary contacts that move together with the main contacts but serve an entirely different purpose.
Instead of carrying load current, auxiliary contacts are designed for low-current control circuits.
They are commonly used for:
- Status indication
- Control logic
- Feedback signals
- Electrical interlocking
- Alarm circuits
- Automation systems
For example, when a motor contactor energizes, an auxiliary contact may send a signal to a PLC indicating that the motor has started successfully.
If you would like a deeper understanding of how auxiliary contacts improve automation, control functionality, and electrical safety, we recommend reading our guide: Auxiliary Contacts Explained: Improving Contactor Control and Safety.
That article explores how auxiliary contacts enhance system performance and why they are widely used in modern control panels.
What is the main difference between main contacts and auxiliary contacts?
Although they operate together mechanically, the differences between them are significant.
| Feature | Main Contacts | Auxiliary Contacts |
|---|---|---|
| Primary Purpose | Switch electrical power to the load | Provide control and signaling functions |
| Current Capacity | High current | Low current |
| Typical Use | Motors, HVAC systems, compressors, pumps, and lighting loads | PLCs, relays, indicator lights, alarms, and interlocking circuits |
| Contact Size | Larger | Smaller |
| Arc Handling | Heavy-duty, designed for power switching | Limited, designed for control circuits |
| Electrical Stress | High | Low |
The easiest way to remember the difference is:
Main contacts control power. Auxiliary contacts control information.
Both are important, but they serve completely different functions within the electrical system.
Why do electrical contactors need auxiliary contacts?
Modern electrical systems require more than simple on/off switching.
Control systems often need confirmation that equipment has started, stopped, or changed state.
Auxiliary contacts provide this capability.
Feedback and Monitoring
Control systems can verify whether a contactor is energized.
Interlocking
Auxiliary contacts help prevent unsafe operating conditions, such as two reversing contactors energizing at the same time.
Automation
PLCs and building automation systems frequently rely on auxiliary contacts to make decisions based on equipment status.
Safety Functions
Emergency stop circuits and fault-monitoring systems often use auxiliary contacts to confirm that equipment has shut down correctly.
Without auxiliary contacts, many automated control systems would become significantly more complex.
Can auxiliary contacts carry the same current as main contacts?
No.
This is one of the most common misconceptions among new technicians.
Main contacts are specifically designed to handle load currents that may range from:
- 20 amps
- 40 amps
- 60 amps
- 100 amps
- Or even higher
Auxiliary contacts typically carry only small control currents.
Attempting to use an auxiliary contact to switch a motor, compressor, or large lighting load can quickly damage the contact surfaces and lead to failure.
Always verify the electrical ratings before using any contact in a circuit.
Where are auxiliary contacts used in real-world applications?
Auxiliary contacts are found in a wide variety of electrical applications.
Motor Starters
Used for holding circuits and interlocking functions.
HVAC Systems
Provide status signals and equipment monitoring.
Industrial Automation
Communicate contactor status to PLCs and control systems.
Lighting Control Panels
Coordinate switching sequences and automation schedules.
Alarm and Monitoring Systems
Trigger notifications when equipment changes operating state.
Although auxiliary contacts rarely receive attention, they often play a critical role in system operation.
How do NO and NC auxiliary contacts work?
Auxiliary contacts are typically available as:
Normally Open (NO)
A normally open contact remains open when the contactor is de-energized.
When the coil energizes:
- Contact closes
- Current flows
- Signal is transmitted
Common designation:
- 13-14
Normally Closed (NC)
A normally closed contact remains closed when the contactor is de-energized.
When the coil energizes:
- Contact opens
- Current stops
- Signal is interrupted
Common designation:
- 21-22
Many contactors include combinations of both NO and NC auxiliary contacts depending on application requirements.
How do you choose the right contact configuration?
Selecting the proper contact arrangement depends on the needs of the control system.
Questions to consider include:
- Is feedback required?
- Will the contact be used for interlocking?
- Do you need a normally open or normally closed signal?
- How many control circuits must be monitored?
- Is an auxiliary contact block required?
Understanding the control objectives helps determine the best contact configuration for the application.
Although they operate together inside a contactor, main contacts and auxiliary contacts perform fundamentally different jobs.
Main contacts handle the power required by motors, HVAC equipment, compressors, pumps, and lighting systems. Auxiliary contacts support control logic, monitoring, automation, feedback, and safety functions.
Understanding the difference between the two helps ensure proper system design, safer operation, and more effective troubleshooting.
At Contactor Depot, we offer a wide selection of contactors, auxiliary contact blocks, and industrial control components designed for HVAC, motor control, lighting, and automation applications.