Wiring a push button switch looks straightforward until you open the terminal cover and find four, five, or six terminals with numbers you weren’t expecting. Most wiring guides show a diagram with two wires and a button. Real industrial push button switches don’t work that way.
This article covers what NO, NC, and 1NO1NC mean in practice, how IEC terminal numbering works, how to wire the three most common control panel configurations, and where things typically go wrong.
What NO and NC Mean
NO stands for Normally Open. NC stands for Normally Closed. These terms describe the state of the contact when the button is not being pressed.
A Normally Open contact has a gap between the contact points at rest. No current flows. Press the button, the gap closes, current flows. Release, the gap opens again.
A Normally Closed contact is the opposite. The points are touching at rest — current flows. Press the button, the contact opens, current stops.
This distinction determines how your circuit behaves when something goes wrong. If a wire breaks or a terminal works loose, what happens?
With a NO contact, a failure means the circuit stays open — the load stays off. With a NC contact, a failure means the circuit opens — the load also turns off, but because the circuit broke, not because someone pressed a button. For a STOP button, this is exactly what you want. The machine stops on any failure. This is the fail-safe principle built into IEC 60947-5-1.
IEC Terminal Numbering
Industrial push button switches follow IEC terminal numbering. Once you know the system, reading an unfamiliar switch is straightforward.
Contact circuits are numbered in pairs. The standard numbering:
NO contact: terminals 13 and 14
Second NO contact: terminals 23 and 24
NC contact: terminals 21 and 22
Second NC contact: terminals 31 and 32
On a 1NO1NC push button switch, you will find four terminals: 13 and 14 (NO) and 21 and 22 (NC).
For illuminated switches, the LED module has its own terminals — typically A1 and A2, or marked + and −. The LED circuit is completely separate from the switching contacts. They share the same button body but are electrically independent. The LED lights up based on whatever you connect to A1 and A2. The contacts at 13-14 and 21-22 switch based on button presses. These are two different circuits in one device.
Wiring Configuration 1: Simple NO Circuit
Terminal 13 connects to the positive side of the control supply. Terminal 14 connects to one side of the load. The other side of the load connects back to the negative or neutral.
At rest: circuit open, load off. Button pressed: circuit closes, load activates. Button released: circuit opens, load deactivates.
Use this for PLC digital inputs, relay coil activation, conveyor start commands, and any function that should only be active when the button is pressed or toggled.
Wiring Configuration 2: Simple NC Circuit
Terminal 21 connects to the positive side of the control supply. Terminal 22 connects to one side of the load. The other side connects to the return.
At rest: circuit closed, load energized. Button pressed: circuit opens, load de-energizes.
For a STOP button, the NC contact is wired in series with the contactor coil circuit. When the coil circuit is complete, the contactor holds the motor running. Pressing STOP opens the NC contact, breaks the coil circuit, drops the contactor, stops the motor. If the wire breaks — same result. The motor stops. That is the fail-safe behavior you want from a STOP button.
Wiring Configuration 3: 1NO1NC Start/Stop Circuit
This is the standard configuration in most industrial motor control panels — two push button switches, one START (NO) and one STOP (NC), plus a contactor with a self-holding auxiliary contact.
The contactor coil circuit runs from control supply positive, through the NC STOP button (21-22), through the NO START button (13-14), to the coil, and back to neutral. The contactor also has an auxiliary NO contact wired in parallel with the START button.
Press START: the NO contact closes, the coil energizes, the contactor picks up, the motor starts, and the contactor’s auxiliary contact closes in parallel with the START button. Release START — the circuit stays closed through the auxiliary contact. The motor keeps running.
Press STOP: the NC contact opens, the coil de-energizes, the contactor drops, the auxiliary contact opens, the motor stops. Release STOP — the motor does not restart because the auxiliary contact is also open.
If power is lost: the coil de-energizes, the contactor drops, the auxiliary contact opens. When power returns, the motor does not restart automatically. This is called no-volt protection, and it is a required safety behavior in most industrial motor control applications.
This three-wire control circuit is why momentary push buttons are standard in industrial motor control rather than latching switches. The contactor provides state retention, not the button.
Wiring an Illuminated Push Button Switch
The contact wiring is identical to non-illuminated versions — terminals 13-14 for NO, 21-22 for NC.
The LED terminals (A1 and A2) connect to a separate pilot circuit. What you connect here determines what the LED indicates.
The most useful configuration for START buttons: connect A1 and A2 to the contactor auxiliary contact output. The LED lights when the contactor is picked up and the motor is running — reflecting machine status, not button status.
Always verify the LED rated voltage matches your pilot circuit voltage before wiring. A 24V LED module wired to a 220V circuit will fail immediately. LED voltage and contact voltage are specified independently and must be matched independently.
Common Wiring Mistakes
Using a NO contact for a STOP button. If the wire falls off, the machine keeps running. Not fail-safe. Always use NC for STOP.
Using a NC contact for a START button. If the wire falls off, the contactor energizes on its own. The motor starts without anyone pressing anything. This is a safety hazard.
Connecting the LED terminals to the contact circuit. The LED module is not designed for contact-level voltages. A1 and A2 are for the pilot circuit only.
Forgetting the seal-in contact. A momentary NO START button wired directly to the contactor coil without a seal-in auxiliary contact in parallel means the motor only runs while you hold the button.
Connecting the wrong terminal pair. Terminals 13-14 are NO and 21-22 are NC. If you connect your STOP button to 13-14, it will do the opposite of what you intend. Always verify terminal numbering against the switch’s wiring diagram before connecting.
Selecting the Right Contact Configuration
START, trigger, jog: NO contact — the circuit should only be active when the button is pressed.
STOP, emergency stop interlock, safety function: NC contact — the circuit must fail to the safe state if wiring is broken.
PLC inputs: almost always NO. PLC input cards detect contact closure — a NO contact closing on press gives a clean edge for the PLC to detect.
Multiple outputs from one button: stack additional contact modules. ACXION push button switches use a modular contact block system — NO or NC contact blocks snap onto the rear of the switch frame, and multiple blocks can be stacked to provide 2NO, 2NC, 1NO1NC, or other combinations from a single button press. If your contact requirements change after the panel is commissioned, the contact block can be swapped without removing the switch from the panel.