A standard push button switch wiring diagram routes control voltage into the common terminal, switching through either the Normally Open (NO) or Normally Closed (NC) contact block to energize a load. For a standard 24V DC motor start circuit, the default pick is a 22mm momentary NO switch, such as the Schneider Electric XB4BA31. You wire the 24V DC+ source to terminal 13, and the switched output from terminal 14 to your contactor coil.

Understanding the physical hardware behind the schematic lines is where most DIYers and junior techs get stuck. This guide walks through the exact symbols, maps them to physical screw terminals, traces a complete 24V control circuit node-by-node, and gives you the exact multimeter tests to prove your wiring is correct before you throw the breaker.

Decoding the Push Button Switch Wiring Diagram Symbols

Before touching a screwdriver, you must translate the schematic symbols into physical hardware. Industrial push button diagrams almost universally follow IEC 60947-5-1 symbol standards, which differ slightly from older NEMA ladder logic styles.

  • Normally Open (NO) Symbol: Depicted as two parallel horizontal lines with a gap between them, and a perpendicular line above the gap representing the movable contact. In its resting state, the circuit is open (infinite resistance).
  • Normally Closed (NC) Symbol: Depicted as two parallel lines touching or overlapping, with a diagonal slash through them. In its resting state, the circuit is closed (near-zero resistance).
  • Mechanical Linkage: A dashed line connecting multiple contact blocks indicates they are actuated by the same physical button press. If you see a dashed line linking an NO and an NC block, pressing the button opens the NC and closes the NO simultaneously.
  • Momentary vs. Maintained: A standard push button is momentary (springs back). If the diagram shows a small detent or latch symbol next to the actuator line, it represents a maintained (latching) switch that stays in position until pulled or pushed again.
Bench Tip: Never assume a green button is NO and a red button is NC based solely on the plastic cap color. The electrical function is determined entirely by the contact block screwed onto the back of the mounting bracket. Always read the terminal markings.

Terminal Mapping and Physical Device Identification

When you flip over a 22mm industrial push button (like the Schneider Harmony XB4 or Eaton HT800 series), you will see numbered terminals stamped into the metal or plastic contact blocks. These numbers are not random; they follow strict IEC conventions to ensure uniform wiring across global panels.

Terminal Number Contact Type Function in Circuit Physical Location
13 NO Common Input / Line Voltage Source Top or Left screw of NO block
14 NO Output Switched Load Side Bottom or Right screw of NO block
21 NC Common Input / Line Voltage Source Top or Left screw of NC block
22 NC Output Switched Load Side Bottom or Right screw of NC block

For control circuits, use 18 AWG (or 1.5mm²) stranded control wire. Always crimp a ferrule onto the stripped wire end before inserting it into these small terminal cages. A bare stranded wire will splay under the screw head, causing a high-resistance joint that will eventually melt under continuous coil current. Torque the terminal screws to 0.8 Nm (approx. 7 in-lbs) to prevent stripping the brass threads.

Node-by-Node Trace: Source to Load

Let us trace a standard 3-wire 24V DC motor start circuit. This circuit uses a momentary NO start button, a momentary NC stop button, and a contactor coil with a holding (seal-in) contact. We will trace the path from the power supply to the ground return.

  1. Node 1: Power Source (+24V DC). The trace begins at the positive terminal of a 24V DC DIN-rail power supply (e.g., Mean Well HDR-60-24).
  2. Node 2: Control Protection. The +24V wire routes through a 2A supplementary breaker or glass fuse to protect the 18 AWG control wiring.
  3. Node 3: Stop Button (NC Common - Terminal 21). The protected +24V enters the NC stop button at terminal 21. Because it is NC, voltage passes straight through to terminal 22 when the button is at rest.
  4. Node 4: Stop Button Output (Terminal 22). The wire leaves terminal 22 and routes to the start button.
  5. Node 5: Start Button (NO Common - Terminal 13). The voltage arrives at the start button. Because it is NO, the path stops here until the operator presses the button.
  6. Node 6: Start Button Output (Terminal 14) & Seal-In Path. When pressed, voltage exits terminal 14. This node splits: one wire goes to the contactor coil (A1), and a parallel wire routes through an NO auxiliary contact on the contactor itself. This auxiliary contact bypasses the start button once the contactor pulls in.
  7. Node 7: Contactor Coil (A1). The +24V energizes the coil's magnetic field.
  8. Node 8: Contactor Coil (A2) to Ground (0V). The current exits the coil at A2 and returns to the negative terminal (0V) of the power supply.
Polarity and Ground Path Note: In 24V DC control systems, the 0V return is your ground path. While it is technically the circuit return, the 0V terminal at the power supply must be bonded to the panel's chassis ground busbar. This ensures that if a +24V wire chafes against the metal enclosure, it creates a dead short that trips the breaker, rather than leaving the panel energized and floating.

Verifying Your Connections with a Multimeter

Never energize a newly wired control panel without cold-testing it first. Grab your multimeter (a Fluke 117 or equivalent True-RMS meter is ideal) and run these two verification sequences.

Test 1: De-Energized Continuity Check

Ensure the main control breaker is OFF. Set your multimeter dial to the Continuity setting (the sound wave icon). The meter should beep when you touch the probes together, confirming < 1 ohm of resistance.

  • Stop Button Test: Place the red probe on terminal 21 and the black probe on terminal 22. The meter should beep continuously. Press the stop button; the beep must stop (OL or infinite resistance).
  • Start Button Test: Place the red probe on terminal 13 and the black probe on terminal 14. The meter should read OL. Press the start button; the meter must beep (< 1 ohm). Release it; it must return to OL.

Test 2: Energized Voltage Drop Check

Turn the control breaker ON. Set your multimeter to DC Voltage (V⎓). Place the black probe firmly on the 0V ground busbar. Keep the red probe on the test points.

  • At Terminal 13 (Start Input): Read should be 24.0V DC (± 5%). If it reads 0V, your stop button is open or wired wrong.
  • At Terminal 14 (Start Output): Read should be 0V at rest. Press the start button; the reading must jump to 24.0V DC. If it reads 12V or fluctuates, you have a loose ferrule or a high-resistance connection upstream.
  • At Coil A1: Should read 24.0V DC when the start button is pressed. If you read 24V at terminal 14 but only 18V at A1, your control wire is too thin for the distance, causing excessive voltage drop.

Choosing the Right Push Button: Decision Tree and Part Picks

Do not guess which switch to buy. Use this decision matrix to select the exact contact block and actuator for your specific application. The part numbers below reference the widely available Schneider Electric Harmony XB4 (22mm metal) line, which is the industry benchmark for industrial control panels.

If your application requires... Then select this contact logic... Concrete Part Pick (Complete Assembly)
Momentary Start / Jog (Circuit activates only while held) 1 NO Contact Block XB4BA31 (Green flush button, 1 NO)
Momentary Stop / E-Stop Reset (Circuit breaks when pressed) 1 NC Contact Block XB4BA42 (Red flush button, 1 NC)
Forward/Reverse Interlock (Need to break one circuit while making another) 1 NO + 1 NC Contact Blocks XB4BA31 + add ZB4BZ102 (NC block) to side
Maintained State / Selector (Switch stays in position, like Hand/Off/Auto) 2 NO Contact Blocks (Cam actuated) XB4BD22 (2-position black selector, 2 NO)
Visual Feedback in Dark Panels (Need to see switch state from 10 feet away) 1 NO Block + 24V LED Module XB4BVB3 (Blue illuminated push button, 24V AC/DC)

Final Recommendation: For 90% of DIY CNC, home automation relay panels, and standard motor starter builds, buy the XB4BA31 for your start functions and the XB4BA42 for your stop functions. They are built with metal bezels, accept standard 18 AWG ferrules, and the contact blocks are rated for 10A at 600V—providing massive overhead for 24V DC control coils that typically draw less than 0.5A. Avoid cheap, unbranded 16mm plastic buttons from online marketplaces for mains-adjacent control panels; their internal leaf springs fail under vibration, leaving your contactor permanently welded shut.