Limit Switch Wiring Diagram: How to Wire NO, NC, and Common Terminals

A limit switch wiring diagram looks simple: three terminals, a few lines, done. Yet the same three-terminal switch is wired three completely different ways depending on the circuit it lives in: straight into a motor or contactor coil, into a PLC input card, or across a pair of switches on a reversing axis. Wire it wrong and the failure is never “it doesn’t work.” It is worse: the machine keeps running when it should have stopped.

This guide walks through the wiring forms of one switch, which contact to choose (NO or NC) and why that choice decides the failure direction of your circuit, how to land the same switch on a PLC or controller input, how to verify your work with a meter, and finally what parameters to check when you are buying the replacement. By the end you will be reading wiring diagrams backward: from circuit to part number.

What a Limit Switch Is — and What This Article Is Not About

A limit switch is an electromechanical device with an actuator (plunger, roller lever, whisker, or rotary shaft) and a set of contacts inside. When a moving machine part, such as a conveyor pallet, a carriage, or a door, pushes the actuator past a set point, the internal snap-action mechanism flips the contacts in under a millisecond, opening or closing an electrical circuit. The name comes from the original job: marking the physical limit of travel so a machine stops or reverses before it damages itself or its load.

One clarification before the diagrams, because it saves you a wasted afternoon: this article is about mechanical contact limit switches, the industrial switches with screw terminals for control wiring. The “limit switch” you find inside a furnace or a garage-door opener is a different device (a thermal switch or a door-position switch) with different wiring. If you are repairing a heater or a door motor, the diagrams below will not match what you have.

For the electromechanical switch, everything wiring-related comes down to three terminals, and one decision about which contact does the stopping.

The Three Terminals: Wiring Forms of One Switch (2-Wire / 3-Pin / 4-Terminal)

Every standard limit switch contact block carries three terminals: COM (common), NO (normally open), and NC (normally closed).

• With the actuator at rest, COM–NC is continuous and COM–NO is open.
• When the actuator is pressed, the contacts change over together: COM–NC opens and COM–NO closes.

Because the moving contact switches between two fixed contacts, the switch is a single-pole double-throw (SPDT) device. You can use NO alone, NC alone, or both. Two circuits, switched at the same instant by one actuator.

That one three-terminal block is what people mean by “2-wire,” “3-wire,” and “4-wire” forms, and the terminology is looser than it looks:

The “4-wire” trap is worth spelling out. In robotics and hydraulic contexts (an FTC robot hub, a dump-trailer controller), people say “4-wire limit switch” when they mean the controller’s cable: four leads, often with a color code such as black for ground and brown for an up-coil. That is a cable, not a terminal count. On the switch body itself, the truth is always the terminal block. Label the terminals, count the terminals, and never trust wire colors.

Wire color is genuinely unreliable here. Consumer guides teach “red is +, black is −,” and beginners get stuck asking which color goes where. An experienced robotics mentor’s answer on a FIRST forum thread was simply “wire color is irrelevant… typically COM and NO connect to ground and signal.” Industrial limit switches have no universal lead-color standard, so two identical-looking switches can ship with different internal conventions. The only reliable identifier is the terminal marking (COM/NO/NC, or numbered 1–2–3–4) plus a continuity check.

Micro switches are the same story in a smaller body: a miniature snap-action switch with the same COM/NO/NC block, commonly rated in the 10–15 A class at 250 VAC (Tameson). If you can wire a full-size limit switch you can wire a micro switch. The terminal logic is identical.

NO or NC? Choosing the Contact That Fails the Right Way

The question every beginner asks, “do I use NO or NC?”, is usually answered with one slogan: use NC for safety. That slogan is half-right and causes real damage when applied blindly, because it ignores what the contact is for in your circuit.

The function decides the contact: stop-jobs versus event-jobs

A limit switch does one of two jobs, and the job decides the contact:

• Stop-jobs: end-of-travel stop, guard interlock, overtravel protection. The circuit runs in the normal state and must drop when the limit is reached. Wire the NC contact in series. Current flows (or the input sits at logic 1) while the machine is inside its travel; opening the NC contact at the limit cuts the circuit.
• Event-jobs: arrive-at-position, home detection, counting, “reached” confirmation. Nothing should happen until the actuator is pressed. Wire the NO contact. The circuit closes (or the input flips to 1) only when the limit is actually reached.

If you force NC onto an event-job, your machine energizes everything except when it should. If you force NO onto a stop-job, you get the failure mode below.

Why stop-jobs should be NC: the broken-wire matrix

The reason is not convention. It is what happens when the wire breaks or a terminal loosens, which is the most common field fault in this circuit (discussion on PLCTalk).

A broken wire cannot fool an NC circuit. It stops. It fools an NO circuit completely: the system believes the limit simply has not been reached yet. This asymmetry is the entire argument for NC on safety-relevant limits, and it is the one row of this table worth remembering. A broken wire fails safe through NC, and fails silent through NO.

That said, NC is not “always better.” NC circuits carry current during normal operation. A contactor coil held energized around the clock is heat and wear, and event-jobs genuinely need NO. The honest rule: match the contact to the function, and when the function is a stop, choose the contact whose failure is also a stop.

Two switches on one reversing motor

The related search “2 limit switches wiring” describes the classic bidirectional-axis circuit: one limit switch at each end of travel, each stopping motion in its own direction. Two mistakes dominate here.

First, each end must stop only its own direction. A common novice design stops all motion when either switch trips. The axis then sits against the CCW limit and can never be driven clockwise away from it. A PLCTalk user building a stepper axis hit exactly this and had it corrected in-thread: the CW limit switch stops CW motion, the CCW limit switch stops CCW motion, and the logic must keep the escape direction alive (PLCTalk, S7-1200 limit switch wiring). On an r/PLC thread about a pair of opposing linear actuators, the same answer came back: “you’d have separate logic for extend vs retract… the retract button will still work” (r/PLC, Limit Switch Logic).

Second, pick the contact per direction job. NC sits in series with each direction’s drive, so a broken wire parks that direction, while the opposite direction remains operable through its own circuit. For a contactor-based reversing starter, each travel limit sits in series with the coil of its own direction, and the two contactor interlocks handle the rest.

One more caution from the field. If a limit switch stops a motor by interrupting the coil circuit of a contactor, the switch contacts only ever carry coil current. That is the correct, long-lived arrangement. If you instead route motor current through the switch itself, you move from the signal domain to the power domain, and the contact ratings in the last section of this article become the whole game.

Wiring a Limit Switch to a PLC or Controller Input

A limit switch feeding a PLC discrete input is the same three-terminal story plus one extra connection that beginners systematically forget: the input’s return path.

The switch is a dry contact. It does not source or sink current, it only opens and closes a circuit. The circuit is completed by the input card’s own supply, and the only wiring question is which polarity arrangement your card expects.

Whichever arrangement your card uses, the loop must close: power leaves the supply, crosses the switch contact, enters the input, and returns to the supply through the input card’s common. Miss the last leg and nothing happens. This is precisely the failure in the PLCTalk thread above. The switch was correctly powered and wired to the input, but the CPU’s input common was never connected to 0 V, so the input never changed state no matter how the lever was pressed. The fix was one wire to the input common.

After the physical wiring, the program must read the contact the way it is wired. Wire the NC contact (the recommended stop-job arrangement from the previous section), and the input sits at logic 1 during normal travel. A tripped limit, or a broken wire, drives it to 0, which stops the axis either way. Program the tag so that 1 = “limit not reached,” and every interlock built on that bit fails safe.

Two practical details. Contact bounce, the rapid make/break chatter as contacts first touch, typically lasts 1–5 ms, and most input cards include a filter in the 4–12 ms range. If your machine sees phantom pulses on a fast axis, check the card’s input filter time before adding software timers (PLCprogramming.io). And the same dry-contact logic applies unchanged to a microcontroller or robot controller. The only difference is the voltage domain (5 V or 3.3 V instead of 24 V), which is why the robotics threads about RoboRIO and REV hubs are solving the same puzzle as the PLC threads.

Verifying Your Wiring: Meter Checks and Symptom Lookup

Before powering anything, three continuity checks settle every wiring question, including “which terminal is which” on an unmarked switch:

1. Identify the terminals (power off). With the actuator at rest, the meter’s continuity mode should beep between COM and NC, and stay silent between COM and NO. Press the actuator: the beep moves to COM–NO. Whatever the labels say, the meter has now told you the truth.
2. Check for shorts. Confirm there is no continuity between the two fixed contacts, and no continuity from any terminal to the switch body (unless the body is a grounded-metal style with a dedicated earth terminal).
3. Confirm the circuit, powered. Measure across the load or at the input. The voltage or logic state must flip when the actuator is pressed. On a PLC input, the card’s input LED is your friend. It should light in one state and extinguish when the lever trips (the same fix that ended the PLCTalk thread).

If it still misbehaves, run the symptom lookup instead of re-reading the diagram:

And when the machine runs but the software disagrees with the hardware, a hardware limit that works while a software limit does not, the problem is in the program, not the wiring. Keep the software limit and the physical switch measuring the same point, and test them together, not separately.

From Circuit Diagram to Part Number: Specs to Check Before You Buy

Reading diagrams forward gets you a working circuit. Reading them backward gets you a correct replacement part. This is the step almost no wiring guide covers, because it is where the part number, not the screwdriver, does the work.

Read the contact symbols before you read the catalog

Every wiring diagram encodes the requirements. Count the switch symbols. Note whether each is drawn with a normally-open or normally-closed contact (the symbol tells you, so don’t guess from the text). Ask whether each is switching power or feeding an input. Two switches and a relay symbol means a two-circuit job; one switch symbol feeding a PLC input means a signal job. The part you buy must match the number of circuits and the contact state each circuit needs.

Contact ratings: the power-domain life-and-death row

If the switch directly interrupts a motor, solenoid, contactor coil, or lamp, its contact rating is the safety boundary of your whole design. Three truths keep buyers out of trouble:

• The rating is load-dependent, not a single number. A switch that happily breaks 10 A of resistive AC load may not break 2 A of DC motor current. DC has no zero-crossing to extinguish the arc, so manufacturers derate DC ratings far below AC ratings.
• Realistic numbers look like this. A typical heavy-duty roller-lever limit switch is rated 10 A at 250 VAC resistive (both NC and NO), 3 A at 480 VAC, and just 0.25 A at 250 VDC (AZ-7100 spec sheet example). A compact vertical-travel unit in the same catalog family rates 5 A at 250 VAC resistive and only 0.2 A at 250 VDC (TZ-8108 spec sheet example). If your load is a DC solenoid or a small DC motor, compare against the DC column, not the 250 VAC headline number.
• Inductive loads need de-rating or an interposing relay. Motor contactor coils and solenoids are inductive. Breaking them arcs and welds contacts that would outlive you on resistive loads (PLCprogramming.io). The standard move: let the switch command a relay or contactor coil, and let that do the power switching. This is also the mechanically right place for the switch in the first place.

Life, actuator, and the replacement path

Two life figures appear on spec sheets and they are not interchangeable. Electrical life, the number of operations at full rated load, is the number that predicts replacement intervals; expect the 500,000-operation class on industrial limit switches. Mechanical life, unloaded switching, runs far higher, often into the millions. If a datasheet quotes only a mechanical figure in the millions, treat it as marketing until an electrical life appears next to it.

The actuator decides how the switch gets touched: roller lever for cams and ramps (and the most common industrial style), plunger for face-on contact, whisker for light multi-directional sensing. Many bodies accept interchangeable heads, so a replacement often swaps the head rather than the whole switch. Check the mounting and the operating direction before ordering.

Replacement then follows two paths: the identical part number, or a parameter-equivalent substitution. For the substitution path, verify circuit count and contact state, rated voltage and current class (AC vs DC), electrical life, IP rating for the environment, and actuator style against the diagram’s requirements. When the job is a signal-level input, the current rating barely matters and the terminal configuration does.

What This Means for Distributors Stocking Replacement Limit Switches

For distributors and wholesalers who sell into repair and maintenance markets, the wiring diagrams above are not background reading; they are the first question of every support call. When a customer says “the limit switch is bad,” the useful reply is not “what model?” but “is this switch wired straight through to the motor or coil, or does it feed a controller, and does it need to stop, or to signal?” That one question routes the customer to the right failure check. The power-side customer is in danger from contact ratings (the burnt-contact row of the symptom table). The signal-side customer is in danger from contact state and terminal configuration (the never-changes row).

That is also the argument for how to stock. A heavy-duty SPDT platform, the same body family across roller-lever and plunger heads with NO and NC both brought out, covers both circuit roles from one product family. The power-through customer gets a rating that backs the claim. The signal customer gets the terminal configuration confirmed. Catalog pages that publish the ratings and life figures next to the model, the way spec sheets do, let a distributor answer the rating question without a phone call. That is exactly the service gap the current wiring-diagram content on the web leaves open: the guides teach you to wire, but almost none of them tell you what to demand of the part you are about to buy. The judgment calls still belong to the customer’s own circuit. This article gives you the questions to ask, not the answers to give on their behalf. Distributors who can ask them in the customer’s language turn a wiring question into a sale.

If you are comparing limit switch families and want spec sheets with the contact ratings, life figures, and actuator options published rather than quoted, OMCH’s limit switch catalog lists the heavy-duty and compact families with full spec tables. Browse the range, or send your circuit details and get a wiring-compatible recommendation.

References

1. PLCprogramming.io. “Limit Switch Explained | How It Works + PLC Wiring.” June 2026.
2. Tameson. “A Comprehensive Guide to Limit Switches.” January 2024.
3. Chief Delphi. “Wiring this Limit Switch” (FIRST Robotics forum thread). February 2025.
4. Arduino Forum. “How to wire limit switch safely and read switch status.” February 2026.
5. PLCTalk. “S7-1200 Limit Switch Wiring.” June 2022.
6. r/PLC. “Limit Switch Logic.” March 2024.
7. OMCH. “Limit Switch” product category.
8. OMCH. “AZ-7100” heavy-duty roller-lever limit switch spec.
9. OMCH. “TZ-8108” compact limit switch spec.