Motor control symbols are the standardized shorthand for electromechanical and solid-state switching devices. The exact symbol you draw or read on a schematic depends entirely on whether your panel was built to North American NEMA (ICS-1) standards or international IEC (60617) standards. Misreading a normally-open time-delay contact as a standard instantaneous contact will cause a machine to sequence out of order, potentially damaging the driven load. Here is the master reference to keep on your bench.
Master Motor Control Symbols Reference Table
The table below maps the most common motor control symbols across both dominant standards. Use this to cross-reference legacy prints with modern component datasheets.
| Component | NEMA Symbol (US/Canada) | IEC 60617 Symbol (Global) | Practical Function & Terminal IDs |
|---|---|---|---|
| Contactor / Relay Coil | Circle with 'M' (Motor) or 'CR' (Control Relay) | Rectangle with diagonal lines or letter 'K' | Energizes the magnetic field. Terminals: A1 (+/L1), A2 (-/L2 or N). |
| Normally Open (NO) Contact | Two parallel lines with a gap and a diagonal slash | Two parallel lines with a gap and diagonal slash | Passes current only when coil is energized. IEC Terminals: 13-14 (or 3-4). |
| Normally Closed (NC) Contact | Two overlapping parallel lines with a diagonal slash | Two overlapping parallel lines with diagonal slash | Passes current when coil is de-energized; breaks on energization. IEC Terminals: 11-12 (or 1-2). |
| Thermal Overload Relay | Box with 'OL' and a heater element (zigzag) symbol | Rectangle with a bimetallic thermal curve line inside | Protects motor from sustained overcurrent. IEC Terminals: 95-96 (NC control), L1-T1 (Power). |
| Time-Delay Contact (On-Delay) | NO/NC symbol with an upward-pointing 'umbrella' or dashpot | NO/NC symbol with a specific arrow pointing inward/delayed | Closes/opens a set time AFTER coil energizes. Used for star-delta transitions. |
| Momentary Pushbutton (Start) | Circle with a mushroom or flat head, NO contact symbol | Similar mechanical actuator symbol over an NO contact | Initiates motor run. Must be held or sealed-in via a holding contact. Terminals: 13-14. |
| Momentary Pushbutton (Stop) | Circle with a mushroom head, NC contact symbol | Mechanical actuator symbol over an NC contact | Breaks the control circuit to de-energize the contactor. Terminals: 11-12. |
| Limit Switch | Rectangle with a roller lever actuator over NO/NC contacts | Similar actuator symbol, often with specific IEC switch designators | Provides positional feedback to the control logic. Terminals: Common (C), NO, NC. |
Regional Standards: NEMA (US) vs. IEC (Global)
When troubleshooting or designing a panel, you must first identify which standard the original engineer used. The physical components and their schematic representations differ in philosophy.
NEMA (National Electrical Manufacturers Association): Governed by standards like NEMA ICS-1. NEMA components are physically larger, built with heavier copper and larger air gaps to handle high fault currents without immediate destruction. Schematics often use device-centric designations (e.g., '1M' for Motor 1, '1CR' for Control Relay 1).
IEC (International Electrotechnical Commission): Governed by IEC 60617 for symbols and IEC 60947 for components. IEC devices are modular, compact, and designed to be protected by upstream fuses or breakers rather than surviving the fault themselves. IEC schematics use strict terminal numbering (e.g., 13/14 for NO, 11/12 for NC) rather than device names.
If you are working on equipment imported from Europe or Asia, or modern US equipment built by global OEMs like Siemens or Schneider Electric, you will encounter IEC symbols. For older US industrial plants, legacy Allen-Bradley or Cutler-Hammer panels will heavily feature NEMA symbols. For a deep dive into the physical differences, the Electrical Engineering Portal's comparison of NEMA and IEC starters provides excellent visual teardowns.
The 'Rows People Get Wrong' Trap
Even experienced technicians misread specific rows in the motor control symbols table. Here are the most common field errors:
- Overload vs. Short Circuit Protection: Row 4 (Thermal Overload) is frequently confused with a standard circuit breaker symbol. An overload relay (OL) is slow-acting. It mimics the heating curve of the motor windings and will tolerate a brief startup inrush current (often 600% of Full Load Amps) for several seconds. A short-circuit device (fuse/breaker) trips instantaneously. If you replace a blown overload heater with a standard fast-acting fuse in the control circuit, the motor will trip on every startup.
- Time-Delay Directionality: In Row 5, the 'umbrella' or dashpot symbol on a NEMA diagram indicates the direction of delay. An umbrella pointing up means 'On-Delay' (delays when energizing). An umbrella pointing down means 'Off-Delay' (delays when de-energizing). Swapping these in a conveyor interlock circuit will cause the downstream belt to stop before the upstream belt clears, resulting in a material jam.
- Mechanical vs. Electrical Interlocks: A dashed line connecting two contactor coils indicates a mechanical interlock (a physical plastic block preventing both contactors from pulling in simultaneously, crucial for reversing starters). A solid line with an NC contact in series represents an electrical interlock. Best practice requires both; relying solely on the electrical interlock risks a welded contact causing a dead-phase short.
Decision Path: Identifying Faded or Unknown Schematics
Legacy prints fade, and panel builders sometimes omit symbol legends. Use this decision tree to positively identify an unknown component symbol on a schematic or an unmarked physical device in the panel.
| Observation / Symptom | If True... | Conclusion / Action |
|---|---|---|
| Symbol has 3 main power terminals (L1/L2/L3, T1/T2/T3) and an A1/A2 coil block. | Yes | It is a Contactor. Check the FLA (Full Load Amps) rating on the side label. |
| Component has a front-facing dial/slider (e.g., 4A-6A) and terminals marked 95/96. | Yes | It is a Thermal Overload Relay. Set the dial to the motor nameplate FLA x 1.0. |
| Schematic shows a coil symbol with a small 'M' inside, but no auxiliary contacts. | Yes | It is a Definite Purpose Contactor (common in HVAC), not a standard IEC industrial contactor. |
| Schematic shows a switch symbol with a wiper arm and multiple dashed positions. | Yes | It is a Selector Switch (Hand-Off-Auto). Verify the cam truth table on the switch body. |
| You are redrawing a completely faded panel in CAD software. | N/A | DEFAULT PICK: Use the IEC 60617 symbol library in EPLAN or AutoCAD Electrical. NEMA-specific symbol blocks are deprecated in modern global CAD suites, and IEC terminal numbering (13/14, 95/96) maps directly to 95% of new replacement parts available in 2026. |
Safe Verification Protocol for Unmarked Components
When symbols are missing, faded, or you suspect the schematic no longer matches the physical panel (a common hazard after years of unpermitted field modifications), you must verify the circuit state electrically. Never assume a contactor is de-energized just because the schematic says the stop button is pressed.
SAFETY FIRST: Motor control panels contain lethal mains voltage (up to 600V AC). Before opening the panel, de-energize the main disconnect. Apply Lockout/Tagout (LOTO). Verify the absence of voltage using a Category III or IV rated multimeter (like a Fluke 87V) on a known live source, then test the panel bus, then test the known live source again to confirm the meter did not blow a fuse during the test.
Once the panel is verified dead, use your multimeter's resistance (Ohms) and continuity settings to map the unmarked components:
- Verify the Coil: Place probes on A1 and A2. A healthy 120V AC coil typically reads between 15Ω and 50Ω. A 24V DC coil will read higher, usually 50Ω to 300Ω. If the meter reads 'OL' (Open Loop), the coil wire is broken internally. If it reads less than 2Ω, the coil is shorted and will blow the control circuit fuse the moment you energize it.
- Map the Auxiliaries: With the contactor manually depressed (using a non-conductive plastic tool or the built-in manual override button), check continuity across the auxiliary terminals. Terminals that show continuity only when depressed are NO (13-14). Terminals that break continuity when depressed are NC (11-12).
- Check the Overload: Measure across 95 and 96. It should read near 0Ω (a dead short). If it reads OL, the thermal overload has tripped and needs to be reset, or the internal bimetallic strip has failed open.
For a comprehensive review of how these symbols translate into functional ladder logic, refer to the All About Circuits guide on ladder diagrams, which bridges the gap between the physical symbols discussed here and the PLC programming logic they represent.
