The symbol for a motor starter is not a single glyph, but a composite representation of a contactor coil, main power contacts, and an overload relay. In North America (NEMA/NFPA 79), it is drawn as discrete electromechanical components linked by dashed lines or alphanumeric tags. Globally (IEC 60617), it utilizes the KM designation for the contactor and F or FR for the thermal overload, relying on standardized terminal numbering rather than literal physical drawings.
Complete Motor Starter Symbol Reference (NEMA vs. IEC)
Motor starters are documented differently depending on the drafting standard your facility uses. The table below maps the core components of a standard 3-phase, non-reversing motor starter across the two dominant global standards. Use this as your primary translation matrix when reading schematics from imported machinery or legacy domestic panels.
| Component | NEMA Symbol Description (US / NFPA 79) | IEC Symbol Description (Global / 60617) | Standard Alphanumeric Tag | Practical Function & Bench Notes |
|---|---|---|---|---|
| Contactor Coil | Circle or rectangle with 'M' or 'CR' inside. Often drawn with parallel lines for AC, solid for DC. | Rectangle with a diagonal line or a semicircle inside. Labeled with the functional code. | NEMA: M (Motor) or C IEC: KM (Contactor Motor) |
The electromagnetic brain. NEMA coils (e.g., Square D 8538) are physically massive and rebuildable. IEC coils (e.g., TeSys D) are sealed, modular, and plug directly into the contactor frame. |
| Main Power Contacts (3-Pole) | Three parallel normally-open (NO) switches linked by a dashed mechanical line. Often shown with arc chutes (small boxes). | Three parallel NO switches, linked by a dashed line. Usually simplified without arc chute graphics. | NEMA: L1/L2/L3 to T1/T2/T3 IEC: 1/3/5 to 2/4/6 |
Switches the high-current 3-phase load. IEC uses strict odd/even terminal numbering (Line is odd, Load is even). NEMA traditionally uses L and T designations. |
| Auxiliary Contact (NO) | Single NO switch symbol, mechanically linked to the main coil. Tagged with the parent coil's name (e.g., M-a). | NO switch symbol. Terminal numbers dictate the function, not the graphic linkage. | NEMA: a or X IEC: 13 / 14 |
Used for holding circuits (seal-in) or PLC feedback. If you see terminals 13 and 14 on an IEC block, it is guaranteed to be Normally Open. |
| Auxiliary Contact (NC) | Single NC switch symbol (with a slash), mechanically linked to the parent coil. | NC switch symbol. Terminal numbers dictate the function. | NEMA: b or Y IEC: 21 / 22 |
Used for electrical interlocks in reversing starters. IEC terminals 21/22 are always Normally Closed. |
| Thermal Overload Relay | Box with a wavy line (heater element) and a mechanical link to an NC contact in the control circuit. | Rectangle with a diagonal line and a small square (thermal trip mechanism), labeled 'F'. | NEMA: OL (Overload) IEC: F or FR |
Protects the motor from sustained overcurrent. NEMA uses discrete heater elements sized by FLA. Modern IEC uses adjustable bimetallic dials or electronic sensors (e.g., Class 10/20/30 trip curves). |
| 3-Phase Motor | Circle with a capital 'M' and 3-phase designation (e.g., 3~). | Circle with a capital 'M' and 3-phase designation. Sometimes includes a square for the terminal box. | NEMA: M IEC: M |
The load. Always verify the nameplate FLA (Full Load Amps) and set the overload relay dial to this exact value, not the breaker size. |
Regional Standards: NEMA, IEC, and Legacy UK
Knowing which standard applies to your panel prevents catastrophic miswiring. A symbol that means 'Normally Closed' in one standard might be misinterpreted if you apply the wrong regional lens.
NEMA (North America) / NFPA 79
Dominant in the US and Canada, NFPA 79 (Electrical Standard for Industrial Machinery) governs these schematics. NEMA symbols are highly literal. A NEMA Size 1 motor starter (rated up to 27A at 230V AC) is drawn exactly as it looks physically: a large coil, heavy main contacts, and separate melting-alloy or bimetallic overload heaters. NEMA schematics rely heavily on 'wire numbers' and dashed lines to show mechanical linkage between the coil and its auxiliary contacts. If you are working on a domestic US panel built before 2010, expect NEMA JIC-style drafting.
IEC (Global) / IEC 60617 & 60947
The International Electrotechnical Commission standard is dominant in Europe, Asia, and increasingly in US facilities using imported OEM equipment (like German packaging machines or Italian CNCs). IEC symbols are abstract and compact. Instead of drawing a literal heater element, IEC uses functional designations. According to Schneider Electric's application guidelines, IEC starters are sized by utilization category (e.g., AC-3 for squirrel cage motors) rather than physical NEMA frame sizes. The schematic relies on terminal numbering (13/14 for NO, 21/22 for NC) rather than graphical dashes to define contact states.
Legacy UK (BS 3939)
Though officially withdrawn in the 1990s in favor of BS EN 60617, the old British Standard 3939 still haunts legacy manufacturing plants in the UK and Commonwealth nations. BS 3939 used unique graphical quirks—such as a circle with a cross inside to denote a motor, and distinct half-circle symbols for relays. If you encounter a schematic where the motor symbol looks like a target, you are looking at BS 3939. Treat these panels with extreme caution; the control logic often uses 415V AC control circuits derived directly from phase-to-phase line voltage, unlike the 120V AC or 24V DC control circuits standard in modern NEMA/IEC panels.
Rows People Get Wrong and Faded Schematic Recovery
Even experienced bench techs misread specific rows in motor starter schematics. Here are the most common traps and how to recover when the physical panel schematic is faded, torn, or missing entirely.
The most dangerous misinterpretation is assuming a contactor symbol represents a full motor starter. A contactor only switches power. A motor starter equals a contactor plus overload protection. If the schematic shows a coil and main contacts but lacks the overload (F/OL) symbol, the branch circuit must rely on a separate Motor Protection Circuit Breaker (MPCB) or electronic overload. Never assume thermal protection exists just because the motor spins.
The Auxiliary Contact Trap (NO vs. NC)
In NEMA schematics, a Normally Open (NO) auxiliary contact is drawn with a physical gap, while a Normally Closed (NC) contact has a diagonal slash through it. Techs often miswire these when replacing a legacy Square D block with a modern Siemens SIRIUS add-on deck. In IEC, the graphic might look identical, but the terminal numbers never lie. If the add-on block is stamped with 13 and 14, it is NO. If it is stamped with 21 and 22, it is NC. Always trust the IEC terminal stamp over your visual interpretation of the schematic graphic.
Overload Heater vs. Electronic Overload
Old NEMA schematics show individual heater coils (e.g., a wavy line inside a box) sized to a specific NEMA heater chart (like the X-class or B-class tables). Modern IEC and updated NEMA schematics just show a single block with a test button symbol. If you are retrofitting a panel and the schematic calls for 'Heater Size X22', do not hunt for discrete heaters. Replace the entire block with an adjustable electronic overload (like a TeSys LRD or Eaton PKZM) and dial the FLA knob to the exact Full Load Amps printed on the motor nameplate.
Safe Interpretation When Markings are Faded or Missing
When the schematic inside the panel door is faded beyond reading, or a previous tech removed it, do not guess the control voltage or contact logic. Use the Trace and Tag method:
- Identify the Coil Voltage Safely: If the coil symbol is illegible, locate the physical contactor. Look at the A1/A2 coil terminals. If the core of the contactor has a visible copper ring (a shading coil) on the outer edges of the E-lamination, it is an AC coil. If the steel core is solid with no copper ring, it is a DC coil. Measure the resistance across A1/A2 with a multimeter (power locked out). A 120VAC coil typically reads 15-50 ohms; a 24VDC coil reads significantly lower (often under 10 ohms).
- Verify the Overload Setting: If the overload symbol is missing, check the physical FLA dial on the block. If it is set to the motor nameplate FLA, it is functioning correctly. If the dial is maxed out or bypassed with a jumper wire, someone defeated the protection. Replace the block immediately before energizing.
- Map the Auxiliaries: Use a multimeter in continuity mode (power off). Press the contactor plunger in manually with an insulated tool. Probe the auxiliary terminals to definitively map which are NO (open until pressed) and which are NC (closed until pressed). Label them with masking tape before re-energizing.
