An electrical schematic is only as useful as your ability to read its legend. A chart of electrical symbols provides the standardized visual vocabulary required to draft, troubleshoot, and wire control panels and electronic circuits. Globally, this vocabulary is split into two dominant frameworks: IEC 60617 (International Electrotechnical Commission), which dominates Europe, Asia, and modern PLC programming, and IEEE 315 / ANSI Y32.2 (often used interchangeably with NEMA conventions), which remains the standard for North American industrial and commercial motor controls.
Misreading a symbol because you are looking at the wrong regional standard is a common cause of wiring faults. Below is the master reference chart, followed by the rules for reading modifiers and the physical limitations of schematic symbols.
The Master Chart of Electrical Symbols (IEC vs. ANSI/IEEE)
How to read this table: This chart maps the 12 most frequently queried components across the two primary standards. The IEC 60617 column favors minimalist, geometric representations (e.g., a simple rectangle for a relay coil). The IEEE 315/ANSI column favors functional, descriptive shapes (e.g., a circle with an internal letter for motors, or specific contact arcs for switches). When reviewing your project's documentation, identify the governing standard on the title block before tracing the circuit.
| Component | IEC 60617 Symbol Description | IEEE 315 / ANSI Symbol Description | Common Application |
|---|---|---|---|
| Resistor | Empty rectangle | Zigzag line | Current limiting, pull-up/pull-down |
| Capacitor (Non-Polarized) | Two parallel lines (one curved if polarized) | Two parallel straight lines | Filtering, decoupling, timing |
| Inductor / Coil | Series of half-circles (loops) | Series of half-circles (loops) | Chokes, relay coils, transformers |
| Diode | Triangle pointing to a line | Triangle pointing to a line | Rectification, flyback protection |
| NPN Transistor | Circle with base, collector, emitter (arrow out) | Circle with base, collector, emitter (arrow out) | Low-side switching, amplification |
| Pushbutton (NO) | Two parallel lines with a gap and a bridging line | Two parallel lines with a gap and a bridging line | Start switches, momentary triggers |
| Pushbutton (NC) | NO symbol with a diagonal cross-line | NO symbol with a diagonal cross-line | Stop switches, safety interlocks |
| Relay / Contactor Coil | Empty rectangle (often labeled K or KM) | Circle with two intersecting arcs or a loop | Electromagnetic switching logic |
| SPDT Switch | Single pole bridging two distinct contact points | Single pole bridging two distinct contact points | Direction control, circuit selection |
| Ground (Earth) | Three decreasing horizontal lines | Three decreasing horizontal lines | Safety earth, chassis reference |
| Motor (3-Phase AC) | Circle with 'M' and '3~' inside | Circle with 'M' inside | Pumps, fans, conveyors |
| Transformer | Two parallel inductor loops with a core line between | Two parallel inductor loops with a core line between | Voltage step-up/step-down, isolation |
Sources: IEC 60617 Graphical Symbols for Diagrams and NEMA / IEEE 315 Standard for Graphic Symbols.
How to Read the Chart: Standards, Modifiers, and Limitations
Which Standard Column Applies to Your Installation?
If you are wiring a residential or commercial building in North America, or working on legacy US industrial motor control centers (MCCs), the IEEE 315 / ANSI Y32.2 column is your governing standard. You will see zigzag resistors and circular motor symbols. If you are programming a modern PLC (like a Siemens S7-1500 or Allen-Bradley ControlLogix), designing automotive harnesses, or working on any equipment destined for the European or Asian markets, the IEC 60617 column applies. Modern CAD software like AutoCAD Electrical and EPLAN defaults to IEC libraries because they map cleanly to IEC 61131-3 ladder logic standards.
How Modifier Symbols Alter Base Values
Just as an ampacity table uses derating rows to modify a wire's base current capacity based on temperature and conduit fill, a symbol chart uses modifier symbols to alter the fundamental behavior of a base component. A base symbol only tells you what the device is; the modifier tells you how it operates.
Modifier Rule of Thumb: A standard Normally Open (NO) contact is just two parallel lines. If you add a dashed line intersecting the contact arm, it becomes a mechanically linked or ganged contact. If you add a small box with a diagonal line to the arm, it becomes a time-delay contact. If you add a thermal element symbol (a small square with a heating loop), it becomes a thermal overload relay. Always look for the secondary lines attached to the primary symbol.
What This Symbol Chart Cannot Tell You
A schematic symbol is a logical abstraction, not a physical datasheet. This chart cannot tell you:
- Physical Pinouts: A relay coil symbol does not tell you that the coil is on pins A1 and A2 of an 8-pin DIN socket.
- Wire Sizing and Torque: A motor symbol does not specify that you need 10 AWG THHN wire or that the terminal lugs must be torqued to 1.2 Nm.
- Enclosure Ratings: A pushbutton symbol does not indicate whether the physical switch is NEMA 4X (watertight) or IP65 rated.
- Proprietary Addressing: In PLC ladder logic, an IEC contact symbol does not reveal the underlying memory address (e.g., Local:1:I.Data.0) without explicit tag annotations.
Frequently Asked Questions About Electrical Symbol Charts
What is the difference between IEC and NEMA electrical symbol charts?
This is a common point of confusion. NEMA (National Electrical Manufacturers Association) primarily defines physical dimensional and performance standards for heavy-duty industrial equipment (e.g., a NEMA Size 4 contactor is physically massive compared to an IEC equivalent). However, for schematic symbols, North America relies on IEEE 315/ANSI Y32.2. IEC, on the other hand, harmonizes both the physical ratings and the schematic symbols under the IEC 60617 and IEC 60947 umbrellas. In modern practice, 'NEMA symbols' usually just refers to the legacy ANSI/IEEE style, while 'IEC symbols' refers to the international geometric style.
How do I read modifier symbols on an electrical schematic chart?
Modifiers are drawn as secondary lines or shapes attached to the primary component. For example, a standard SPDT (Single Pole Double Throw) switch is drawn with a single moving arm bridging two contacts. If that arm has a small 'X' drawn through it, it indicates a 'break-before-make' or 'make-before-break' transition, depending on the exact drafting standard. Dashed lines almost always indicate a mechanical interlock or a physical linkage between two separate components (like two contactors that cannot be closed simultaneously). A small 'delay' arc (a curved line with a hook) indicates a pneumatic or electronic time delay on a relay contact.
Where can I find a printable chart of electrical symbols for PLC ladder logic?
PLC ladder logic uses a specialized subset of IEC 60617, formally defined under IEC 61131-3. You can find printable, standardized ladder logic symbol charts directly from major automation vendors. Rockwell Automation (Allen-Bradley) and Siemens both publish free 'Quick Reference Guides' for their respective Studio 5000 and TIA Portal software. Search for 'IEC 61131-3 ladder logic symbol cheat sheet' or download the instruction reference manual from your PLC vendor's support portal, which will include the exact graphical representations for timers (TON/TOF), counters (CTU/CTD), and compare blocks.
