An electrical coil symbol represents a wound conductor designed to generate a magnetic field when energized. Depending on the specific component—inductor, relay, solenoid, or transformer—and the governing regional standard (IEC vs. ANSI), the schematic geometry shifts dramatically from looping semicircles to stark rectangles. Below is the definitive reference for identifying these symbols on the bench or in the field.
Master Electrical Coil Symbol Reference Table
The table below maps the most common coil types to their geometric representations under the two dominant global standards. Use this to quickly decode schematic prints before taking measurements.
| Component Type | IEC 60617 Geometry (Global/EU) | ANSI/IEEE 315 Geometry (North America) | Standard Designator | Typical DC Coil Resistance (DCR) |
|---|---|---|---|---|
| Fixed Inductor | Three or four adjacent semicircles (loops) | Three or four adjacent semicircles (loops) | L (e.g., L1, L2) | 0.1Ω – 15Ω |
| Relay / Contactor Coil | Empty rectangle (or rectangle with a diagonal line) | Circle, or semicircle paired with a specific letter code | K or CR (e.g., K1, CR3) | 50Ω – 2,500Ω |
| Solenoid Valve Coil | Rectangle with a diagonal line and an outward-pointing arrow | Circle with 'SOL' inside, or a rectangle with an arrow | Y or SOL (e.g., Y1, SOL2) | 20Ω – 800Ω |
| Transformer Winding | Adjacent semicircles with parallel straight lines (core) | Adjacent semicircles with parallel straight lines (core) | T (e.g., T1-PRI, T1-SEC) | 1Ω – 150Ω (VA dependent) |
| Motor Field Winding | Circle with 'M' and internal semicircle loops | Circle with 'M' and internal semicircle loops | M (e.g., M1-F1, M1-F2) | 5Ω – 100Ω |
| AC Contactor Coil (Shaded Pole) | Rectangle with a small intersecting loop (shading ring symbol) | Circle with a small intersecting loop or '~' symbol | KM (e.g., KM1) | 10Ω – 300Ω (AC impedance varies) |
Regional Standards: IEC 60617 vs. ANSI/IEEE 315 vs. Legacy UK
Before troubleshooting a control panel, you must identify which drafting standard the OEM used. Mixing up IEC and ANSI conventions is the leading cause of miswired replacement components in industrial settings.
IEC 60617 (International / European / Modern Global OEMs)
The International Electrotechnical Commission (IEC) standardizes schematic symbols globally, heavily favored in Europe, Asia, and by modern multinational OEMs. The defining trait of IEC 60617 for coils is its reliance on rectangles. A relay coil, a solenoid, and a basic resistor all start as simple rectangles. The distinction is made via internal annotations (like a diagonal line for a relay) and the alphanumeric designator prefix (K for relays, R for resistors). This rectangular approach was adopted to simplify early CAD drafting and remains the standard for PLC I/O diagrams today.
ANSI/IEEE 315 & NEMA (North America)
In the United States and Canada, industrial control panels built to UL 508A standards frequently rely on ANSI/IEEE 315 and NEMA conventions. These standards favor circles and semicircles. A relay coil is typically drawn as a circle or a semicircle, distinctly separating it visually from the rectangular or zig-zag symbols used for resistors. If you are working on legacy North American machinery (e.g., older Allen-Bradley or Square D panels), expect to see circular coil symbols and 'CR' (Control Relay) designators rather than the IEC 'K'.
Legacy UK (BS 3939)
Before harmonizing with Europe, the UK used BS 3939. While officially withdrawn and replaced by IEC equivalents, you will still encounter BS 3939 symbols in older British industrial plants and marine installations. BS 3939 coil symbols closely resemble early ANSI circles but often include unique internal cross-hatching to denote specific AC vs. DC winding types. If a schematic looks like a hybrid of ANSI circles and unfamiliar hatching, assume BS 3939 and verify with a multimeter.
The "Rows People Get Wrong" Troubleshooting Notes
Even experienced technicians misread specific coil symbols when scanning complex schematics quickly. Here are the most common points of failure and how to resolve them.
Confusion 1: The IEC Relay Coil vs. IEC Resistor
Under IEC 60617, a standard resistor is drawn as an empty rectangle. A standard relay coil is also drawn as an empty rectangle.
The Fix: Look at the designator prefix. If it says R14, it is a resistor. If it says K3, it is a relay coil. Furthermore, trace the lines exiting the component. A resistor will sit inline with a load or act as a pull-down. A relay coil will almost always have a parallel flyback diode (in DC circuits) or an RC snubber network (in AC circuits) wired across its terminals to suppress inductive kickback.
Confusion 2: AC vs. DC Coil Designations
Schematics rarely draw entirely different symbols for AC and DC coils; instead, they use small modifiers.
The Fix: Look for a tilde (~) inside or next to the coil symbol to denote AC. For DC coils, look for a solid straight line (—) or a polarity marker (+ and -) on the coil terminals. If the symbol includes a small intersecting loop (a shading ring), it is definitively an AC contactor coil designed to prevent the 50/60Hz zero-crossing chatter.
Safe Interpretation When Markings Are Faded or Missing
In harsh environments (mining, marine, heavy manufacturing), panel prints degrade, and physical coil labels burn off. When you cannot rely on the symbol or the sticker, use this bench-proven recovery protocol to identify the component safely:
- The Flyback Diode Test: Inspect the physical wiring or trace the PCB. If you find a diode (e.g., a 1N4007) wired in reverse-parallel across the component's terminals, you are looking at a DC coil (relay or solenoid). AC coils do not use standard flyback diodes; they use MOVs or RC snubbers.
- DCR Measurement Thresholds: Disconnect the component and measure its DC Resistance (DCR) with a multimeter.
- 0.1Ω to 15Ω: Likely a power inductor, transformer primary, or a heavy-duty AC contactor coil (which will show low DC resistance but high AC impedance).
- 50Ω to 2,500Ω: Almost certainly a DC relay or small solenoid coil. For context, a standard Omron MY2N 24VDC relay has a coil resistance of roughly 650Ω, while a 12VDC automotive relay sits around 75Ω.
- Open Loop (OL): The coil is burned out (internal thermal fuse tripped or wire snapped). Replace the component.
- Trace the Mechanical Linkage: If you suspect a faded symbol represents a relay coil, look for dashed lines on the schematic pointing away from the rectangle. In both IEC and ANSI, dashed lines indicate mechanical linkage to remote contacts (NO/NC). If the rectangle has dashed lines pointing to switch contacts on other pages of the schematic, it is a relay or contactor coil, regardless of how faded the primary symbol appears.
For a deeper dive into standard schematic drafting rules and component designators, refer to the IEC official symbols database or the All About Circuits reference chapter on electrical symbols. Always cross-reference your physical panel layout with the OEM schematic before energizing any replaced coil components.
