The fundamental symbol for coil in electrical and electronic schematics is a series of adjacent semi-circles (often called loops or "bumps") representing wound conductive wire. However, a bare coil symbol on a drawing is inherently ambiguous. Depending on the standard, context, and surrounding annotations, that exact same shape could represent a 5mH RF choke, a 24VDC ice-cube relay, a 480VAC motor stator winding, or a solenoid valve. Misinterpreting which coil you are looking at on a schematic can lead to catastrophic component failure or severe shock hazards.
This reference guide provides the exact symbol variations across global standards, explains the physical realities of the components they represent, and details how to safely verify a coil's identity when panel markings are faded or missing.
The Complete Coil Symbol Reference Table
The table below maps the most common coil-based components to their graphical representations across the three dominant global standards: IEC (International), ANSI/IEEE (North American electronics), and NEMA (North American industrial control). Use this as your primary bench and field reference.
| Component Type | IEC 60617 Symbol Description | ANSI/IEEE 315 Symbol Description | NEMA ICS 1 Industrial Standard | Typical Bench/Jobsite Application |
|---|---|---|---|---|
| Fixed Inductor (Air Core) | 4 adjacent semi-circles (no underlying line). | 4 adjacent semi-circles (no underlying line). | Rarely used in NEMA power schematics; defaults to IEC/IEEE. | RF filtering, high-frequency switching power supplies, snubber circuits. |
| Fixed Inductor (Iron/Ferrite Core) | 4 semi-circles with a solid straight line parallel underneath. | 4 semi-circles with a solid straight line parallel underneath. | Same as IEEE; sometimes denoted with a specific core material code. | Line reactors, buck/boost converters, EMI chokes. |
| Relay / Contactor Coil | A simple rectangle (often with the designator inside, e.g., "K1"). | 4 semi-circles, sometimes enclosed in a box or marked with "CR" (Control Relay). | Strictly a rectangle. Contactors use a rectangle with specific NEMA size codes. | Motor control centers (MCCs), PLC output isolation, HVAC control boards. |
| Motor Stator Winding | A circle containing a single coil loop or the letter "M". | A circle containing 3 overlapping loops (for 3-phase) or a single loop. | A circle with "M" and specific NEMA frame/HP designators adjacent. | VFD output diagrams, across-the-line starters, servo drives. |
| Transformer Winding | Two sets of 4 semi-circles facing each other, separated by a core line. | Two sets of semi-circles with a core line; dots indicate phase polarity. | Two rectangles or overlapping circles depending on single vs. 3-phase. | Control power transformers (CPTs), isolation transformers, gate drive transformers. |
Regional Standards and Legacy Panel Variants
Knowing which standard applies to your region—and more importantly, which standard was in vogue when a specific machine was built—is critical for accurate schematic interpretation. The IEEE 315 standard and IEC 60617 are the modern baselines, but legacy equipment often defies both.
IEC 60617 (International & Modern European)
The IEC standard heavily favors geometric abstraction over pictorial representation. For control circuits, IEC almost entirely abandons the "loops" symbol for relay and contactor coils, replacing it with a simple rectangle. The coil is identified by its alphanumeric designator (e.g., K1 for a relay, KM1 for a contactor, Y1 for a solenoid valve). If you are working on modern European machinery, CNC equipment, or PLC panels built after 2005, expect to see rectangles for all electromagnetic coils. The semi-circle loops are strictly reserved for passive inductors and transformer windings.
ANSI/IEEE 315 (North American Electronics & Power)
Prevalent in US-based electronics, power distribution single-line diagrams, and older control panels, the ANSI/IEEE 315 standard retains the pictorial semi-circles for almost all wound components. A relay coil is drawn as the familiar four loops, often with a diagonal line or a specific label like "CR" (Control Relay) or "M" (Motor Starter). Phase polarity dots on transformer windings are strictly enforced in IEEE to prevent catastrophic out-of-phase paralleling.
NEMA ICS 1 (North American Industrial Control)
NEMA standards govern heavy industrial motor control in the US. NEMA aligns closely with IEC regarding the use of rectangles for relay and contactor coils, but NEMA schematics are uniquely dense with auxiliary contact interlocks, mechanical linkage dashed lines, and specific NEMA size ratings (Size 0 through Size 6) printed directly adjacent to the coil symbol.
Old UK Standard (BS 3939)
If you are troubleshooting legacy British machinery, marine vessels, or old textile equipment, you may encounter BS 3939. This deprecated standard sometimes represented relay coils as a circle with a single internal loop, or used unique shading patterns to denote magnetic cores. While officially superseded by harmonized IEC standards in the late 1990s, BS 3939 symbols still hide inside the control cabinets of equipment built prior to 1995.
The "Rows People Get Wrong" and Faded Schematic Safety
Even experienced technicians misread specific coil symbol variations. Here are the most common schematic traps, followed by a critical safety protocol for dealing with degraded documentation.
1. The "Box" Rule: Relay vs. Inductor
The Mistake: Assuming any symbol with four semi-circles is a passive inductor.
The Reality: In ANSI/IEEE schematics, if those four loops are enclosed in a rectangular box, or if they have a dashed line connecting them to a set of mechanical switch contacts, it is an electromechanical relay or contactor coil. Applying a continuous DC current to a relay coil without accounting for its duty cycle or flyback diode requirements will burn out your driving transistor.
2. The Arrow Direction: Variable vs. Tapped
The Mistake: Confusing a variable inductor with a tapped inductor.
The Reality: If an arrow crosses diagonally through the semi-circles, it is a variable inductor (the core can be physically adjusted to change inductance). If an arrow points directly at the junction between two loops without crossing them, it is a tapped inductor (a fixed component with a wire brought out from the middle of the winding, common in autotransformers and multi-tap line reactors).
3. The Core Material Line: Magnetic vs. Non-Magnetic
The Mistake: Ignoring the line parallel to the coil loops.
The Reality: A solid line indicates an iron or ferrite core (high inductance, prone to saturation). A dashed line indicates a powdered iron core (lower inductance, high saturation threshold). No line means an air core (used in high-frequency RF where core losses would be unacceptable). Swapping an iron-core choke for an air-core choke in a buck converter will result in immediate current spike failures.
In industrial environments, UV exposure, oil mist, and heat often fade schematic ink or obliterate physical component nameplates. Never assume a coil's operating voltage based purely on a degraded schematic symbol. Applying 120VAC to a faded symbol that was actually a 24VDC relay will cause an explosive failure and arc flash.
Safe Verification Procedure:
- De-energize and Lock/Tag: Shut off all power sources to the panel. Verify dead with a tested CAT III/IV multimeter.
- Measure DC Resistance: Disconnect one leg of the coil and measure resistance. A standard 24VDC ice-cube relay (e.g., Omron MY2N) typically reads 600–700 Ω. A 120VAC version of the exact same physical relay reads around 10,000 Ω due to the finer wire and higher turn count required for AC impedance.
- Trace the Power Source: Follow the physical wire from the coil terminal back to the power supply or transformer secondary. Do not trust the wire color alone; verify the source voltage with a meter once power is safely restored for testing.
- Look for Flyback Protection: If a diode is wired in reverse-parallel across the coil terminals, it is almost certainly a DC coil. AC coils typically use RC snubber networks (a resistor and capacitor in series) or MOVs for suppression.
Understanding the symbol for coil is only the first step. True electrical troubleshooting requires cross-referencing the schematic symbol with the physical reality of the component's DC resistance, AC impedance, and suppression circuitry. Always defer to the physical component's datasheet and local AHJ codes when modernizing or repairing legacy control panels.
