The electrical symbol for coil varies drastically depending on whether you are reading an IEC (International), NEMA (North American), or IEEE/ANSI schematic. Misinterpreting a contactor coil for an inductor can lead to catastrophic wiring errors on the bench or jobsite. Below is the definitive reference for identifying inductor, relay, contactor, and motor starter coils in the wild, followed by practical troubleshooting for degraded schematics.
Complete Coil Symbol Reference Table
Because text-based schematics cannot render complex vector graphics, this table describes the exact geometric construction of each symbol. Use this as your master key when cross-referencing physical drawings.
| Component Type | IEC 60617 Geometry | NEMA / IEEE 315 Geometry | Standard Designator Prefix |
|---|---|---|---|
| Inductor / Choke | Rectangle (sometimes with a core line below) | Three or four contiguous semicircles (loops) | L |
| Relay Coil | Rectangle with 'K' or 'KA' inside | Circle or rectangle with 'CR' (Control Relay) inside | K / CR |
| Contactor Coil | Rectangle with 'KM' inside | Circle or rectangle with 'M' or '1M' inside | KM / M |
| Motor Starter Coil | Rectangle with 'KM' and thermal overload link | Circle with 'M' and adjacent heater element symbol | M / MS |
| Solenoid Valve Coil | Rectangle with 'Y' or 'YA' inside | Rectangle with 'SOL' or a diagonal line inside | Y / SOL |
| Transformer Winding | Two adjacent rectangles (core line between them) | Two sets of contiguous semicircles facing each other | T |
Regional Variants and the 'Rows People Get Wrong'
Your region dictates the standard your local Authority Having Jurisdiction (AHJ) and panel builders will use. Assuming a single standard is universal is the fastest way to miswire a control circuit.
Which Standard Applies to You?
- IEC 60617 (International / Europe / Modern Global): Relies heavily on the 'box' or rectangle method. Almost all coils are drawn as rectangles, differentiated only by the alphanumeric code inside (K, KM, Y). This is the standard for modern PLC I/O drawings and global machinery.
- NEMA / IEEE 315 (North America): Uses the classic 'semicircle' loops for inductors and transformers, and circles or boxes with specific letter codes (CR, M) for electromechanical coils. You will see this on legacy US industrial equipment and HVAC schematics.
- Old UK (BS 3939): Largely obsolete but still found in pre-1990s British infrastructure. It used a mix of semicircles and unique mechanical linkages. If you encounter this, treat it as a legacy system and map it to IEC 60617 for replacement parts.
The Rows People Get Wrong
Common Pitfall 2: Inductor vs. Transformer Winding in NEMA. A single set of three loops is an inductor. Two sets of loops facing each other is a transformer. If a solid line is drawn between the NEMA loops, it indicates an iron core; no line means an air core. Missing that core line changes your expected inductance calculations by orders of magnitude.
Safe Interpretation When Markings Are Faded or Missing
On the jobsite, you are rarely handed a pristine PDF. You are usually staring at a sun-faded schematic taped inside a 25-year-old lathe cabinet, or holding a physical contactor with a burned-off label. Here is how you identify the coil safely when the symbols and text fail you.
Step 1: De-energize and Verify Dead. Never probe a control circuit assuming it is safe. Lock out the main disconnect, and use a known-good CAT III multimeter to verify 0V AC/DC across the coil terminals and to ground.
Step 2: The Resistance Test (DC vs AC Coils). Set your multimeter to the Ohms (Ω) setting and measure across the coil terminals (A1 and A2). This reveals the coil's nature:
- DC Coils: Rely purely on wire resistance to limit current. A 24V DC relay coil will typically read between 300Ω and 800Ω. A 12V DC solenoid might read 12Ω to 40Ω. The resistance is relatively high.
- AC Coils: Rely on inductive reactance to limit current once the armature pulls in. Their raw DC resistance is deceptively low. A 120V AC contactor coil might read just 15Ω to 30Ω on a multimeter. If you apply 120V DC to this coil, it will draw massive current and burn out instantly because it lacks inductive reactance in a DC circuit.
Step 3: The Shading Ring Check. If you are looking at a physical, unlabeled contactor and need to know if it is AC or DC, look at the steel face of the armature (the moving part). AC contactors have a copper 'shading ring' (a thick wire loop embedded in the pole face) to prevent the armature from buzzing and dropping out at every zero-crossing of the 50/60Hz sine wave. DC contactors do not have shading rings. According to All About Circuits, this physical difference is the most reliable field identifier when labels are destroyed.
Frequently Asked Questions About Coil Symbols
What is the electrical symbol for coil on a European schematic?
European schematics follow the IEC 60617 standard. The electrical symbol for a coil is almost universally a simple rectangle. The specific type of coil is identified by the letter code inside the rectangle: 'K' or 'KA' for a standard relay, 'KM' for a power contactor, and 'Y' for a solenoid valve. Inductors and chokes are also rectangles, but are designated with the letter 'L'.
How do I tell a relay coil symbol from a contactor coil symbol?
In North American (NEMA/IEEE) schematics, a relay coil is typically a circle or box labeled 'CR' (Control Relay), while a contactor coil is labeled 'M' or '1M' (Motor Starter/Contactor). In IEC schematics, both are rectangles, but a relay is labeled 'K' and a contactor is labeled 'KM'. Physically, contactor coils are wired in series with power contacts rated for high amperage, while relay coils drive low-amperage logic contacts.
Why does the inductor coil symbol have a line through it or under it?
A line drawn parallel to the coil symbol (underneath in NEMA, or through/adjacent in some IEC variants) represents the magnetic core material. A solid straight line indicates a ferromagnetic core (like iron or steel), which dramatically increases inductance. A dashed line indicates a powdered iron or ferrite core. If there is no line at all, it represents an air-core inductor, which has much lower inductance and is typically used in high-frequency RF applications.
What does a box with a diagonal line mean inside a coil symbol?
A rectangle with a diagonal line drawn through it usually represents a thermal overload relay heater element or a solenoid valve coil, depending on the standard. In older NEMA diagrams, a box with a diagonal line adjacent to a motor starter coil (M) represents the thermal overload block that physically trips the contactor if the motor draws too much current. For precise identification, consult the IEEE 315 standard documentation for graphic symbols, which maps these specific internal modifiers to their exact component functions.
Is there a specific electrical symbol for coil with thermal overload protection?
Yes. In IEC schematics, a motor starter coil (KM) is often drawn with a small box or dashed line connecting it to a thermal overload relay symbol (usually a box with a bimetallic strip curve inside, designated 'F' or 'FR'). In NEMA schematics, the motor starter coil (M) is shown alongside the overload heaters (drawn as small boxes with a loop or 'Z' inside), linked by a dashed mechanical line indicating that the heater will physically trip the coil's latch mechanism.
