The symbol of a coil in an electrical schematic typically appears as a circle with two connection terminals (NEMA/ANSI) or a rectangle (IEC 60617). It represents an electromagnetic winding—most commonly a relay coil, contactor coil, solenoid, or inductor. Understanding these symbols is critical for tracing control logic and diagnosing faults in motor control centers and automation panels.
The Complete Coil Symbol Reference Table
Use this reference table to identify coil symbols across different component types. The table maps the physical component to its graphical representation in the two dominant global standards.
| Component Type | NEMA / ANSI / IEEE 315 Symbol | IEC 60617 Symbol | Typical Application & Notes |
|---|---|---|---|
| Relay / Contactor Coil | Circle with two terminal lines (A1, A2) | Rectangle with two terminal lines | Control circuit actuation. Often labeled with a device tag (e.g., CR1, KM1). |
| Inductor / Choke | Series of semicircles (humps) | Series of semicircles (humps) | Power filtering, current limiting. Both standards use humps, but IEC may add a solid line beneath to indicate a magnetic core. |
| Transformer (2-Winding) | Two overlapping or adjacent circles | Two adjacent rectangles or overlapping circles | Voltage step-up/step-down or isolation. IEC prefers rectangles for windings in modern diagrams. |
| Solenoid Valve Coil | Circle with a diagonal line or arrow | Rectangle with a diagonal line or arrow | Fluid/pneumatic control. The diagonal indicates the mechanical action on the valve spool. |
| Thermal Overload Heater | Circle with a diagonal slash or box inside | Rectangle with a diagonal slash | Motor protection. Not a magnetic coil; it is a bimetallic heating element (e.g., Allen-Bradley 193 series). |
| Polarized / Latching Coil | Circle with a permanent magnet symbol inside | Rectangle with a permanent magnet symbol | Latching relays that maintain state without continuous power. |
Regional Variants: NEMA, IEC, and Legacy Standards
The shape of the coil symbol changes depending on the drafting standard your region or facility adopts. According to the NEMA Electrical Diagrams standards, North American schematics rely heavily on the circle to represent any electromagnetic operating coil. This device-centric approach means a relay coil, a contactor coil, and a timer coil all look identical on paper; you must rely on the device tag (e.g., 1CR for a control relay, 1M for a motor starter) to know what the coil actually operates.
In contrast, the IEC 60617 standard, which dominates Europe, Asia, and modern global OEM equipment, uses a rectangle for operating coils. The IEC philosophy is modular and function-based. A rectangle represents the "block" of the relay, and the contacts associated with that coil are drawn elsewhere on the schematic but linked via a cross-reference grid (e.g., Coil K1 on page 4 operates contacts K1/12 on page 7).
Legacy UK (BS 3939): If you are troubleshooting older British machinery, you may encounter BS 3939 symbols. This standard used circles similar to NEMA but often included internal cross-hatching or specific letter codes inside the circle to denote AC versus DC operation. BS 3939 was officially withdrawn in favor of IEC 60617, but it remains prevalent in unupdated legacy panels.
Rows People Get Wrong (and How to Fix Them)
Even experienced technicians misinterpret specific coil variations. Here are the most common schematic traps and how to navigate them.
1. AC vs. DC Coil Markings
A standard circle or rectangle does not inherently tell you if the coil requires Alternating Current or Direct Current. In NEMA diagrams, an AC coil is often denoted by a small sine wave drawn inside or immediately adjacent to the circle. A DC coil features a solid straight line.
The Physics Trap: If you wire 24VAC to a 24VDC relay coil, the coil will likely burn out. A DC coil relies entirely on the wire's DC resistance ($R$) to limit current. An AC coil relies on inductive reactance ($X_L = 2\pi fL$) to limit current; its actual wire resistance is very low. Applying DC to an AC coil removes the reactance, causing a massive current spike that melts the winding.
2. Inductor vs. Relay Coil
Beginners often confuse the relay coil (circle/rectangle) with the inductor (humps). Remember the context: if the symbol is in the low-current control circuit (typically 24V or 120V) and its state changes contacts elsewhere, it is a relay coil. If it is in the high-current power circuit (e.g., series with a VFD output or a power supply filter) and has no associated contacts, it is an inductor.
Safe Interpretation When Markings Fade
On the jobsite, panel labels fade, schematic folders go missing, and dust obscures device tags. When you cannot read the symbol or the label, you must identify the coil electrically. Using a digital multimeter (DMM) like a Fluke 87V, set it to measure resistance (Ohms) and test across the A1 and A2 terminals.
- 24VDC Relay Coil (e.g., Omron MY2N): Expect a reading between 600Ω and 700Ω. The high resistance limits the 24V DC current to roughly 35mA.
- 120VAC Contactor Coil (e.g., Schneider TeSys D): Expect a much lower reading, typically 15Ω to 25Ω. The low DC resistance is normal because the AC impedance is dominated by the magnetic circuit's reactance when energized.
- Inductor / Choke: Will read near zero (typically < 1Ω or in the milliohm range). It is essentially a short piece of thick wire.
- Thermal Overload Heater: Will read near zero ohms, but physically it will be a thick resistive wire or bimetallic strip, not a spool of fine copper wire.
If your DMM reads "OL" (Open Loop) across a coil, the internal winding is broken, or the internal thermal fuse (common in 120VAC contactor coils) has blown. The coil must be replaced; it cannot be repaired. For deeper diagnostic techniques, refer to the Fluke relay testing guide for step-by-step bench procedures.
Frequently Asked Questions
What does the symbol of a coil with a diagonal line through it mean?
A coil symbol with a diagonal line through it (or a diagonal line inside the circle/rectangle) represents a thermal overload relay heater element, not a magnetic operating coil. In motor control circuits, this element heats up proportionally to the motor's current draw. If the motor overloads, the heat bends a bimetallic strip, mechanically tripping the contactor. It does not generate a magnetic field to pull in contacts.
How do I tell a relay coil symbol from an inductor symbol?
Look at the shape and the circuit location. In NEMA standards, a relay coil is a circle, while an inductor is a series of semicircles (humps). In IEC standards, a relay coil is a rectangle, while an inductor remains a series of humps. Furthermore, relay coils are found in control circuits (thin wires, low current) and have associated contact symbols mapped to them. Inductors are found in power circuits (thick wires, high current) and do not operate mechanical contacts.
Why does my contactor coil symbol have a box around it?
If you see a standard coil symbol enclosed in an additional dashed or solid box, it typically indicates a shielded assembly, a magnetic latching mechanism, or an integrated solid-state driver. In modern IEC schematics, a solid box around a rectangle might denote an electronic relay or a solid-state contactor (SSR) where the "coil" is actually an optocoupler LED and the internal logic circuitry, rather than a simple copper winding.
