The electrical coil symbol is the universal schematic shorthand for any component that converts electrical energy into a magnetic field to perform mechanical work or store energy. Whether you are tracing a control circuit on a machine schematic or designing a custom relay board, misreading a coil symbol leads to miswired logic, blown fuses, or fried control transformers. Below is the definitive reference for identifying, interpreting, and replacing electrical coils across global standards.

The Complete Electrical Coil Symbol Reference Table

Schematics use distinct geometric shapes and designators to differentiate between energy storage (inductors), control logic (relays), and heavy power switching (contactors). Use this table to decode the symbols on your prints.

Component IEC 60617 Symbol ANSI/NEMA Symbol Standard Designator Real-World Application
Inductor (Air Core) 4 connected semi-circles 4 connected semi-circles L RF filtering, high-frequency chokes
Inductor (Iron Core) Semi-circles with solid parallel line Semi-circles with solid parallel line L Power supply filtering, buck converters
Control Relay Coil Rectangle (often with diagonal line) Circle or Rectangle K, CR, KA Logic branching, PLC output multiplication
Motor Contactor Coil Rectangle with mechanical linkage slash Circle with 'M' or 'C' inside KM, M, C Starting/stopping 3-phase AC motors
Solenoid Valve Coil Rectangle with diagonal arrow pointing out Rectangle with diagonal arrow Y, SOL Pneumatic/hydraulic directional control
Transformer (2-Winding) Two sets of semi-circles separated by core line Two overlapping circles or semi-circle sets T Stepping down 480V to 120V for control circuits

Regional Standard Variants: IEC vs. ANSI/NEMA vs. Old UK

The physical footprint and schematic representation of a coil change drastically depending on the engineering standard used to draft the print. Knowing which standard you are looking at prevents catastrophic wiring errors.

Rule of Thumb: If the schematic uses rectangles for relay coils and alphanumeric tags like KM1 or KA2, you are looking at an IEC 60617 print (common in Europe, Asia, and modern global machinery). If it uses circles with letters inside (like 1M or CR3), you are looking at a North American NEMA/ANSI print.
  • IEC 60617 (Global/Modern): Coils are almost exclusively drawn as rectangles. A relay coil is a simple rectangle. A contactor coil is a rectangle with a small slash or mechanical linkage indicator. Terminals are universally labeled A1 (positive/hot) and A2 (negative/neutral).
  • ANSI/NEMA (North America): Historically relies on circles for motor control coils (e.g., a circle with an 'M' for a motor starter) and rectangles for control relays. Terminals on NEMA contactors are often just unmarked screw terminals on the coil block, or labeled with numbers specific to the manufacturer's wiring diagram rather than a universal A1/A2 standard.
  • Old UK BS 3939 (Legacy): Obsolete since the 1990s but still found in older British plants. Used a circle for relays and a rectangle with a specific internal hash pattern for inductors. If you encounter this, treat it as IEC but verify physical terminal markings with a meter, as the schematic-to-physical mapping is notoriously inconsistent.

Rows People Get Wrong: Common Symbol Misinterpretations

Even experienced technicians misread specific coil variants when skimming a dense schematic. Here are the most common traps and how to avoid them.

1. AC Coil vs. DC Coil

Schematically, an AC relay coil and a DC relay coil might both be drawn as a simple rectangle. The trap is assuming interchangeability. The Fix: Look for a diode symbol drawn in parallel with the coil (a flyback diode). If a diode is present, it is a DC coil. AC coils do not use flyback diodes; instead, they rely on a physical copper shading ring embedded in the armature to prevent AC hum. Applying DC to an AC coil will cause it to overheat and burn out within minutes because DC lacks the inductive reactance that limits AC current.

2. Contactor vs. Control Relay

Both switch loads via a magnetic coil, but their symbols dictate their physical capabilities. A control relay (CR/K) is designed for low-current logic (typically under 10A). A contactor (M/KM) has arc chutes and heavy-duty contacts for motor loads. The Fix: Never substitute a control relay for a motor contactor, even if the coil voltage matches. The inrush current of a motor will weld the relay contacts shut on the first start cycle.

3. Autotransformer vs. Standard Transformer

A standard transformer symbol shows two distinct, separated winding sets. An autotransformer symbol shows a single continuous winding with a tap point. The Fix: Autotransformers do not provide galvanic isolation. If your circuit requires isolation for safety or noise rejection, an autotransformer will fail the requirement, regardless of the voltage step-down ratio.

Identifying Unmarked or Faded Coils on the Bench

Safety Warning: Always de-energize the panel, lock out/tag out the main breaker, and verify zero voltage with a tested CAT III or CAT IV meter before probing coil terminals. Control circuits often backfeed from multiple sources.

When you pull a relay or contactor from a legacy panel and the label is faded, melted, or missing, you must electrically identify the coil before applying power. Use a digital multimeter (DMM) in resistance (Ohms) mode across the coil terminals (A1 and A2).

  • Reads 'OL' (Open Loop): The internal copper winding is broken. The coil is burnt out and must be replaced.
  • Reads near 0 Ohms: The winding is shorted internally. Replace immediately; applying voltage will blow the control circuit fuse.
  • Reads 5 to 20 Ohms: This is typically a 120VAC or 240VAC coil. The low resistance is normal because the coil's impedance (reactance) at 60Hz limits the current, not the DC resistance.
  • Reads 100 to 1,500 Ohms: This is typically a 24VDC or 12VDC coil. DC coils rely purely on the wire's DC resistance to limit current.

Source reference: For detailed troubleshooting procedures on magnetic motor controllers, consult the All About Circuits relay and contactor guide or standard IEC 60617 symbol documentation.

Decision Tree: Selecting the Right Replacement Coil

When a coil burns out, do not guess the replacement. Follow this decision path to select the exact part number for your bench or panel.

Condition / Question If YES If NO
Is the load a motor or high-inductive load > 1HP (750W)? Go to Path A (Contactor) Go to Path B (Relay)
Path A: Is the existing hardware DIN-rail mounted with A1/A2 markings? IEC Standard. Buy Schneider Electric LC1D09 (or equivalent TeSys D series) matched to your voltage (e.g., LC1D09M7 for 220VAC). NEMA Standard. Buy Eaton C25DND230 (Definite Purpose Contactor) or Square D 8502 Type S, matched to FLA ratings.
Path B: Is the control voltage 24VDC? Buy Omron MY2N DC24 (8-pin) or MY4N DC24 (14-pin) with matching DIN socket. Check AC voltage. For 120VAC, buy Omron MY2N AC100/110.
Does the schematic show a flyback diode symbol across the coil? Ensure your DC relay has a built-in diode (e.g., Omron MY2N-D2 DC24) or add an external RC snubber/diode module. Standard coil is fine. Do not add a diode to an AC coil circuit.

Final Default Pick: If you are building a new 24VDC control panel from scratch and need a universal, highly reliable control relay coil, standardize on the Omron MY2N DC24. It is the industry benchmark for 8-pin logic branching, handles 10A per pole, and the A1/A2 coil terminals are clearly marked, eliminating the guesswork between NEMA and IEC wiring practices.