The standard electrical symbol for a coil depends entirely on its function and your regional drafting standard. For a basic inductor coil, the universal symbol is a series of three or four adjacent semi-circles (loops). For a relay or contactor coil, the NEMA (US) standard uses a circle or a rectangle with a specific letter label (like 'CR' or 'M'), while the IEC (international) standard uses a rectangle, sometimes featuring a diagonal line or specific letter codes like 'K'.
Standard Electrical Symbols for Coils (NEMA, IEC, BS)
Before tracing wires or ordering replacement parts, identify which drafting standard your schematic follows. The table below maps the most common coil types to their respective regional symbols. This reference aligns with NFPA 79 for US industrial panels and IEC 60617 for global standards.
| Component Type | NEMA / ANSI (US) | IEC 60617 (Global) | Old BS 3939 (Legacy UK) |
|---|---|---|---|
| Inductor / Choke | 3 or 4 adjacent semi-circles (loops) | 3 or 4 adjacent semi-circles (loops) | 3 or 4 adjacent semi-circles (loops) |
| Relay Coil (Control) | Circle labeled 'CR' or rectangle with 'CR' | Rectangle labeled 'K' or 'KA' | Circle with a diagonal line, labeled 'K' |
| Contactor Coil (Motor) | Circle labeled 'M' or '1M' | Rectangle labeled 'KM' | Circle with a diagonal line, labeled 'C' |
| Solenoid Valve Coil | Circle labeled 'SOL' or 'Y' | Rectangle labeled 'Y' | Rectangle with a diagonal line |
| Transformer Winding | Two sets of 3 loops facing each other | Two sets of 3 loops facing each other | Two sets of 3 loops with a core line |
Regional Differences and Legacy Schematics
Knowing which standard applies to your region prevents catastrophic miswiring. In North America, industrial control panels are predominantly drafted using NEMA conventions. Here, the circle is the dominant shape for electromagnetic coils. If you see a circle with 'M' inside, you are looking at a motor starter contactor coil (e.g., an Eaton C25DND230). If you see 'CR', it is a control relay.
In Europe, Asia, and most modern global installations, IEC 60617 is the governing standard. IEC abandoned the circle for coils decades ago in favor of the rectangle. A rectangle labeled 'KM' is your motor contactor, and 'KA' is an auxiliary relay. The Electrical Engineering Portal provides excellent side-by-side visual comparisons of these divergent standards.
Legacy UK Schematics: If you are maintaining older British machinery (pre-1990s), you may encounter BS 3939 symbols. This standard used a circle with a distinct diagonal slash through it to denote a relay coil, and a separate rectangle for a solenoid. While BS 3939 has been officially superseded by IEC equivalents, maintenance electricians still frequently encounter these 'slash-circle' symbols on legacy panels in older UK factories.
Rows People Get Wrong (And How to Avoid Mistakes)
Even experienced technicians misread schematic symbols when working quickly. Here are the most common interpretation errors:
- Confusing the IEC Rectangle with a PLC Block: In IEC schematics, a simple rectangle is a coil. However, a rectangle with multiple connection points on both sides and internal text like 'CPU' or 'DI' is a PLC module. Rule of thumb: A coil symbol will only have two primary power wires (usually designated A1 and A2) entering the shape.
- Missing the Flyback Diode Marker: On DC circuits (12V or 24V), a relay coil symbol will often have a small triangle with a bar across it drawn parallel to the coil loops or rectangle. This indicates an integrated or external flyback diode. If you replace a DC relay (like a Finder 38.51) and ignore this symbol, wiring it with reverse polarity will instantly short the power supply or destroy the driving transistor.
- Inductor vs. Transformer Windings: A standalone inductor is 3 or 4 loops. If you see two sets of loops placed side-by-side or top-to-bottom, it is a transformer winding, not a simple choke. Treating a transformer primary as a simple inductor will result in a dead short when you apply AC voltage.
- Assuming 'M' Always Means Motor: In NEMA, 'M' inside a circle is a motor contactor coil. But in some older pneumatic/hydraulic schematics, 'M' can denote a manual valve actuator. Always trace the output lines; if they go to a 3-phase bus, it is an electrical contactor.
Practical Verification and Multimeter Testing
When schematic markings are faded, smudged, or entirely missing, you must safely interpret the physical component. If you find a cylindrical or rectangular component with two terminals (often labeled A1 and A2, or simply two spade connectors), it is highly likely a coil.
Safe Interpretation Protocol for Faded Markings:
- De-energize the panel: Lock out and tag out the main breaker. Verify zero voltage with a known-working CAT III or CAT IV multimeter.
- Trace the wires: If one wire goes to a neutral bus or 24VDC negative, and the other goes to a switch, pushbutton, or PLC output, the component is a coil.
- Measure Resistance: Set your multimeter to Ohms (Ω) and measure across the two terminals. Coils have specific, predictable resistance ranges based on their voltage.
Expected Resistance Benchmarks (Copper Windings at 20°C):
- 24VDC Control Relay (e.g., Omron MY2N-D2): Expect ~1,150 Ω. If it reads OL (open), the internal wire is broken. If it reads < 10 Ω, the winding is shorted.
- 120VAC Contactor Coil (e.g., Eaton C25DND230): Expect ~15 Ω to 30 Ω. AC coils have much lower DC resistance than DC coils of the same physical size because their impedance is primarily inductive (reactance) when energized.
- 24VAC Solenoid Valve: Expect ~8 Ω to 12 Ω.
By combining the physical resistance measurement with the wire tracing method, you can definitively identify a coil even if the schematic is completely destroyed.
Frequently Asked Questions About Coil Symbols
What is the electrical symbol for a relay coil on a PLC wiring diagram?
On a PLC wiring diagram, the physical relay coil connected to the PLC output is usually drawn using the standard NEMA circle (labeled 'CR') or IEC rectangle (labeled 'KA'). However, inside the PLC programming software (like RSLogix or TIA Portal), the 'coil' symbol is represented as a set of parentheses ( ) or a circle on the right side of a logic rung, which represents a software bit, not a physical wire.
How do I safely interpret a coil symbol when the schematic is faded or missing?
If the drawing is illegible, do not guess. De-energize the circuit, locate the physical component, and look for the terminal designations 'A1' and 'A2' (the IEC standard for coil terminals). Measure the DC resistance across these terminals. A reading in the tens or thousands of ohms confirms it is a coil. Cross-reference the physical part number printed on the housing (e.g., Schneider TeSys LC1D09) to find the exact voltage and replacement specs online.
What does a coil symbol with a diagonal line through it mean?
A coil symbol (circle or rectangle) with a diagonal line through it typically denotes a latching relay or a mechanically held contactor. Unlike a standard coil that drops out when power is removed, a latching coil uses a brief pulse to change state and stays in that state mechanically until a second pulse (often to a separate coil or a reversed polarity) resets it. In older BS 3939 UK schematics, the diagonal line simply meant 'standard relay'.
Is the symbol for a solenoid valve coil different from a standard relay coil?
Yes, in most modern standards. In NEMA, a solenoid valve coil is a circle labeled 'SOL' or 'Y', whereas a relay is 'CR'. In IEC, both use the rectangle shape, but the solenoid is labeled 'Y' (often with a small arrow pointing outward to indicate mechanical actuation of the valve spool), while the relay is labeled 'KA'. Always check the label inside the shape to distinguish between an electrical switching device and a fluid power actuator.
