The electrical symbols coil representation changes drastically depending on whether you are reading an IEC 60617 (Global/European) or NEMA/IEEE 315 (North American) schematic. A coil symbol can represent a passive inductor, an electromechanical relay, a heavy-duty contactor, or a solenoid valve. Misinterpreting these symbols leads to incorrect replacements, blown fuses, or control circuit failure.

The Master Electrical Symbols Coil Reference Table

Before tracing a circuit, identify which standard the schematic follows. North American industrial prints heavily favor NEMA (circles), while modern global and OEM equipment defaults to IEC (rectangles).

Component NEMA / ANSI Symbol Shape IEC 60617 Symbol Shape Typical Designation Practical Application
Inductor / Choke Four connected semi-circles (humps) Rectangle with a diagonal line L (e.g., L1) Filtering, energy storage, buck/boost converters
Relay Coil (Control) Empty Circle Rectangle (often with diagonal line or 'K') CR (NEMA) / K (IEC) Logic branching, signal isolation, low-current switching
Contactor Coil (Power) Circle with 'M' or 'C' inside Rectangle with 'KM' inside M (NEMA) / KM (IEC) Switching 3-phase motors, heavy resistive loads
Solenoid Valve Rectangle with a diagonal arrow Rectangle with a diagonal arrow (similar to NEMA) SOL or Y Pneumatic/hydraulic actuation, water valves
Transformer Winding Two sets of humps facing each other Two overlapping circles or adjacent rectangles T (e.g., T1) Voltage step-up/step-down, isolation
Bench Tip: If you see a circle on a schematic, you are almost certainly looking at a North American NEMA-style control print. If every component is a rectangle with alphanumeric tags inside (like -K1 or -KM2), you are reading an IEC 60617 print, common on European OEM machinery like Siemens or Festo systems.

Regional Standards: NEMA Circles vs. IEC Rectangles

The divergence in coil symbols stems from fundamentally different drafting philosophies. NEMA (National Electrical Manufacturers Association) and IEEE 315 use shape-based identification. A circle universally means "this is an electromagnetic coil that consumes power to create a magnetic field," regardless of what it switches. The contacts it operates are drawn elsewhere on the page but linked by the same device number (e.g., Coil 1M operates contacts 1M).

IEC 60617, the international standard, uses function-based rectangular blocks. The shape of the coil is almost always a simple rectangle. The specific function is dictated entirely by the alphanumeric code inside or next to the box. For example, a rectangle labeled K1 is a control relay, while KM1 is a motor contactor. Understanding these regional drafting differences is critical when troubleshooting imported machinery or working on multinational job sites.

The "Rows People Get Wrong" Field Guide

Even experienced technicians misread specific coil variations. Watch out for these common schematic traps:

  • Relay vs. Contactor Coil: In NEMA prints, an empty circle is a control relay (typically < 10A). A circle with an 'M' is a motor contactor. Swapping a control relay for a contactor coil in a replacement scenario will result in an immediate blown control fuse due to the massive inrush current of a contactor coil.
  • AC vs. DC Coil Symbols: Schematics rarely differentiate AC and DC coils visually. You must read the text next to the symbol. ~120V or 120VAC means alternating current. =24V or 24VDC means direct current. Installing a 24VDC coil on a 120VAC circuit will vaporize the coil wire instantly.
  • Inductor vs. Transformer: An inductor has two terminals (one continuous winding). A transformer symbol will show two distinct coils (four terminals minimum) separated by a core line (solid for iron, dashed for air/ferrite).
  • Latching vs. Standard Relay: A standard relay coil is just a circle/rectangle. A latching relay coil will have a small mechanical latch symbol (a hook or cross) attached to the side of the coil box, indicating it requires a pulse to set and a reverse pulse to reset.

Decision Tree: Identifying Unknown or Faded Coils

When you are troubleshooting a dead machine and the coil's nameplate is faded, burned, or missing, do not guess the voltage. Follow this decision path to safely identify the coil and select a replacement.

Safety Warning: Always de-energize the panel, lock out/tag out the main disconnect, and verify zero voltage with a calibrated CAT III/IV multimeter before touching coil terminals. Control circuits often retain lethal voltages even when the main motor disconnect is off.
Step Action / Measurement Result & Interpretation
1. Visual Core Inspection Look at the stationary iron core face of the contactor/relay. Shading ring present (copper loop): It is an AC coil.
Solid, flat core face: It is a DC coil.
2. Measure DC Resistance Set DMM to Ohms (Ω). Measure across the coil terminals (A1 to A2). 10Ω - 50Ω: Likely 120VAC or 240VAC.
200Ω - 1000Ω: Likely 24VDC.
Open (OL): Coil is burned out internally.
3. Trace the Control Power Trace the wires feeding A1/A2 back to the control transformer or power supply. Check the secondary rating of the control transformer (e.g., 120V) or the DIN-rail power supply label (e.g., 24V).
4. Check Flyback Protection Look for a diode wired in reverse-parallel across the coil terminals. Diode present: Confirms DC coil.
RC Snubber or MOV present: Confirms AC coil.

Source reference for coil troubleshooting thresholds: Macromatic Technical Resources on Relay Coil Diagnostics.

Default Replacements and Sourcing

When you have identified the coil type but cannot find the exact OEM part number, terminate your search by defaulting to these industry-standard, globally available workhorses. Do not leave the machine down waiting for a proprietary OEM coil.

  • For an unknown 24VDC Control Relay: Default to the Omron G2R-1-S DC24. It is a 10A, single-pole, plug-in relay with a 650Ω coil resistance. It fits standard DIN-rail sockets and includes a built-in LED indicator and mechanical flag for easy troubleshooting.
  • For an unknown 120VAC Contactor (up to 10HP/30A): Default to the Eaton C320 Series (e.g., C320CE1B). The 120VAC coil (Part # C320DK) draws roughly 14 ohms of resistance and features a robust shading ring to prevent 120Hz contact chatter. It is a direct drop-in replacement for most legacy NEMA Size 1 and IEC equivalent frames.
  • For an unknown 24VAC Solenoid Valve: Default to a standard ISO 4401 / DIN 43650 Form A connector coil. These are universally interchangeable across pneumatic brands like SMC, Parker, and Festo, provided the physical spool diameter matches (typically 13mm or 16mm).

By matching the schematic symbol standard (NEMA vs IEC) and verifying the physical coil characteristics (AC shading rings vs DC resistance), you can accurately diagnose and replace any electromagnetic coil without relying on faded nameplates.