Finding the correct symbol for a coil depends entirely on whether you are reading a schematic diagram or tracing physical wires on a jobsite. Schematic symbols represent the electromagnetic function, while physical terminal markings dictate how you wire the component to a control circuit. Below is the definitive reference for coil symbols across IEC and ANSI standards, followed by physical terminal mappings and bench-testing protocols for unlabeled parts.
Schematic Symbol for a Coil: IEC vs. ANSI/IEEE Standards
In electrical drafting, a coil is universally represented by a series of adjacent semicircles (often called 'humps' or 'loops'), symbolizing the physical winding of wire around a core. However, the exact geometry and annotation rules differ depending on whether your facility follows the international IEC 60617 standard or the North American ANSI/IEEE 315 standard. The table below maps the most common coil symbols you will encounter in control and power schematics.
| Component Type | IEC 60617 Symbol Geometry | ANSI/IEEE 315 Symbol Geometry | Common Use Case & Annotation |
|---|---|---|---|
| Air-Core Inductor | Three or four adjacent semicircles on a straight line. | Three or four adjacent semicircles, sometimes with a straight line below. | RF filters, high-frequency chokes. Labeled with 'L' and inductance value (e.g., L1 10mH). |
| Iron-Core Inductor (Choke) | Semicircles with a solid straight line directly above or below the loops. | Semicircles with a solid straight line directly below the loops. | Power supply filtering, ballast circuits. The solid line denotes a high-permeability magnetic core. |
| Relay / Contactor Coil | A rectangle (often with a diagonal line or 'K' inside) OR semicircles with a thick line. | Semicircles with a solid straight line below, or a circle with 'CR' (Control Relay). | Motor starters, control logic. Labeled with device tag (e.g., K1, CR1) and voltage (e.g., 24VDC). |
| Motor Winding | Circle with 'M' and three internal semicircles (for 3-phase) or a single coil symbol. | Circle with 'M' and internal coil loops, or specific NEMA letter designations. | AC/DC motors. Terminals inside the circle map to physical peckerhead markings (U, V, W). |
| Transformer Winding | Two sets of semicircles facing each other, separated by two parallel vertical lines. | Two sets of semicircles facing each other, separated by two parallel vertical lines. | Step-up/step-down isolation. The parallel lines denote a laminated iron core. Dots indicate phase polarity. |
For a deeper look into how these symbols integrate into larger ladder logic diagrams, the All About Circuits schematic symbol reference provides excellent visual examples of complex relay logic. Note that modern CAD software like AutoCAD Electrical or EPLAN often defaults to IEC symbols globally, even in North American projects, unless the template is specifically forced to JIC/NEMA standards.
Physical Terminal Markings and Wire Colors
When you open a control panel or a motor peckerhead, you will not see schematic 'humps'. Instead, you will see alphanumeric terminal designations. Misinterpreting these physical markings is a primary cause of short circuits and blown control fuses. The following table details the standard physical terminal IDs for coils and windings, alongside typical regional wire color codes for the control circuits feeding them.
| Component | Standard Terminal IDs | IEC Control Wire Color (Typical) | US/NEC Control Wire Color (Typical) | Function & Polarity Notes |
|---|---|---|---|---|
| Control Relay / Contactor Coil | A1 (+/Line), A2 (-/Neutral) | Blue (DC), Black or Red (AC) | Red (AC Hot), Blue (DC+), White/Black (Neutral/-) | A1 and A2 are the universal IEC designations for the coil itself. Polarity matters only for DC coils with integral suppression diodes. |
| Single-Phase Motor (Capacitor Start) | U1, U2 (Main/Run) Z1, Z2 (Start) |
N/A (Power circuit) | Black (Hot), White (Neutral) | U1/U2 is the run winding (lower resistance). Z1/Z2 is the start winding (higher resistance, in series with centrifugal switch). |
| 3-Phase AC Motor (Wye/Delta) | U1, V1, W1 (Starts) U2, V2, W2 (Finishes) |
Brown, Black, Grey (L1, L2, L3) | Black, Red, Blue (L1, L2, L3) | IEC standard numbering. NEMA uses T1-T9. Connecting U1-W2, V1-U2, W1-V2 creates a Delta configuration. |
| Control Transformer | H1, H2 (Primary/High) X1, X2 (Secondary/Low) |
N/A (Power circuit) | Black (Primary), Red (Secondary) | H always denotes the high-voltage winding, X denotes low-voltage. H1 and X1 share the same instantaneous polarity (marked with a dot on schematics). |
For authoritative motor terminal markings, refer to the NEMA MG 1 standard for motors and generators, which dictates the T-lead numbering system (T1 through T9) used heavily in North America, contrasting with the U/V/W IEC system used in Europe and Asia.
Rows People Get Wrong (and Faded Label Workarounds)
Even experienced technicians make specific errors when interpreting coil markings, particularly when dealing with multi-function components or degraded equipment. Here are the most common pitfalls and the exact bench-testing procedures to resolve them.
Mistake 1: Wiring Mains to Relay Contacts Instead of the Coil
On a standard IEC 10A control relay, the coil terminals are marked A1 and A2. The normally open (NO) power contacts are marked 13 and 14. A frequent jobsite error is applying 120VAC to terminals 13 and 14, assuming they are the coil inputs because they are located at the top of the relay. This instantly destroys the internal contact bridge. Always verify A1/A2, which are typically located on the top or bottom edge of the relay body, separate from the stacked contact decks.
Mistake 2: Confusing Transformer H and X with Primary and Secondary
The 'H' winding is always the high-voltage winding, and 'X' is always the low-voltage winding, regardless of power flow direction. If you are using a control transformer as a step-up (feeding 120V into the X winding to get 240V out of the H winding), the physical markings do not change. H1/H2 will still output the 240V. Assuming H always means 'input' will lead to dangerous miswiring in reverse-feed scenarios.
Safe Interpretation When Markings are Faded or Missing
When you inherit an old panel with a relay or motor where the nameplate is painted over or sun-faded, never guess the coil terminals. Use a digital multimeter (DMM) in resistance (Ohms) mode to map the component safely.
- Identifying a Relay Coil vs. Contacts: Set your DMM to Ohms. Measure across suspected coil pins. A 24VDC relay coil will typically read between 20Ω and 60Ω. A 120VAC coil will read between 100Ω and 300Ω. Conversely, measure across the contact pins (e.g., 13/14). With the relay de-energized, an NO contact will read 'OL' (infinite), and an NC contact (21/22) will read less than 0.5Ω. The coil is the only path that will show a mid-range resistance value.
- Identifying Motor Start vs. Run Windings: Measure across the winding pairs. The run winding (U1/U2) uses thicker wire and will show a very low resistance (e.g., 2Ω to 5Ω). The start winding (Z1/Z2) uses thinner wire and will show a noticeably higher resistance (e.g., 10Ω to 20Ω). If you measure a short (0.1Ω) or an open (OL) between any winding and the motor casing, the winding insulation has failed and the motor must be replaced.
When bench-testing an unknown coil by briefly applying voltage to see if it actuates, always use a momentary pushbutton and stand clear of the contacts. When you break the circuit to an inductive coil, the collapsing magnetic field induces a massive voltage spike (flyback voltage) that can exceed 1000V for a fraction of a millisecond. This can arc across your test probes or destroy your DMM if connected in parallel during the break. Always ensure a flyback diode (for DC) or an RC snubber (for AC) is present in the final circuit.
The 'Drop Test' for Unknown AC vs DC Coils
If you have identified the coil terminals (A1/A2) via resistance testing, but do not know if it is rated for AC or DC, you can perform a low-voltage drop test. Apply 12VDC to the coil. If it is a DC coil, it will pull in firmly and hold with a steady, silent magnetic field. If it is an AC coil, it will likely chatter violently, hum loudly, and fail to seal completely because it lacks the DC resistance profile and the shading ring (a small copper loop embedded in the AC core face designed to prevent the magnetic field from dropping to zero at the 50/60Hz zero-crossing). For a comprehensive guide on electromagnetic standards, the IEC symbols and standards portal provides the foundational definitions for these component classifications.
Regional Standards and Code Compliance
Understanding which standard applies to your region is critical for both designing new panels and troubleshooting legacy systems. While globalization has pushed IEC standards into many North American OEM machines, local electrical codes still dictate specific wiring practices.
- IEC 60617 (International / Europe / Modern Global OEMs): This is the current global standard for graphical symbols. It uses the A1/A2 coil designation, U/V/W motor leads, and relies heavily on reference designators (e.g., -K1 for a relay, -F1 for a fuse). If you are building a panel for export or working on modern European-imported machinery (like Siemens or ABB drives), this is your governing standard.
- ANSI/IEEE 315 & NEMA (North America): Governs most US and Canadian industrial facilities. You will see 'CR' for control relays, 'M' for motor starters, and T-leads (T1-T9) for motors. NEMA standards also dictate physical contactor sizing (NEMA Size 0 through 5), which correlates to horsepower ratings rather than the IEC utilization categories (AC-1, AC-3).
- BS 3939 (Legacy UK): If you are maintaining older infrastructure in the UK, you may encounter the withdrawn BS 3939 standard. In this legacy system, relay coils were often drawn as simple circles with the device letter inside, and motor windings used A, B, C designations instead of U, V, W. Always verify the drawing revision date; if it predates the late 1990s, cross-reference the physical wiring against the schematic, as previous maintenance crews may have retrofitted IEC components into a BS 3939 panel without updating the prints.
Ultimately, while the symbol for a coil on a page tells you how the circuit functions logically, the physical terminal markings and regional color codes dictate how you safely execute the work. Always de-energize, lock out, and verify dead with a tested meter before probing any coil terminals in a live panel.
