The standard electrical contactor symbol on a schematic consists of an electromagnetic coil linked to main power contacts and auxiliary control contacts. However, the exact geometry and terminal numbering change drastically depending on whether the drawing follows IEC 60617 (global standard) or NEMA ICS 19 (North American standard). Below is the direct reference for identifying these symbols, decoding their terminal pinouts, and safely troubleshooting them when physical markings fade.
The Complete Electrical Contactor Symbol Reference
The table below maps the physical components of a 3-pole contactor (like a Schneider TeSys LC1D or Eaton XTCE) to their schematic representations. Use this as your primary legend when reading motor control circuit diagrams.
| Row | Component | IEC 60617 Symbol Description | NEMA ICS 19 Symbol Description | Standard Tag |
|---|---|---|---|---|
| 1 | AC Coil | Rectangle with diagonal lines inside; no polarity marks | Circle or rectangle with 'M' or 'C' inside | K1, KM1 |
| 2 | DC Coil | Rectangle with a solid black fill or polarity (+/-) marks | Circle with polarity marks or diode symbol | K1, KM1 |
| 3 | Main Power NO | Two parallel lines, one with a diagonal slash (normally open) | Two parallel lines bridged by a diagonal line | 1-2, 3-4, 5-6 |
| 4 | Main Power NC | Two parallel lines crossed by a diagonal slash (normally closed) | Two parallel lines bridged, with a cross through it | Rare on 3-pole |
| 5 | Auxiliary NO | Same as Main NO, but drawn smaller, linked to coil via dashed line | Same as Main NO, smaller, linked via dashed line | 13-14, 43-44 |
| 6 | Auxiliary NC | Same as Main NC, smaller, linked to coil via dashed line | Same as Main NC, smaller, linked via dashed line | 21-22, 31-32 |
In both IEC and NEMA schematics, the mechanical linkage between the coil (Row 1/2) and the contacts (Rows 3-6) is indicated by a dashed line. If you see a contact symbol without this dashed line linking it to a specific coil, it is a standalone relay or a manual switch, not a contactor auxiliary.
Decoding IEC vs NEMA Terminal Numbering
While the schematic symbols tell you how the circuit logic works, the physical terminal numbers tell you where to land the wires. The IEC standard uses a highly structured two-digit numbering system for auxiliary contacts that NEMA does not strictly enforce (NEMA often relies on wire line numbers instead). Understanding the IEC system is critical because modern global equipment, even when installed in North America, predominantly uses IEC terminal markings.
In the IEC system, the first digit indicates the sequence number of the contact (1, 2, 3, etc.), and the second digit indicates the function of the contact.
| IEC Terminal Pair | Function Code (2nd Digit) | Contact State | Typical Application |
|---|---|---|---|
| 13 - 14 | 3 and 4 | Normally Open (NO) | Sealing/holding circuits, run-indicator lights |
| 21 - 22 | 1 and 2 | Normally Closed (NC) | Electrical interlocks, fault-reset circuits |
| 31 - 32 | 1 and 2 | Normally Closed (NC) | Secondary interlocks, stop-circuit feedback |
| 43 - 44 | 3 and 4 | Normally Open (NO) | PLC feedback inputs, secondary run indicators |
| 15 - 18 | 5 and 8 | NO, Time-Delayed (Make) | Star-delta transition timing, soft-start bypass |
| 25 - 28 | 5 and 8 | NC, Time-Delayed (Break) | Safety interlock drop-out delays |
For main power terminals, IEC and NEMA largely agree: Line side is 1 (L1), 3 (L2), 5 (L3) and Load side is 2 (T1), 4 (T2), 6 (T3). Always verify the physical casing stamp, as some older NEMA contactors use L1/L2/L3 and T1/T2/T3 lettering instead of numbers.
Regional Standard Variants and Schematic Rules
When reading schematics from imported machinery or legacy panels, you will encounter regional variations in how the electrical contactor symbol is drawn. According to All About Circuits, recognizing these regional quirks prevents dangerous misinterpretations during troubleshooting.
IEC 60617 (Global / Modern EU / Modern North America)
IEC symbols are minimalist. The coil is a simple rectangle. The state of the contact (NO or NC) is determined strictly by the slash across the parallel lines. IEC schematics read from left to right, top to bottom, and heavily rely on the two-digit terminal numbering system detailed above.
NEMA ICS 19 (Legacy North America)
NEMA symbols are more pictorial. Coils are often drawn as circles with a letter inside (like 'M' for Motor starter or 'C' for Contactor). NEMA schematics traditionally use "ladder logic" formatting with vertical power rails (L1 on the left, L2 on the right) and horizontal rungs. Instead of IEC terminal numbers, NEMA drawings assign a unique "wire number" to every continuous electrical node.
Old UK Standard (BS 3939)
If you are maintaining legacy British equipment (pre-1990s), you may encounter BS 3939 symbols. In this standard, the contactor coil was often drawn as a circle with a 'K' inside, and the contacts were drawn using a specific "hump" or "bridge" notation rather than the modern parallel-line-and-slash method. Schneider Electric and other manufacturers phased these out decades ago, but they still appear in old factory documentation.
Rows People Get Wrong & Troubleshooting Faded Markings
Even experienced technicians misread specific rows in the symbol reference table or miswire the physical block. Here are the most common errors and how to fix them.
The Rows People Get Wrong
- Row 5 vs Row 6 (Auxiliary NO vs NC): The most frequent mistake is wiring a safety interlock into a 13-14 (NO) terminal block when the schematic calls for a 21-22 (NC) block. Physically, the auxiliary blocks on contactors like the Eaton XTCE are color-coded or stamped, but if you are adding a side-mount aux block in the field, it is easy to grab an NO block (usually black or green) instead of an NC block (usually red or black with different stamps). Always verify the schematic slash direction.
- Row 1 vs Row 2 (AC vs DC Coil): Schematics often omit the polarity marks on DC coils if the drawing is rushed. If you apply 24V AC to a 24V DC coil, the contactor will chatter violently, overheat, and burn out the winding within minutes because the AC zero-crossings cause the magnetic field to collapse 120 times a second. Always check the coil label for the ~ (AC) or ⎓ (DC) symbol.
- Confusing Main (Row 3) with Aux (Row 5): Main contacts are designed for high current (e.g., 9A to 800A) and have arc chutes. Auxiliary contacts are typically rated for only 1A to 10A at control voltages. Never wire a 5A solenoid valve through an auxiliary 13-14 contact; it will weld the contacts shut.
Safe Interpretation When Markings are Faded or Missing
Before performing any continuity or resistance checks on a contactor, you must de-energize the panel, apply lockout/tagout (LOTO), and verify the absence of voltage with a properly rated CAT III or CAT IV multimeter. Never assume a control circuit is dead just because the motor is off.
When industrial environments degrade the printed terminal labels on a contactor, you must electrically verify the pinout before re-terminating wires. Use your multimeter in resistance (Ω) and continuity mode to map the device:
- Identify the Coil: Set your meter to Ohms. Probe the A1 and A2 terminals. A healthy 24VDC coil (like on a TeSys LC1D09) will read between 15Ω and 25Ω. A 120VAC coil will read higher, typically 100Ω to 300Ω. If it reads OL (open), the coil is burnt. If it reads 0.1Ω, it is shorted.
- Map the Main Poles: With power OFF and the contactor de-energized, probe across 1 and 2, 3 and 4, 5 and 6. It should read OL. Manually press the contactor armature down with an insulated tool. The reading should drop to < 0.1Ω. This confirms Rows 3/4.
- Map the Auxiliaries: Probe the small terminals on the side or front-mounted aux blocks. Find the pair that reads OL when resting, and < 0.1Ω when the armature is pressed—this is your NO (13-14 or 43-44). Find the pair that reads < 0.1Ω when resting, and OL when pressed—this is your NC (21-22 or 31-32). Label them immediately with a permanent marker or Brady wire labels.
By cross-referencing the physical multimeter readings with the schematic symbol logic, you can safely restore control to legacy panels even when the manufacturer's silk-screening has been entirely burned away by decades of heat and oil.
