The electrical symbol for a contactor consists of three distinct schematic elements: the electromagnetic coil, the main power contacts, and the auxiliary control contacts. While the physical hardware operates identically worldwide, the way these components are drawn on a schematic differs sharply depending on whether you are reading a North American (NEMA/ANSI) or International (IEC 60617) standard. Below is the direct translation guide you need to read, draw, and troubleshoot contactor circuits without guessing.
The Complete Contactor Symbol Reference Table
Before tracing wires, you must identify which drafting standard your schematic follows. North American prints typically use NEMA/ANSI Y32.2 symbols, while European, Asian, and modern global OEM prints use IEC 60617. This table maps the exact graphical representations and standard reference designators for both.
| Component | IEC 60617 Symbol Description | NEMA / ANSI Y32.2 Symbol Description | Standard Reference Designator |
|---|---|---|---|
| Electromagnetic Coil | Rectangle (often with 'K' or 'KM' inside) | Circle (often with 'M' or 'C' inside) | IEC: K / NEMA: M or C |
| Main Power Contacts (NO) | Two parallel lines with a diagonal slash and gap | Two parallel lines with a perpendicular gap and slash | 1L1-2T1, 3L2-4T2, 5L3-6T3 |
| Main Power Contacts (NC) | Two parallel lines with a diagonal slash, crossed by a solid line | Two parallel lines with perpendicular gap, crossed by solid line | Rare on main poles; usually aux |
| Auxiliary Contacts (NO) | Same as main NO, but tagged with 2-digit IEC code (e.g., 13-14) | Same as main NO, tagged with 'NO' or 'A' | 13-14, 43-44, 83-84 |
| Auxiliary Contacts (NC) | Same as main NC, tagged with 2-digit IEC code (e.g., 21-22) | Same as main NC, tagged with 'NC' or 'B' | 21-22, 31-32, 51-52 |
| Mechanical Interlock | Dotted line linking two coil rectangles | Dotted line linking two coil circles | N/A (Mechanical linkage) |
Hobbyists frequently search for 'NEC contactor symbols.' The National Electrical Code (NFPA 70) governs physical installation, wire sizing, and safety clearances. It does not dictate schematic symbols. When working in the US, you are actually looking for NEMA ICS standards and ANSI Y32.2 for graphical symbols.
Regional Variants and 'Rows People Get Wrong'
Even when you know the baseline standards, regional legacy systems and common drafting shortcuts cause massive confusion on the bench. Here is how to navigate the variants and avoid the most frequent misinterpretations.
Regional Standard Variants
- IEC 60617 (Global Standard): Dominates modern OEM equipment. Uses rectangles for coils and a strict two-digit numbering system for auxiliary contacts. The first digit indicates the sequence number of the contact on the block, and the second digit indicates the function (1-2 for NC, 3-4 for NO, 5-6 for NO time-delayed, 7-8 for NC time-delayed).
- NEMA / ANSI Y32.2 (North America): Uses circles for coils. Auxiliary contacts are typically labeled simply as 'NO' or 'NC' next to the symbol, or use letters like 'A' (NO) and 'B' (NC). Main terminals use L1/T1 nomenclature.
- Old UK (BS 3939 - Withdrawn): If you are troubleshooting a plant built in the UK before 1995, you will encounter BS 3939. In this obsolete standard, the contactor coil was often drawn as a circle with a literal 'C' inside, and auxiliary contacts lacked the modern IEC function-digit logic. Always verify the print revision date; if it predates 1996, expect BS 3939 conventions.
The 'Rows People Get Wrong' Notes
When reading the reference table above, these specific areas cause 90% of troubleshooting errors:
- Confusing Contactors with Relays: Graphically, an IEC contactor coil and an IEC control relay coil are identical rectangles. The only difference is the reference designator. Contactors use 'K' or 'KM' (e.g., KM1), while control relays use 'KA' or 'CR'. If you see a rectangle labeled 'CR1', it is a low-current control relay, not a motor contactor. Do not route 3-phase mains through it.
- Misreading the IEC Aux Digit Codes: A contact labeled '13-14' is Normally Open (NO). A contact labeled '21-22' is Normally Closed (NC). Beginners often assume '13-14' means pins 13 and 14 on a physical terminal block. In reality, it is a logical schematic identifier. The physical pin might be labeled '13' and '14' on a Schneider TeSys D block, but on a Siemens 3RT20, the physical layout might differ while the logical schematic remains 13-14.
- The 'Blowout Coil' Ghost: In older DC motor schematics, you may see a small coil symbol drawn in series with the main power contacts. This is not a second contactor; it is a magnetic blowout coil used to extinguish DC arcs. Treating it as a separate control coil will break your understanding of the circuit.
Safe Interpretation When Markings Are Faded or Missing
Schematics are only half the battle. On a 15-year-old industrial panel, the physical contactor nameplate is often baked off by heat, and the terminal stickers are peeled away. When you cannot match the physical hardware to the electrical symbol for the contactor, you must safely map the device using a multimeter.
Before performing any continuity testing on a contactor, you must de-energize the panel, apply Lockout/Tagout (LOTO), and verify zero voltage on all incoming lines (L1, L2, L3) using a CAT III or CAT IV rated meter. Capacitors in the drive circuit and snubber networks across the coil can retain lethal charges.
Follow this bench-proven sequence to map an unreadable 3-pole contactor (like an Eaton XTCE or Allen-Bradley 100-C):
- Locate the Coil (A1 / A2): Look for two smaller terminals, usually offset from the main power lugs. Set your multimeter to resistance (Ohms). An AC coil (e.g., 120VAC) will typically read between 15Ω and 50Ω. A DC coil (e.g., 24VDC) will read much higher, often 100Ω to 400Ω. If it reads OL (open), the coil is burned. If it reads 0.1Ω, it is shorted.
- Map the Main Poles (L1-T1, L2-T2, L3-T3): These are the largest physical lugs. With the contactor de-energized and at rest, they should read OL (open). Manually press the plastic armature down with an insulated screwdriver. The meter should drop to less than 0.5Ω. If it reads higher, the main contacts are pitted or carbon-fouled and the contactor must be replaced.
- Identify the Auxiliary Block: Aux contacts are usually smaller screw terminals on the side or top. Use the IEC function-digit rule to verify them. Probe pairs of terminals while pressing the armature. Terminals that read OL at rest and <0.5Ω when pressed are your NO contacts (function digit 3-4). Terminals that read <0.5Ω at rest and OL when pressed are your NC contacts (function digit 1-2).
- Watch for Snubber False Positives: Many modern contactors have an RC snubber or a flyback diode wired directly across the A1/A2 coil terminals to protect PLC outputs. If you measure across A1/A2 with a multimeter in continuity/diode mode, the meter might beep or show a voltage drop, leading you to falsely believe the coil is shorted. Always use the standard Ohms range to measure the actual coil winding resistance.
Practical Schematic Reading: Cross-Referencing and Hardware Selection
Modern IEC schematics rarely draw the contactor coil and its contacts in the same physical location on the page. Instead, they use a cross-reference grid system to keep the power circuit (usually drawn vertically on the left) and the control circuit (drawn horizontally on the right) logically separated.
When you see the electrical symbol for a contactor coil on page 4, column 2, it will be labeled K1 /4.2. This tells you that the main power contacts for K1 are located on page 4, column 2, while the auxiliary NO contact used for the holding circuit might be located at /5.1 (page 5, column 1). If you ignore the grid reference and assume the contacts are drawn directly below the coil, you will completely misread the logic sequence.
When replacing a contactor based on a schematic, never substitute a NEMA-rated device for an IEC-rated device without checking the utilization category. The schematic symbol might look identical, but the hardware differs vastly:
- IEC Contactors (e.g., Schneider LC1D09, Siemens 3RT2015): Rated by AC-3 (squirrel cage motor starting). They are compact, optimized for specific loads, and typically rated for 100,000 electrical cycles at their exact AC-3 current rating.
- NEMA Contactors (e.g., Eaton C25, Allen-Bradley 100-A): Sized by NEMA frame (Size 00, Size 0, Size 1). They are physically much larger, heavily overbuilt, and designed for broader voltage ranges and harsher environments, often surviving 1,000,000+ cycles.
Always match the utilization category (AC-1 for resistive, AC-3 for inductive motors, AC-5a for fluorescent lighting) printed on the physical hardware to the load described in the schematic, regardless of how the electrical symbol for the contactor is drawn on the page.
