When you open an industrial control panel, the schematic on the inside of the door is your only reliable map. The motor starter symbol is the Rosetta Stone for that map, representing the combination of a contactor (the muscle) and an overload relay (the brain). However, a symbol drawn to North American NEMA standards looks fundamentally different from one drawn to international IEC standards. Misreading these symbols leads to miswired control circuits, tripped breakers, and burned-out coils.

Below is the direct reference for decoding these symbols, followed by the exact protocols for tracing them when the physical markings on the starter are faded or missing.

The Complete Motor Starter Symbol Reference Table

This table maps the schematic symbols you will see on drawings to their physical counterparts on the DIN rail. Note that NEMA (ANSI/IEEE 315) favors functional simplicity, while IEC (IEC 60617) uses strict alphanumeric terminal designations.

Component NEMA / ANSI Symbol Style IEC Symbol Style & Designation Function in Practice
Contactor Coil Simple rectangle or circle. Labeled 'M' (Motor) or 'CR' (Control Relay). Rectangle with diagonal lines. Labeled with '-K' prefix (e.g., -K1, -K2). The electromagnetic coil (usually A1/A2 terminals) that pulls in the main power contacts when energized.
Main Power Contacts Three parallel NO (Normally Open) contacts, often linked by a dashed mechanical line. Three separate NO contacts. Labeled with line/load numbers (1-2, 3-4, 5-6) or L1/T1. Switches the 3-phase high-current power to the motor. Rated for high inrush (AC-3 utilization).
Overload Thermal Element Box with a line through it, or a heater symbol in series with the power lines. Thermal element symbol in series with main contacts. Labeled '-F' (e.g., -F1). Heats up proportionally to motor current. Bimetallic strip or eutectic alloy melts/bends to trigger a trip.
Overload Trip Contact NC (Normally Closed) contact in the control circuit, labeled 'OL'. NC contact labeled '95-96'. Cross-referenced to the '-F' overload device. Wired in series with the contactor coil. Opens when the thermal element trips, dropping the coil to stop the motor.
Auxiliary NO Contact NO contact, often used for sealing/holding circuits. NO contact labeled '13-14'. Cross-referenced to the '-K' contactor. Closes when the starter pulls in. Used to maintain the circuit after the momentary start button is released.
Auxiliary NC Contact NC contact, used for electrical interlocks. NC contact labeled '21-22'. Opens when the starter pulls in. Used to prevent two reversing contactors from engaging simultaneously.

NEMA vs. IEC: Which Standard Applies to Your Panel?

Before you start tracing wires, you must identify which standard the panel builder used. Mixing NEMA and IEC logic on a single drawing is a recipe for disaster.

How to tell at a glance: If the schematic uses simple geometric shapes with letters like 'M', 'CR', and 'OL', you are looking at a NEMA/ANSI drawing. If the schematic uses strict alphanumeric codes (Q for power switches, F for protection, K for contactors) and two-digit terminal numbers (13/14, A1/A2), you are looking at an IEC drawing.
  • NEMA (National Electrical Manufacturers Association): Dominant in the US and Canada. NEMA standards (like NEMA ICS 2) historically focused on physical frame sizes (Size 00 through Size 8) and extreme mechanical robustness. The symbols are simpler but assume the technician knows the physical layout of standard US starters (like Allen-Bradley Bulletin 500).
  • IEC (International Electrotechnical Commission): The global standard (IEC 60947-4-1), dominant in Europe, Asia, and increasingly common in modern US OEM panels. IEC focuses on utilization categories (e.g., AC-3 for squirrel cage motors) and modular components. The symbols are highly detailed and strictly cross-referenced.
  • Old UK (BS 3939): If you are working on legacy equipment in the UK pre-1990s, you may encounter BS 3939 symbols. These used different cross-referencing and contact representations. Fortunately, modern UK panels have fully migrated to IEC 60617, but always verify the drawing revision date if the symbols look "off".

Rows People Get Wrong: Common Schematic Misreads

Even experienced electricians and controls technicians fall into specific traps when reading motor starter symbols. Avoid these three critical errors:

  1. Confusing the Overload Heater with the Overload Trip Contact: This is the most common troubleshooting mistake. The heater (thermal element) is drawn in the 3-phase high-voltage power circuit. The trip contact (usually 95/96) is drawn in the low-voltage (120V/24V) control circuit. If a motor won't start, do not check the control circuit voltage at the heater; check it at the 95/96 terminals.
  2. Misidentifying Auxiliary vs. Main Contacts: On IEC drawings, main contacts are usually unlabeled or simply marked L1/T1, L2/T2, L3/T3. Auxiliary contacts always have two-digit functional numbers. If you see '13/14' on the drawing, it is a low-current auxiliary contact (usually rated 10A max). Never wire a high-current load through an auxiliary contact.
  3. Ignoring the Mechanical Linkage Dashed Line: On NEMA drawings, a dashed line connecting multiple contacts means they are mechanically linked—when the coil pulls in, all those contacts move simultaneously. If you replace a contactor and forget to reinstall the physical mechanical interlock block between two reversing contactors, the dashed line on the schematic won't save you from a catastrophic phase-to-phase short.

Faded or Missing Markings: Safe Interpretation Protocol

In older panels, UV exposure, heat, and oil mist destroy the physical labels on contactors and overload relays. When the schematic says "K1" but the physical starter is completely blank, follow this safe interpretation protocol to verify the wiring matches the drawing.

SAFETY FIRST: Never trace or measure continuity on control circuits while energized. De-energize the panel, apply Lockout/Tagout (LOTO), and verify dead with a properly rated CAT III/IV multimeter before opening terminal covers.
  1. Locate the Coil Terminals: Find A1 and A2. Apply your multimeter in resistance mode. A healthy AC contactor coil (e.g., 120VAC) will typically read between 15 and 50 ohms. A DC coil will read significantly higher. If it reads open (OL), the coil is burned out regardless of what the schematic says.
  2. Decode the IEC Two-Digit Numbers: If the physical labels are gone, look at the terminal block molding. IEC auxiliary contacts follow a strict rule. The first digit is the sequence number (1, 2, 3...). The second digit is the function:
    • 1 or 2: Normally Closed (NC). Example: 21/22.
    • 3 or 4: Normally Open (NO). Example: 13/14.
    • 5 or 6: Fault/Overload specific. Example: 95/96 (NC overload trip).
  3. Verify the Mechanical State: Use a small insulated screwdriver to manually press the contactor's plunger. Use your multimeter in continuity mode to verify which terminals close and which open. This physically confirms if a mystery contact is NO or NC, overriding any faded schematic assumptions.

Decision Path: Selecting the Right Starter for Your Application

When you are reading a schematic to specify a replacement motor starter, or designing a new panel, use this decision matrix to terminate your search with a concrete part number. This path assumes a standard 3-phase AC induction motor application.

Application Parameter Condition Required Specification
Panel Standard US Legacy / Heavy Industry NEMA Sized Starter (Size 0, 1, 2...)
Panel Standard Global OEM / Modern US / Compact IEC Utilization Category AC-3
Motor Load Type Standard starting, stopping at speed AC-3 Rating (e.g., 9A at 400V)
Motor Load Type Jogging, plugging, rapid reversal AC-4 Rating (Requires heavier contactor)
Control Voltage 120V AC (Standard US control) Coil suffix for 110-120V 50/60Hz
Control Voltage 24V DC (PLC output driven) Coil suffix for 24V DC (Solid state or low power)

The Concrete Pick: What to Buy

If you are building a modern panel, upgrading an OEM machine, or replacing a failed unit in a standard industrial environment (up to 10 HP / 7.5 kW at 480V AC), default to the Schneider Electric TeSys D-Line series. It is the global benchmark for IEC starters, widely available, and perfectly documented.

Exact Part Number to Order: LC1D09G7 (Contactor) + LRD10 (Overload Relay)

  • LC1D09: TeSys D contactor, 9A AC-3 rating at 440V (covers up to 4kW/5HP, step up to LC1D18 for 10HP).
  • G7: The coil voltage code for 120V AC 50/60Hz. (Use BD for 24V DC if driven by a PLC).
  • LRD10: TeSys LRD thermal overload relay, adjustable trip range 4.0A to 6.0A. (Select the specific LRD model that brackets your motor's Full Load Amps found on the nameplate).

By mapping the schematic symbols to these specific IEC terminal designations and part numbers, you eliminate the guesswork. The drawing tells you the logic; the IEC standard tells you the wiring; the specific part number gets the machine running.