The Complete Magnetic Starter Symbol Reference Table
Use this table to decode the specific graphical elements and terminal designations you will encounter on a motor control schematic. This covers both the physical component and its schematic representation.
| Component / Function | NEMA Symbol Representation (NFPA 79) | IEC Symbol Representation (IEC 60617) | Terminal Designations & Practice Notes |
|---|---|---|---|
| Contactor Coil | Circle with 'M' or 'CR' inside, often boxed. | Rectangle with diagonal line or 'KM1' alphanumeric tag. | NEMA: Labeled by device. IEC: A1 (positive/hot) and A2 (neutral/return). Expect 15-50 ohms resistance on 120VAC coils. |
| Main Power Contacts (NO) | Three parallel NO switches ganged together, boxed. | Three parallel NO switches, labeled -KM1, often with slash marks for 3-pole. | Line side: L1, L2, L3 (or 1, 3, 5). Load side: T1, T2, T3 (or 2, 4, 6). Carries full motor FLA. |
| Thermal Overload Relay (Heater) | Box with a wavy line or 'OL' inside, in series with main contacts. | Rectangle with overlapping boxes, tagged -F1 or -F2. | Wired directly below T1/T2/T3. Sized to 100-115% of motor Full Load Amps (FLA). |
| Overload Trip Contact (NC) | Single NC switch labeled 'OL' or 'M' in the control circuit. | Single NC switch tagged with the overload relay number (e.g., -F1). | Terminals 95-96. Must be wired in series with the contactor coil to drop out power on a thermal trip. |
| Auxiliary Contacts (NO/NC) | Small NO/NC switches attached to the main contactor box. | Switches tagged with the contactor ID (e.g., -KM1) with 2-digit codes. | NO: 13-14. NC: 21-22. Used for holding circuits, PLC feedback, and interlocks. Rated for control current only (<2A). |
Regional Variants: NEMA (US) vs. IEC (Global) vs. Old BS (UK)
The way a magnetic starter symbol is drawn tells you immediately where the machine was designed and what physical footprint to expect inside the enclosure. Never assume a NEMA-rated physical starter will fit in an IEC-designed panel without checking the dimensions.
NEMA (North America - NFPA 79 / ANSI)
NEMA symbols are highly graphical, often enclosing the coil and contacts in a dashed or solid box to indicate they are part of the same physical assembly. NEMA starters are physically massive, built with heavy copper contacts and robust arc chutes. They are classified by 'Size' rather than exact amperage:
- Size 00: 2 Amps
- Size 0: 9 Amps
- Size 1: 27 Amps
- Size 2: 45 Amps
- Size 3: 90 Amps
IEC (Global - IEC 60617 / IEC 81346)
IEC symbols are minimalist and functional. Instead of drawing boxes around related components, IEC uses alphanumeric cross-referencing. If the coil is labeled -KM1, every auxiliary contact and main pole associated with that contactor will also carry the -KM1 tag, even if they are drawn on page 4 of the schematic and the coil is on page 1. IEC starters are compact, DIN-rail mounted, and rated by utilization category (e.g., AC-3 for standard squirrel-cage motor starting).
Old BS (UK Pre-2000s)
Older British Standard schematics (BS 3939) look very similar to IEC but often use legacy reference designations like C1 for contactors and OL1 for overloads, rather than the modern IEC 81346 -KM and -F prefixes. If you are retrofitting a machine built in the UK before the harmonization of EN standards, expect these legacy tags.
Rows and Symbols People Get Wrong (And How to Fix Them)
The Mistake: Wiring the 95-96 Overload Trip Contact into the Main Power Line
Beginners often see the overload relay symbol on the schematic and assume its NC trip contact (terminals 95 and 96) is meant to break the main 3-phase motor power. They wire the 480V motor feed through the 95-96 terminals.
The Reality: The 95-96 contact is a low-current auxiliary switch rated for roughly 2A to 10A max. If you put motor Full Load Amps (e.g., 20A) through it, the contact will weld shut or catch fire when the overload trips. The Fix: The 95-96 NC contact belongs exclusively in the low-voltage control circuit (120VAC or 24VDC), wired in series with the contactor coil (A1/A2). When the thermal heater element trips, 95-96 opens, de-energizing the coil, which in turn drops the heavy-duty main power poles (L1-T1, etc.).
The Mistake: Confusing Auxiliary (13-14) with Main (L1-T1) Contacts
On a schematic, a standard NO contact looks identical whether it is a main power pole or an auxiliary holding contact. The Fix: Look at the terminal numbering. If the terminals are labeled 13 and 14 (IEC) or lack L/T designations (NEMA), it is an auxiliary contact. Do not route motor current through 13-14; use it only for the seal-in (holding) circuit or PLC feedback.
Decision Path: Which Starter and Symbol Standard to Pick
Use this decision matrix to select the correct physical starter and schematic standard for your project. This eliminates the 'it depends' ambiguity and terminates in a concrete hardware selection.
| Application Scenario | Required Standard | Concrete Hardware Pick (2026 Availability) |
|---|---|---|
| New North American industrial panel (Heavy duty, high fault current, spacious enclosure) | NEMA (NFPA 79) | Eaton XTCE Series (e.g., XTCE027B for a 27A Size 1 equivalent). Offers NEMA ruggedness with modern footprint. |
| Global export machinery or compact MCC (DIN-rail mounted, space-constrained, IEC schematics) | IEC (IEC 60617) | Schneider TeSys D (LC1D series) with LRD thermal overload. (e.g., LC1D25 + LRD22 for a 25A AC-3 motor). |
| Retrofitting an existing legacy US panel (Replacing a burnt-out Square D or Furnas starter) | NEMA (Match existing) | Siemens Sirius 3RT10 (NEMA rated equivalents) or direct OEM replacement from the original panel builder. |
Safe Interpretation When Markings Are Faded or Missing
On older machinery, the physical terminal markings on the magnetic starter (the stamped numbers on the contactor and overload relay) are often obscured by grease, arc dust, or UV fading. Never guess the wiring based on physical position alone. Follow this multimeter verification sequence before energizing the panel.
- De-energize and Lockout: Shut off the main disconnect. Verify zero voltage at L1, L2, and L3 using a CAT III or CAT IV rated meter (e.g., Fluke 117). Test your meter on a known live source before and after.
- Identify the Coil (A1/A2): Set your multimeter to Ohms. Probe the smaller terminals on the top or side of the contactor. You are looking for a resistance reading between 10 and 100 ohms (for 120VAC/240VAC coils) or 200-500 ohms (for 24VDC coils). If it reads 'OL' (open), the coil is burnt out. If it reads near 0 ohms, the coil is shorted.
- Trace the Main Poles: With the contactor manually depressed (using a non-conductive tool to push the armature down), check for continuity between the top three large terminals and the bottom three large terminals. These are your L1-T1, L2-T2, L3-T3 main power poles.
- Map the Auxiliaries and Overload Trip: Probe the smaller side-mounted or front-mounted terminals. With the contactor at rest, find the pair that shows continuity (this is your NC, likely 21-22 or the 95-96 overload trip). Manually depress the contactor; the continuity should break, and a new pair (NO, likely 13-14) should show continuity. Label them immediately with a fine-tip paint pen.
For deeper reference on schematic standards and utilization categories, consult the NFPA 79 Electrical Standard for Industrial Machinery for North American NEMA requirements, and review the differences between NEMA and IEC motor starters to ensure your physical hardware matches your schematic symbols.
