Thermal Overload Device Symbol: IEC vs NEMA Reference & Wiring Guide

The thermal overload device symbol represents a protective mechanism—typically a bimetallic strip or eutectic alloy heater—that physically bends or melts when motor current exceeds the Full Load Amps (FLA) for a sustained period. On a schematic, this symbol is split into two distinct electrical functions: the power-circuit heater element and the control-circuit trip contact. Understanding how these symbols map to physical terminals is critical for wiring motor starters and troubleshooting control panels.

Thermal Overload Device Symbol Reference Table

Below is the complete mapping of thermal overload symbols to their physical terminals and schematic functions. This table bridges the gap between the line diagram on the panel door and the physical thermal overload relay bolted to the contactor.

Symbol Function	IEC 60617 Visual	NEMA ICS 2 Visual	Physical Terminals	Practical Meaning in Circuit
Heater Element (Power)	Rectangle with an inward-bowing line	Box with a zigzag heater coil	L1/L2/L3 to T1/T2/T3	Senses motor current. The inward bow represents the bimetallic strip bending under heat. Carries full motor FLA.
NC Trip Contact (Control)	Standard NC contact symbol (often linked by a dashed line to the heater)	Standard NC contact symbol, explicitly labeled 'OL'	95 and 96	Wired in series with the motor contactor coil. Opens to drop the contactor when the heater trips.
NO Fault Indication Contact	Standard NO contact symbol (linked to heater)	Standard NO contact symbol, labeled 'OL'	97 and 98	Closes only when tripped. Used to send a fault signal to a PLC input or illuminate a red HMI indicator.
Manual/Auto Reset Switch	Small toggle symbol adjacent to the NC contact	Rarely shown on basic schematics	Physical dial on relay face	Determines if the NC contact resets automatically when cool, or requires a manual push to re-arm.

Regional Standard Variants: IEC vs. NEMA vs. Old UK

While the physics of a thermal overload remain constant, the way engineers draw them depends heavily on the regional standard governing the schematic.

IEC 60617 (Global / Europe / Modern US)

The IEC standard uses a minimalist approach. The heater element is drawn as a rectangle with a single line bowing inward. The control contacts are drawn separately on the control circuit page, linked to the heater via a dashed mechanical line or a common alphanumeric tag (e.g., F2 for the overload, with F2-95 and F2-96 for the contacts). IEC schematics assume the reader knows that the power and control circuits are electrically isolated but mechanically linked.

NEMA ICS 2 (North America Legacy)

NEMA schematics, heavily used in older US industrial plants, tend to be more literal. The heater element is often drawn with a distinct zigzag resistor/heater coil symbol inside a box. NEMA ladder diagrams frequently place the NC overload contact directly in the rung alongside the contactor coil, labeling it explicitly as OL rather than using cross-reference alphanumeric tags. If you are troubleshooting a panel built in the US before 2005, expect NEMA-style literalism.

Old UK (BS 3939)

Largely superseded by IEC 60617, old British Standard schematics used a variation of the IEC symbol but often enclosed the entire device (heater and contacts) in a single dotted boundary box to indicate a single physical component. You will only encounter this in legacy municipal water or transit infrastructure in the UK. Treat the symbols inside the box exactly as you would modern IEC symbols.

The "Rows People Get Wrong" Field Notes

When reading overload schematics or wiring physical relays, journeyman electricians and hobbyists consistently make the same three mistakes. Avoid these to prevent immediate nuisance tripping or, worse, a failure to protect the motor.

Decision Tree: Selecting and Identifying Overload Relays

Use this decision path to terminate your component selection with a concrete part number and trip class. Do not guess; match the application to the technology.

Application Condition	Technology Choice	Concrete Default Pick (2026 Market)
Standard 3-phase induction motor, < 50A, clean indoor environment, fixed speed.	Bimetallic Thermal Overload (Class 10 or 20)	Schneider TeSys LRD (e.g., LRD14 for 7-10A FLA). Set dial to 100% of motor nameplate FLA.
Motor is in a high-vibration environment, or subject to frequent jogging/inching (high inrush cycles).	Eutectic Alloy Melting Relay	Allen-Bradley 193-EA series. Eutectic alloy does not suffer from bimetallic fatigue during heavy jogging.
Wide ambient temperature swings (-30°C to +50°C), need precise phase-loss protection, or VFD bypass applications.	Solid-State Electronic Overload	Eaton PKZM0 or TeSys LRE. Electronic sensing ignores cabinet heat and detects phase loss in < 3 seconds.
Motor > 50A (e.g., 100HP+ compressor).	CT-Driven Electronic Overload	Siemens 3RU2 with external current transformers. Direct-mount bimetallics are too bulky and expensive above 80A.

The Universal Default: If you have a standard 5HP to 15HP 480V 3-phase motor in a standard industrial panel and no special constraints, buy a bimetallic TeSys LRD series relay, set to Class 10 trip, dialed to exactly the motor nameplate FLA. It is the industry workhorse, universally understood, and cheap to replace.

Safe Interpretation When Markings Are Faded or Missing

Industrial environments are harsh. UV exposure, oil mist, and heat frequently destroy the stamped terminal diagrams on the face of an overload relay. Never guess terminal functions based on physical position, as NEMA and IEC pinouts differ.

Follow this multimeter verification procedure to safely identify the control contacts without relying on the faded symbol:

1. De-energize and Lock Out: Open the main disconnect. Verify zero voltage on L1, L2, and L3 using a CAT III rated meter. Thermal overload power terminals carry line voltage when the contactor is pulled in.
2. Verify Power Path: Set your Fluke 87V to Ohms (Ω). Measure across L1 to T1, L2 to T2, and L3 to T3. You should read a very low resistance (typically 0.1Ω to 0.5Ω depending on the heater size). If you read Open Line (OL), the internal heater element is burned open and the entire relay must be replaced.
3. Identify the NC Control Contact (95-96): Switch the meter to Continuity (beep mode). Probe the pairs of small terminals on the front or side. You will find one pair that beeps continuously. These are your NC terminals (universally 95 and 96 on IEC, though NEMA may use different numbering). Wire your contactor coil stop circuit through these.
4. Identify the NO Fault Contact (97-98): Find the pair of terminals that reads OL (open). These are your NO fault indication terminals.
5. Perform the Mechanical Trip Test: Locate the physical "TEST" button on the relay face (usually a small red or black plastic tab). Press it firmly with a flathead screwdriver. This mechanically simulates the bimetallic strip bending.
   • The NC pair (95-96) must immediately stop beeping (open).
   • The NO pair (97-98) must immediately start beeping (close).
6. Reset: Press the blue or red "RESET" button. The continuity states must return to their original condition. If they do not, the internal mechanical linkage is fused or broken; discard the relay.