Heating Cable / Heat Trace Symbol
Definition: The Heating Cable / Heat Trace symbol represents a self-regulating or constant-wattage resistive cable installed along pipes, roofs, or floors that converts electrical energy to heat, shown in wiring diagrams and electrical schematics as a two-terminal load element (Line L and Neutral N) governed by installation standards such as IEC 60519-1 (industrial electroheat) and NFPA 70 / NEC Article 426 (fixed outdoor electric deicing and snow-melting equipment).
Also known as: heat trace, heat tracing cable, electric heat tape, pipe heating cable, self-regulating heater, parallel resistance heater, frost protection cable.
What the Heating Cable / Heat Trace symbol means
The Heating Cable / Heat Trace symbol denotes a resistive electrical element — either a constant-wattage parallel-core cable or a self-regulating polymer-core cable — that is wrapped around or attached to a pipe, roof edge, gutter, floor slab, or vessel to prevent freezing, maintain process temperatures, or provide floor comfort heating. In a wiring diagram, the symbol appears as a two-terminal load drawing power from Line (L) and Neutral (N) of an AC supply, often through a thermostat or ground-fault protection device.
Heat-trace cables are deployed in industrial process piping, residential plumbing freeze protection, roof de-icing systems, and electric underfloor heating. The symbol in a wiring diagram identifies the heater load's power rating, supply voltage (typically 120 V or 240 V AC), and control interface (thermostat contact, controller relay output), allowing electricians and engineers to size protection devices and calculate current draw.
How to identify the Heating Cable / Heat Trace symbol
The heating cable symbol is drawn as a rectangular or elongated block with a wavy or zigzag internal line indicating resistive heating, similar to a heating element symbol, but labelled to indicate its linear or distributed nature. In simplified one-line diagrams it may appear as a simple resistor rectangle with a temperature annotation. Two terminals extend from the block: L (line/phase) on the left and N (neutral) on the right, representing the two conductors of the AC supply connected to the cable's cold-lead ends.
Function in a circuit
A heating cable functions as a distributed resistive load: electric current flowing through the resistive core generates Joule heat (P = I²R) along the cable's length. Self-regulating cables automatically reduce power output as temperature rises (the polymer core increases resistance at higher temperatures), providing inherent overheat protection. Constant-wattage cables deliver a fixed watts-per-metre regardless of temperature and require external thermostatic control to prevent overheating.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60519-1 covers safety requirements for industrial electroheat installations including heat tracing. IEC 62395-1 and IEC 62395-2 specifically address resistance heating cables for industrial applications. The IEC symbol for a heating element is a rectangle with a resistive-heating annotation (series resistor block). |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 / NFPA 70 NEC Article 426 covers fixed outdoor electric deicing; NEC Article 424 covers fixed indoor electric heating equipment. In North American wiring diagrams, the heating cable is represented as a resistance heating element symbol with the designator EH (electric heater) or HS (heater element). |
| Key difference | IEC and ANSI/IEEE use essentially the same rectangular heating-element block symbol for heat tracing; the main practical difference is in system standards — IEC 62395 vs. IEEE 515 (IEEE Standard for the Testing, Design, Installation, and Maintenance of Electrical Resistance Heat Tracing for Industrial Applications). |
Terminals / pins
| Pin | Name |
|---|---|
| a | L |
| b | N |
Typical values
Typical power output: 5–60 W/m (self-regulating) or 10–25 W/m (constant-wattage). Supply voltage: 120 V or 240 V AC (single-phase). Maximum circuit length: 30–150 m depending on cable type and supply voltage. Operating temperature range: −60 °C to +85 °C (cable surface). Ground-fault protection: 30 mA GFCI or 5 mA heat-trace GFPE required by NEC.
Where the Heating Cable / Heat Trace symbol is used
- Residential pipe freeze protection: heat trace applied to water supply pipes in unheated crawl spaces or attic runs to prevent freezing at temperatures below 0 °C
- Roof and gutter de-icing: constant-wattage cable installed in zigzag pattern on roof edges and inside gutters and downspouts to prevent ice dam formation
- Industrial process pipe maintenance: self-regulating heat trace on chemical, oil, or water process pipes to maintain fluid viscosity or prevent freezing in outdoor installations
- Electric floor heating: embedded parallel-resistance mat or cable beneath tile or hardwood flooring for comfort heating or snow-melt in entryways
- Vessel and tank freeze protection: heat trace wrapped around storage tanks, valves, and instrumentation (pressure transmitters, flow meters) to maintain operability in cold climates
- Fire sprinkler system pipe protection: heat trace on dry-pipe or pre-action sprinkler pipes in unheated spaces such as loading docks and parking structures
Example
In a residential freeze-protection wiring diagram, a 240 V self-regulating heat-trace cable (rated 15 W/m) appears as the heating cable symbol connected through a 30 mA GFCI breaker and a pipe thermostat (set to activate at 3 °C). The Line (L) terminal connects to the hot conductor of the GFCI output, and the Neutral (N) terminal returns to the panel neutral bar.
Key facts
- The Heating Cable / Heat Trace symbol represents a two-terminal resistive heating load with Line (L) and Neutral (N) connection points, powered by 120 V or 240 V AC single-phase supply.
- NEC Article 426 (NFPA 70) requires ground-fault protection of 30 mA (or 5 mA for personnel protection) on all fixed outdoor electric deicing and snow-melting circuits using heat trace.
- Self-regulating heat-trace cables automatically reduce their power output as ambient temperature rises because their conductive polymer core increases in resistance with heat, providing inherent overheat protection without a thermostat.
- IEC 62395-1 and IEEE 515 are the primary design and installation standards for industrial resistance heat-tracing systems, covering cable testing, circuit design, and installation practices.
- Constant-wattage heat trace delivers a fixed power output (watts per metre) regardless of temperature and must be controlled by a thermostat or temperature controller to prevent surface damage or fire.
- Typical self-regulating heat-trace output ranges from 5 to 60 W/m; the cable's maximum exposure temperature (MET) must exceed the pipe insulation temperature to avoid degradation.
- The Heating Cable symbol in wiring diagrams always includes a control device — typically a line-sensing thermostat or electronic controller — in series between the supply breaker and the heating cable cold-lead terminals.
- In industrial P&ID and electrical drawings, heat trace is indicated by a dashed line parallel to the pipe with an 'ET' (electric tracing) callout and a circuit number referencing the heat-trace panel.
Diagrams that use this symbol
Frequently asked questions
What does the heating cable symbol mean in a wiring diagram?
The heating cable symbol represents a resistive electric heat-trace element that converts electrical energy to heat along its length. It is shown as a two-terminal load (Line L and Neutral N) and is always paired with overcurrent protection and usually a thermostat or GFCI in the wiring diagram.
What does the heating cable / heat trace symbol look like?
The heat trace symbol appears as a rectangular block — similar to a resistor or heating element — with a wavy or zigzag internal line and two terminals labelled L (line) and N (neutral). In P&ID drawings it is often shown as a dashed line running parallel to a pipe, annotated with 'ET' for electric tracing.
What standard governs heat trace wiring diagrams?
In North America, NEC Articles 424 (fixed indoor heaters) and 426 (fixed outdoor deicing) in NFPA 70 govern heat-trace installations. IEEE 515 covers industrial resistance heat-tracing design and installation. Internationally, IEC 62395-1 and IEC 62395-2 apply to resistance heating cables for industrial applications.
What is the difference between self-regulating and constant-wattage heat trace?
Self-regulating heat trace automatically reduces power as temperature rises because its polymer core resistance increases with heat, so it cannot overheat itself. Constant-wattage heat trace delivers a fixed watts-per-metre output regardless of temperature and requires an external thermostat to prevent overheating or fire.
What ground-fault protection is required for heat trace circuits?
NEC Article 426 requires ground-fault protection for heating equipment — typically a 30 mA GFCI breaker or a dedicated 5 mA heat-trace GFPE device. This protection detects leakage current through degraded cable insulation before it reaches a level that causes fire or shock hazard.
What voltage does a heating cable operate at?
Most residential heat-trace cables operate at 120 V AC or 240 V AC single-phase. Industrial heat-trace systems often use 240 V or 277 V single-phase. The supply voltage and cable resistance determine the circuit length; 240 V supply allows approximately twice the circuit length compared to 120 V for the same cable type.
What is the designator for a heating cable in a schematic?
In North American electrical drawings, heating elements are commonly designated EH (electric heater) or HS (heater section). In IEC industrial schematics the designator EK (electroheat element) or a locally defined tag referencing the heat-trace circuit number is used.
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