Type K Thermocouple Extension & Compensating Cable
TC-K-EXT/COMP
Moves the reference junction from the sensor head to a stable terminal, with a known error.
Thermocouple & Compensating Cable
A thermocouple measures the temperature difference between the hot junction and the point where the thermocouple metal meets copper. Extend the circuit with ordinary copper and you
A thermocouple measures the temperature difference between the hot junction and the point where the thermocouple metal meets copper. Extend the circuit with ordinary copper and you simply move that reference point to wherever the copper starts — usually a terminal block at an uncontrolled temperature. Extension and compensating cable exists to move the reference junction to a known, stable place, and the alloy in the conductor decides how much error that move introduces.
That is why the alloy matters and why the two grades behave differently. Extension cable uses the same alloys as the thermocouple itself; compensating cable uses cheaper substitute alloys chosen to reproduce the thermoelectric EMF over a limited range. Voltcore supplies both for Type K, together with Pt100 RTD signal cable and high-temperature PTFE and FEP insulated grades, all identified to IEC 60584-3 colour coding.
4 constructions in Thermocouple & Compensating Cable
KX and KCA Alloy Conductor Compensating Cable
TC-KX/KCA
Same-alloy KX or substitute-alloy KCA, with the tolerance range stated on the datasheet.
Pt100 RTD 3-Wire & 4-Wire Signal Cable
RTD-PT100-3/4W
Screened triples and quads that keep lead resistance out of the temperature reading.
PTFE / FEP Insulated High-Temperature Extension Cable
TC-K-PTFE/FEP
Insulation that survives the route when a PVC sheath would already have failed.
Extension cable or compensating cable — what is the real difference?
Extension cable, coded KX for Type K, is made from the same nominal alloys as the thermocouple: a nickel-chromium positive leg and a nickel-silicon negative leg. Its EMF matches the thermocouple curve across the whole range, within the tolerance class, so the pair of junctions it creates at each end of the cable cancels whatever temperature they share.
Compensating cable, coded KCA for Type K, replaces those nickel alloys with copper and a copper-nickel alloy chosen to reproduce the Type K EMF over a limited span. It is cheaper and has a much lower loop resistance, but the match only holds over the stated range — roughly 0 °C to 150 °C — and the error grows quickly once the cable runs hotter.
- KX extension: same alloys as the sensor, tolerance Class 1 ±1.5 °C or Class 2 ±2.5 °C, valid over a wider span.
- KCA compensating: copper and copper-nickel legs, lower cost and lower loop resistance, valid over a narrower span.
- Outside the stated range the compensating cable is not merely less accurate — it is wrong, and by an unpredictable amount.
- The two grades must never be joined in one circuit; the added junction of dissimilar metals becomes another thermocouple.
How much error does lead resistance add to a Pt100?
Resistance temperature detectors are resistive devices, so every ohm in the leads reads as temperature. A Pt100 to IEC 60751 changes by 0.385 Ω per kelvin, so 1 Ω of lead resistance is about 2.6 °C of error and 3 Ω is nearly 8 °C. That is the problem the third and fourth wires exist to solve.
A two-wire connection adds both lead resistances directly into the measurement and is only acceptable over a metre or two. Three-wire works because the instrument measures the difference between the leads and cancels them, provided all three leads have equal resistance — which means equal length, equal cross-section and sound terminations. Four-wire is genuinely immune: a constant current flows through one pair while a high-impedance voltmeter reads the other, so no lead current means no lead voltage drop.
- Two-wire: acceptable only for very short runs or where 5 °C of error is tolerable.
- Three-wire: the industrial standard; cancel the leads by keeping their resistances equal.
- Four-wire: laboratory and calibration accuracy; lead resistance is irrelevant to the reading.
- Size the conductors so that lead resistance stays low — 1.5 mm² gives 1.33 Ω per 100 m lead.
Which insulation for high-temperature extension cable?
The alloy is rarely the limiting factor; the insulation is. PTFE runs continuously to 260 °C and survives short excursions above that, FEP to about 200 °C with easier processing and a smoother finish, and both hold their electrical properties across the range where a PVC sheath would already have failed.
Above those temperatures the answer changes material rather than grade: glass fibre with a silicone or varnish impregnation, or a mineral-insulated construction. Remember that the insulation rating governs the cable, not the junction temperature — the cable only has to survive the temperature along its route, which is usually well below the process temperature at the sensor head.
- PTFE: continuous 260 °C, chemically inert, stiff and prone to cold flow under clamp pressure.
- FEP: continuous 200 °C, more flexible and easier to strip than PTFE.
- Silicone rubber with glass fibre braid: flexible to about 180–200 °C with good abrasion resistance.
- Keep the cable away from the hot zone near the sensor head; the last 300 mm often decides the cable's life.
Request pricing across Thermocouple & Compensating Cable
Include cross-section, core count, voltage grade, route length and installation method — those five determine price more than anything else on the page.
- Engineer-reviewed before pricing
- Reference drawings issued with the quotation
- Free sample up to free sample up to 5 m
