Wiring Industrial Thermocouples: Basic tips and suggestions

One thing Kele Industrial has learned in this industry: even though there are established standards for thermocouple color codes and types, the most basic rule for installation is often counterintuitive — especially for those with an electrical wiring background. (In the U.S., when wiring a thermocouple, RED is always negative. Foreign countries use other color codes.)

It’s usually easy to spot when the wiring is incorrect: connect the thermocouple directly to an instrument, and as the process heats up, the readings show a drop in temperature. As the process cools, the readings indicate a rise. The thermocouple is wired backwards and reversing the wiring connections at the analog input will correct the direction of the reading.

Here are some basic wiring diagrams with rules to follow, and some suggestions on specifying the right thermocouple wire.

The success of paralleling a single thermocouple to two receiver instruments is subject to burnout detection compatibility and grounding. Burnout detection in one instrument can cause spikes in the second instrument. Grounding issues can cause an offset error. The use of a duplex thermocouple (two thermocouple elements in one sheath) is used when paralleling thermocouples does not work.

There are three basic thermocouple wiring diagrams shown at the right:

Top: How to wire one thermocouple to one instrument
Middle: How to wire one thermocouple to two different receiving instruments (paralleling)
Bottom: How to wire multiple thermocouples to a single instrument through a switch

As you can see, none of the diagrams are really complicated, but there are some rules you need to follow to do it properly.

In the United States, when you’re connecting thermocouple wires to instrumentation, RED is always negative. The other color-coded wire is always positive.
Match the thermocouple extension wire to the thermocouple type you’re using.
Use thermocouple wire to make thermocouple elements, or to connect thermocouples to instrumentation. ONLY thermocouple extension wire should be used to connect thermocouples to instrumentation; copper wire should never be used.
If you’re wiring a thermocouple into a switch or junction box, the terminal blocks do NOT need to be the same as the thermocouple materials. Just know that any difference in temperature between the positive and negative contacts will become an error in the signal.
Do not run thermocouple leads in conduits that carry power wiring. And do not run conduit carrying thermocouple leads parallel to electrical busbars or heavy power-carrying conduit. Cross them at right angles.

MgO (or mineral insulated) style thermocouples can be ordered with either a grounded or ungrounded junction. An ungrounded junction has a slower thermal response than a grounded thermocouple but will work without a grounding problem encountered with many instruments that have a non-isolated analog input.

There are also a few tips to consider when you’re buying thermocouple wire. It starts with matching the wire type to the thermocouple type, but to get the best performance, also consider these:

When you’re buying thermocouple wire, choose wire insulation that’s compatible with your application environment. For applications that require moisture resistance, use Teflon, PVC, Kapton or Tefzel. For high-temperature applications, use fiberglass, vitreous silica, and ceramic fiber.
If there’s going to be frequent flexing of the leadwire, use a stranded conductor wire to connect the thermocouples.
To provide protection against physical abuse to the wiring, use metal overbraids or flexible armor.
To connect sensors to computers and protect against EMF stray signals, use leadwire with aluminum Mylar shields and drain wires.

Kele Industrial offers a wide variety of thermocouples, such as spring-loaded, head mounted, and tubeskin thermocouple assemblies. You may also be interested in our offering of instrumentation that use thermocouple inputs, such as single or dual loop controllers and temperature transmitters.