One thing I’ve learned in this industry: Even though there are standards to thermocouple color codes and types, the most basic rule for installation (In the US, when you’re wiring a thermocouple, RED is always negative) is non-intuitive to anybody who’s ever done electrical wiring.
It’s pretty easy to tell when you’ve done it wrong: You wire the thermocouple directly into an instrument, and as the process gets hotter, readings say it’s cooling down. As your process cools down, the readings show a rise in temperatures. If you reverse the wiring at a junction box, it’ll read in the right direction, but you’ll have errors because of the false junctions.
Here are some basic wiring diagrams from the reference section of the Lesman catalog, with rules to follow, and some suggestions on specifying the right thermocouple wire.
There are three basic thermocouple wiring diagrams listed:
- How to wire one thermocouple to one instrument
- How to wire one thermocouple to two different receiving instruments
- 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 wire to the thermocouple type you’re using.
- Use thermocouple wire to make thermocouple elements, or to connect thermocouples to instrumentation. Thermocouple extension wire should ONLY be used to connect thermocouples to instrumentation, and copper wire should never be used.
- If you’re wiring a thermocouple into a switch or junction box, the contacts 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 buss bars or heavy power-carrying conduit. Cross them at right angles.
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 and leads in flexible armor.
- To connect sensors to computers and protect against EMF stray signals, use leadwire with aluminum Mylar sheids and drain wires.
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Lesman catalog reference section (Includes the diagrams shown here plus unit conversion tables, media properties, ratings and standards, and a glossary.)
Lesman Tech Tips 
#1 by Jamie Lockhart on February 22, 2019 - 9:52 am
Trying to hook up a thermometer that has t1 and t2 leads. Trying to figure out which one goes to positive and negative on a thermocouple.
#2 by Ken on June 4, 2018 - 6:59 am
Hello,
Question.
How critical can it be if we use a r type extension wire for a R type TC.
We are using b extension wire for the R type TC.
Can it make a HUGE difference?
#3 by danstips on June 11, 2018 - 1:53 pm
BX (copper alloy) extension wire uses a different copper alloy that RX/SX extension wire. I am unaware of data that defines the error when BX (copper alloy) wire is used with a Type R thermocouple. So without numbers to go by, it isn’t possible to assign a magnitude such as HUGE or minimal.
Be aware that a Type B thermocouple can use copper-copper wire and maintain spec’d limit of error if the extension wire junction with the thermocouple remains at less than 122/150/212 DegF depending upon one’s reference source. So there can be copper-copper BX wire or copper vs PCLW630 copper alloy BX wire. But either would create a error.
#4 by Patrick on October 16, 2016 - 9:08 pm
Is there a special kind of switch required for this application? If I want to switch between 2 thermocouples going into a PID, will a standard dpdt switch do the trick?
#5 by danstips on October 31, 2016 - 1:40 pm
A standard DPDT toggle switch will switch from T/C A to T/C B and back again, assuming the wiring is the correct type thermocouple wire on all sides.
The receiver might detect an open circuit during the switching when neither T/C is connected and the receiver might go into fault mode or burn-out (1st potential issue) momentarily.
Any difference in temperature between the input and output sides of the switch will be a temperature error. (2nd potential issue). No, I don’t know the magnitude nor the polarity because that depends on the temperature difference.
A Google search for thermocouple switch will bring up over a dozen commercially available mechanical ‘1 of X’ thermocouple switches designed for a specific thermocouple type which eliminates the 2nd issue. I don’t know if any of them avoids the first issue.
#6 by Chadd on January 12, 2016 - 2:44 pm
Is it alright to use a solid TC wire to a TC terminal block and then use stranded TC (same type) to your monitoring device?
#7 by danstips on January 14, 2016 - 8:37 am
The thermocouple circuit doesn’t know or care whether the thermocouple wire is solid or stranded and the circuit will work with either type of wire equally well.
Whether there are corporate preferences/standard/specification requiring one or the other is something else, but I can’t answer that.
#8 by ash on June 25, 2012 - 6:50 am
Good explanation.Although not so sophisticated but should be kept in mind.