In general, what are the downsides of using 20AWG typ "R" TC extension cable against 16 AWG extension cable? We normally use 16 AWG type R thermocouple cable, fiberglass coated, but a contractor just made an installation with 20 AWG cable, and some of the older plant people say that this can create problems, signal losses, etc...

I would believe that losses shouldn't be an issue because practically no current flows through the cable, but are there other downsides when using thinner cable?

Our cable runs are typically between 20 (min) and 200 (max) meters long.

 

As long as you are using the proper cable, you should have no issues. Make sure the cable, the thermocouple, and the metering device are set for the same TC type. For example you do NOT want to mix a J TC with K wire.

Another thing that I have seen to cause problems in the past when connecting cables, is that the installer inadvertantly crosses the wires. This just creates a new TC junction.

 

Some measuring devices limit the effective resistance on the analog input, in order to maintain the stated accuracy.

Resistance Vs Wire Diameter table here for T/C wire (bottom of page H-8):
http://www.omega.com/temperature/pdf/tc_colorcodes.pdf

Type R 20g resistance is 0.190 ohms per double ft.
Type R 16g resistance is 0.073 ohms per double ft.

200 (double) m = 656 (double) feet * 0.190 = 125 ohms + probe + panel wiring

Note Omega's statement (below) about total loop resistance.

http://www.omega.com/prodinfo/ThermocoupleWire.html

"What is the maximum length of thermocouple wire?
There are many factors that can impact the usable length of a thermocouple.

As a guideline, under 100 feet with 20 AWG or thicker wire in an area free of electromagnetic interference usually is fine.

Two of the main factors in determining useable thermocouple length are total loop resistance and preventing electrical noise getting into
the signal. Because different thermocouple wires are made of different materials, the resistance will vary based on the type as well as the wire diameter and length.

The allowable loop resistance is affected by the input resistance of the amplifier circuit to which it is attached. But as a guideline,
typically the objective is to keep the total loop resistance under 100 ohms. Loop resistance is determined by multiplying the length in
feet by the resistance per double feet (remember 1 foot length of run includes 1 foot from each of the two t/c wires) as shown in "Resistance Vs Wire Diameter" table for thermocouple type and gauge. Remember in your calculations to include the probe (when used/applicable) in addition to the wire length.

The second major factor in running a thermocouple wire is to keep it away from any electromagnetic fields. Thermocouple wire
creates a low voltage signal and should not be run near power wires, motors, etc. To help minimize noise pickup, a metal over braid
or twisted shielded wire is commonly used."

David

 

I have experienced more problems with "oversized" TC extension cable then I have with the smaller sizes. This became a real problem in a unit where 14AWG was spec'd. Because of the stiffness of the wire the larger diameters, which take up more space in TC heads and won't "squish down" as readily, the bigger sizes were more likely to rub against the cap causing grounds.

I also found that they're more likely to pop out from under screw terminals because it's harder to make a loop for the installer, so they tend not to bother. I find the smaller diameters are easier to work with and provide more reliable terminations.