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Discussion Starter · #1 ·
Hey you guys. I am having my first project that have thermocouples in it. I need to wire type J wire from the thermocouple to a control panel. I was wondering what's the longest distance that type J wire can go with out having electrical noise? It needs to be wired to a 4-20ma converter?


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Guy said:
Hey you guys. I am having my first project that have thermocouples in it. I need to wire type J wire from the thermocouple to a control panel. I was wondering what's the longest distance that type J wire can go with out having electrical noise? It needs to be wired to a 4-20ma converter? Sent from my iPhone using Tapatalk
I have seen it run hundreds of feet.
 

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Hey you guys. I am having my first project that have thermocouples in it. I need to wire type J wire from the thermocouple to a control panel. I was wondering what's the longest distance that type J wire can go with out having electrical noise? It needs to be wired to a 4-20ma converter?


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The thermocouple signal is millivolts.

Don't freak out about noise, it depends on the controlled process, what is being measured or controlled may be slower and not respond to noise anyway.

But if you are concerned, put the signal conditioner NEAR the measured process, this will keep you mV exposure to a minimum. The signal conditioner will measure the TC millivolts, and convert to a robust 4-20mA signal that can go longer distances with more noise immunity.
 

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Discussion Starter · #4 ·
The thermocouple signal is millivolts.

Don't freak out about noise, it depends on the controlled process, what is being measured or controlled may be slower and not respond to noise anyway.

But if you are concerned, put the signal conditioner NEAR the measured process, this will keep you mV exposure to a minimum. The signal conditioner will measure the TC millivolts, and convert to a robust 4-20mA signal that can go longer distances with more noise immunity.

So you are telling me that is the shielded wire (4-20ma) that I should be worried about. The type j can go on with out any noise interference in a dedicated conduit or even with other wires in the same conduit?



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Your 4-20 analog signal is more stable than the millivolt signal as previously stated. The shielding prevents signifigant interference however I would not run either cable in a conduit with line voltage if that was the question. I have run tc wire over 300+ feet with no issues.

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So you are telling me that is the shielded wire (4-20ma) that I should be worried about. The type j can go on with out any noise interference in a dedicated conduit or even with other wires in the same conduit?
Because thermocouples are often used in high-temperature applications with a moderate error-band, you're not talking about something that requires quick sampling and response times, so a brief blip of noise is not likely to affect it.

That said, it's not really a good idea to run any instrumentation with power conductors because why risk creating errors? Though I'm not sure what rules actually prohibit it.

Because a thermocouple is a voltage signal it's subject to degradation with distance and splices and it can be a pain to maintain accuracy. Current signals are not subject to this same problem, which is why IMM said put the signal-conditioner as close to the thermocouple source as possible.

Once you convert to 4-20mA, you have a Class 2 circuit and need to run it according to article 725.
 

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So you are telling me that is the shielded wire (4-20ma) that I should be worried about. The type j can go on with out any noise interference in a dedicated conduit or even with other wires in the same conduit?



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The opposite.
4-20 is more immune to noise.
Unshielded TC mV is less immune to noise.
 

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I also see you have other posts. (480/120 same conduit).

It appears that you are the installing electrician on large industrial machinery, or process.

Can you consult the OEM of the equipment? They may be of the best resource of what critical, and what is not.

They may know if the thermocouple is just a general device of low significance, or if the thermocouple must be the most important measurement of the entire project.

I assume that your prints are only a schematic, and there is no conduit or harness charts showing grouping, or routing of conductors.
 

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We have them configured both ways. Some of the TC extension wire runs for hundreds of feet with power cables without any trouble. We use multi pair shielded TC extension cable for bulk thermocouple locations instead of having a bunch of transmitters. If its just a thermocouple here or there we will use a transmitter. Never had any issues with them other than the TC itself crapping out and dying. If you do have to run a significant distance make sure you use extension grade wire instead of thermocouple grade wire. It will save lots of $$
 

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Also note that the entire run must be thermocouple wire, and it must be the correct type.

Thermocouple wire is color-coded to match the type of thermocouple. In your case (type J), it'll be red and white.

To add to the confusion, red is always negative; the opposite of standard DC.

I've run basic unshielded thermocouple wire more than 500' without issue, and I've run it in the same pipe as 120AC.

As noted above, if it's possible, I'd put the signal converter as close to the thermocouple as I could. Not only is a 4-20 signal 'tougher', but thermocouple wire is a LOT more expensive than twisted shielded.
 

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micromind said:
To add to the confusion, red is always negative; the opposite of standard DC.

This is true for US standards, but our plant is German ran and located in California so we see different devices. I'd say our rule of thumb is to put a magnet on one of the conductors, the magnetic one is the negative.
 
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