RTD 3 Wires Max Cable Length

This should clear any <...doubt...>.

The alternative to the installation as described is to mount the transmitter close to the RTD and transmit the temperature as an industry-standard 4-20mA signal. The proverbial "two bits of wet string" will convey 4-20mA.

Several Motor Protection Relays offer an RTD module that is mounted near the motor and then has a fiber optic cable connection back to the relay. That’s what I like to use. Virtually unlimited distance.

The information I have seen says:

• If you have to keep the length long, use 4-wire RTD cable.
• For long lengths be prepared to have induced signals on the wire, so use shielded cable.
• Put the transmitter near the RTD and use a much more standard 4-20mA, 0-20mA, or similar signal method.

--JMM

What resistance can your RTD input handle?

A major vendor’s PLC/PAC analog input spec for Pt100 RTD states a maximum of 40 ohms resistance in the lead wires.

400m distance is 800m out-and-back.

20g copper is 0.0333 ohms/meter, which is 26.6 ohms over 800m, the working limit.

22g copper is 0.0530 ohms/meter, which is too high a resistance for this AI at 42.4 ohms over 800m.

I’d be looking at a temp transmitter.

3 wire RTD measurement assumes that the wiring voltage drop across each lead is equal. A 4 wire measurement will produce the best accuracy.

In theory, 400 meters of cabling length will not significantly affect 3 or 4 wire measurements. In practice, you may start getting false readings at these lengths. The unknowns of non-linear common-mode rejection, filtering, noise sources, and measurement source ranges makes it impossible to predict if this is a signal run too far.

I know this thread is more than three years old but I'll just give my answer anyway.

The rule of thumb is 200 feet or 61 meters for three-wire RTD's. The actual distance depends on the size of the wire. 18AWG is the usual size.

You also need to use a shielded three-wire cable, not three separate wires. This avoids differences in manufacturing and also ensures that the lengths are as equal as can possibly be. The shield protects against interference especially if you use the whole 200 feet.

The right thing to do, however, is to use a temperature transmitter with a 4-20mA output. Rule of thumb for 4-20mA current is 1000 meters so this should fit your requirements nicely. Use a two-wire shielded cable #18AWG.

Note: using the three-wire method does not "eliminate" the lead resistance. It just minimizes it. We once experimented using the three-wire method and found that the error due to lead wire resistance increases with distance. At 200 feet, the error can exceed one degree Celsius.

Remember also that copper wire is also temperature sensitive. A long wire run that is exposed to the sun will cause your lead resistance to change increasing your error.

regards,

Vulcan