Sensor cables & effects of its extensions
For accurate measurement of sensor data using the right cables & extending it in the right way is important. Below details will help you to select the right wire and help you understand the right method of extending it.
Cable Specifications
Conductor cross section (gauge): 0.34 mm2 (AWG 22)
Material insulation: Polypropylene (PP)
Electrical resistance (at 20 °C (68 °F)): ≤ 0.058 Ω/m (0.018 Ω/ft)
Weather resistance
Sample: Belden 70202E (3 core) and Belden 70203E (4 core)
Cable Length: There are no fixed rules, site conditions, wire size, cable routings, and other local conditions are to be considered with the below suggestions.
- 0-5 V DC Sensors: This should not be an issue if these are extended up to 50 feet but should not be extended beyond 100 feet for accurate results.
- 4-20 mA Sensors: Technically it can be extended up to 500 feet but if feasible should not be extended beyond 100 feet.
- RTD Sensors: With RTD sensors the cable can be extended up to 100 feet but the resistance of the connecting cable should be considered and accordingly compensated to measure the right Temperature.
Suggestion:
- In case you are using sensors with Analog to Modbus Converter then its suggested to extend the RS485 cable instead of extending all the sensor wires. This not only saves cost & time but also helps in extending the solution life.
- Ensure connections are weatherproof.
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Calculating de-rating factor while extending sensor wires
In most cases, sensor wires need to be extended to reach the measuring/monitoring equipment. In this process, the sensor output is reduced due to the extended wire and needs a de-rating factor to minimize the effect of extension.
This effect occurs because wires have resistance. Higher the length more will be the drop, also the voltage drop is more pronounced in active sensors (sensors that require 12 Vdc to operate) than in passive sensors, because there is more current flowing in the ground wires of the active sensors.
De-rating factor: Actual output of sensor with original wire / Output at end of extended wire