When installing ETI sensors and thermistors, you may find that you need to extend the leads greater than what’s already provided from the factory. Because the Snow Switch sensors operate at a higher voltage and use higher current than the thermistors, the recommended wire gauge differs between the two. To avoid confusion, we’ve created this writeup to explain our recommendations for extending the leads on ETI’s Snow Switch sensors and thermistors. For Snow Switch sensors, ETI recommends #18 AWG wire for up to 500 ft and #12 AWG wire for anything greater than 500 ft. On several product datasheets, #12 AWG wire is limited to 2000 ft because the average sensor runs 24 volts at 250 mA of current to limit voltage drop and low current issues.
Regarding the temperature sensors (thermistors), the requirement differs because the thermistor runs at 5 volts and approximately 2 mA of current, which is a negligible line loss. The main difference resides in the line resistance. ETI controllers use a 100k ohm thermistor with a negative temperature coefficient, so when the temperature decreases, the resistance increases. Since the thermistors are used for snow melt and freeze protection, the range of resistance used is 2.7Meg ohms (-40F °) to 75.2k ohms (+90F °). This means a single digit degree change will be marked by a resistance change of between 800k ohm per degree on the cold end to 2k ohms per degree on the hot end.
Since #22 AWG wire (which is what is specified on the thermistor) has a resistance value of 16.5 ohms per 1000 ft, it can be observed that the wire gauge will not make a considerable difference in the measurement of temperature. This means it is acceptable to use a #22 AWG wire to extend the cable.
#18AWG wire is 6.51 ohms per 1000 feet of wire and #12 AWG is 1.62 ohms per 1000 feet. It is worth mentioning because the #22 AWG may be the most cost-effective solution compared to the #18 or #12 AWG wire.
With all of that said, if the wire meets the work requirements for insulation and protection of the wire and meets ETI’s wire recommendations, it is acceptable to use #22 AWG or larger to extend the wires for the thermistor, and #18 AWG wire for up to 500 ft and #12 AWG wire for anything greater than 500 ft for ETI’s Snow Switch sensor cables