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I recently worked on a project that involved a 300 feet cable run from one part of a building to another at our plant. Its a simple 120V circuit that originates in a control panel, travels 150 feet to the NO contacts of a temperature controller, and back to the input card of a PLC inside the control panel...for a total of 300 feet. All the circuit is doing is detecting if the temperature has dropped below the setpoint which in turn turns the heater on.

Now as to why there is no remote I/O rack located near the temp controller is a story for another day. I am fully aware that this is not a well designed system but this is what we have for now.

First thing I encountered in my design was sizing the cable. I have designed power circuits in the past and figuring out voltage drop in power circuits is fairly straight forward as you know the power rating of the load and can easily calculate the current for a 3-phase 480V system.

Not so straight forward for control circuits apparently, because I am not sure what my load is (is it the I/O card?) and how much current it's pulling. Since I don't know the current flowing through the cable, I can't figure out the voltage drop. Even if I were to ignore voltage drop for a second, how could I size the conductor without knowing the current.

Based on an experienced colleague's suggestion, I ended up using 16AWG. But I'd like to know the specifics. Please advise.

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  • \$\begingroup\$ The type of control signal you mention should have a very low current, but you could check with the input card's datasheet? \$\endgroup\$ – user253751 Sep 25 '18 at 0:01
  • \$\begingroup\$ If you ever want to measure, you can use clamp on ammeters to avoid disturbing the function of a system. As far as the wire size, I'd check if there is a fuse on that output+input on the PLC. You probably ended up using a securex pair or something similar, and your colleague is right, there's not much reason to go larger and not always much reason to go smaller. If you decided to use wire that had lower ampacity than the rating of the fuse, I would add another fuse inline to protect the wire though. \$\endgroup\$ – K H Sep 25 '18 at 0:32
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If all it will ever carry is a control signal, then consider the resistivity of the wire. Also find the manual for the PLC and determine it's 120v input impedance; it should be in the specifications. Plug these into the following.

Wire resistivity:

\$\frac{4.016Ω}{1000ft} \cdot 300ft = 1.2Ω\$.

If the PLC's 120v input presents a load of 10kΩ, then use the voltage divider rule:

\$120v\cdot \frac{10k\Omega}{1.2\Omega + 10k\Omega} = 119.98v\$

That said, I'd shy away from using very small wire for long control signals, even though it is much less expensive. Typically, traditional hook-up wire is used (like 18 or 16 gauge), as to be robust and tough in service. Thinner wire usually has thinner insulation, and thus a lower voltage rating. If anywhere near 480v, then use 600v-rated wire. It has more (and better) insulation.

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