To be able to use your DC power supply panel out in the field, at some point, you are going to have to buy some cable or wire to hook up your sensors. Using current loops gives you some flexibility in terms of the exact configuration of the cable you use. Here is a picture of some of the cable types I have used and have sitting around in my shop.
From left to right:
The light blue cable is a rated Category 5E (CAT5e) cable that I used for the phones and Ethernet wiring in my house when I put in my structured wiring system. It consists 4 pair of insulated #24 gauge solid wires, with a fairly tight spec on the number of twists per inch and other physical characteristics required to let it be rated CAT5e and work at the high transmission rates associated with Ethernet. It’s listed as type CMR, which I believe stands for Communications Riser Cable, which means it will resist spreading fire from floor to floor if properly installed but does not have low smoke producing characteristics like plenum cable (Type CMP).
The yellow cable is general purpose power limited circuit cable that is rated for direct burial that I used for wiring audio through my house as a part of my structured wiring system. It consists of 1 pair of insulated #16 gauge stranded wires with a twist nominally every 3 inches and is Type CM/CL3/FPL/PLTC, which means it is general purpose cable that will resist fire, but is not rated to resist spreading fire from floor to floor like Type CMR and does not have low smoke producing characteristics like type CMP.
The dark blue cable is a plenum rated (Type CMP) cable consisting of 3 insulated #18 gauge stranded wires, twisted together with a shield. This cable is often used for control, instrumentation, security, and fire alarm applications that require three wires. The three wire type current loop transmitters I mentioned in a previous post are one example of such a device.
The gray cable is a power limited circuit communications cable that consists of 4 insulated #18 gauge stranded wires twisted together with a shield with a twist nominally every 4 inches. It is type CMR/CL3R/FPLR, similar to the light blue cable.
All of the cables have their pertinent rating information printed on the jacket …
… which I believe is required by code and if nothing else, would allow an inspector to verify that the installed cable meets the applicable requirements. Some cables also have running foot marks printed on their jackets (at 12″, it says 1FT, at 24″ it says 2FT, etc.) which is handy if you know how long of a piece you need. Some even have zone numbers and device numbers (as in a string of characters like “DEVICE A B C D E F ZONE 1 2 3 4 5 6 7 8 9 0” so you can easily mark the appropriate zone and device number associated with a particular cable on multi-cable pulls.
Technically, all of these cables would would work with the 4-20 ma current loops that the DC power supply panel is intended to be used with, but there are reasons you might pick one over the other, which I will go into a bit in the next post.
Senior Engineer – Facility Dynamics Engineering