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    Monitoring wires in with PV conductors

    I'm trying to get a bid out to a client and this small matter of not wanting to tell them to run more pipe came up.

    They are going to install, most likely, several #0 THHN conductors from a solar array combiner box back to where the charge controllers are located. Assuming they spec the same solar panels as on another job I know of, the maximum array voltage is about 110VDC and the maximum current in each of those probably #0's is 50 amps.

    I need several pair of #24 for sensors on the roof and want to spec a Cat5 with CMR insulation (because it is cheap and in freakin' abundance ...) to go in the same piece of pipe, assuming the usual issues of conductor fill, etc. aren't exceeded.

    Anyone see anything wrong with that?
    Julie in Austin

    Born to brew, forced to work ...

    #2
    Originally posted by tallgirl View Post
    Anyone see anything wrong with that?
    Unless there's documentation saying otherwise, I believe you're okay as long as the CAT5 is rated for the higher voltage.
    Master Electrician
    Electrical Contractor
    Richmond, VA

    Comment


      #3
      Julie,
      There will be an issue if the sensors are Article 725 Class 2 or 3 circuits.

      Also if you have any bends in the conduit, don't expect the Cat 5 survive the pull.
      Don, Illinois
      (All code citations are 2017 unless otherwise noted)

      Comment


        #4
        Originally posted by don_resqcapt19 View Post
        Julie,
        There will be an issue if the sensors are Article 725 Class 2 or 3 circuits.

        Also if you have any bends in the conduit, don't expect the Cat 5 survive the pull.
        Nope, neither of those class circuits. The one proposed sensor is a back-of-solar-panel temperature sensor, with a possible solar irradiance sensor being added.

        I'm not expecting there to be any problems with the Cat 5 -- I've pulled more Cat5 than I can shake a stick at in my life and as long as I didn't actually PULL on it, I've yet to break a single conductor. Plus the radii for any bends are going to be huge on account of it's a really big piece of pipe -- 8KW low voltage solar array and all the 156% derating rules that go with solar.
        Julie in Austin

        Born to brew, forced to work ...

        Comment


          #5
          Originally posted by tallgirl View Post
          Nope, neither of those class circuits. The one proposed sensor is a back-of-solar-panel temperature sensor, with a possible solar irradiance sensor being added.

          I'm not expecting there to be any problems with the Cat 5 -- I've pulled more Cat5 than I can shake a stick at in my life and as long as I didn't actually PULL on it, I've yet to break a single conductor. Plus the radii for any bends are going to be huge on account of it's a really big piece of pipe -- 8KW low voltage solar array and all the 156% derating rules that go with solar.
          It is not a good idea to install analog sensor signals in the same conduit as power conductors. Sensor signals for temp sesnors (T/C, RTD, Thermisor) are mv signals and don't play well with 125V, 50A power.

          Comment


            #6
            Nope. The 110 volts/50 amps make it a Class 1 circuit for sure. This is no different than any other application. You can't run CL3 or CL2 or communications wiring in the same raceway or boxes as Class 1. Bite the bullet and run a separate raceway.

            -Hal

            Comment


              #7
              Even if it were code compliant I would not do it for all the reasons stated.

              You could have issues down the road. Plus I don't see how you can gurantee no damage to the cat 5 cables. Seems like a accident waiting.

              Comment


                #8
                Hey Tallgirl where ya been,

                I'd agree with the rest if it's a vote!

                I would think that the cat 5 and other sensor wires should be of the same voltage rated to be in the same conduit.
                I don't think it's practical.
                It is DC though. I don't think that would pose a problem for the analog sensor unless it is pulsed DC.

                Comment


                  #9
                  Originally posted by markstg View Post
                  It is not a good idea to install analog sensor signals in the same conduit as power conductors. Sensor signals for temp sesnors (T/C, RTD, Thermisor) are mv signals and don't play well with 125V, 50A power.
                  It's not AC power, it's DC power. I'd never put low voltage DC in an anything with high power AC. The DC is switched at a fairly high frequency -- much too high to have any impact on the DC signals.
                  Julie in Austin

                  Born to brew, forced to work ...

                  Comment


                    #10
                    Originally posted by tallgirl View Post
                    It's not AC power, it's DC power. I'd never put low voltage DC in an anything with high power AC. The DC is switched at a fairly high frequency -- much too high to have any impact on the DC signals.
                    What do you mean when you say the The DC is switched at a fairly high frequency ? Do you mean when it is input to an inverter to produce the AC output? What is the current level that is switching?

                    If there is any high frequency present in the conductors (whether riding on 60 hz or DC) you can have noise issues from coupling into low amplitude dc signals for transducers.
                    - Resistance is Futile ..... (if less than < 1 ohm) -

                    Comment


                      #11
                      This is what I get for not pushing the "Submit" button until hours later.

                      The only time I've seen =sensors=, these aren't controls, it's analog voltages in the 0 to 5 volt or so range, run outside of the same conduit as PV conductors is on high voltage (500 to 1000VDC) string inverters, and that's because CMR / CMP is 600 volt rated insulation, and high voltage inverters can exceed 600 volts, so it's a straight up code violation. For this system, Voc is 3 x 29.9V or 89.7 volts, temperature correction is -0.12V/*C reference 25C, record low is 9F or -12C, Vtc is 37C * 3 * 0.12V/C or 13.3V for maximum Voc of 103 volts. The other system that's just the same has a recorded maximum Voc of 97 volts. So ... this isn't even a "voltage" thing.

                      Or are y'all concerned about =physical= damage? I'm trying to get out to see the house to see how the pipe was run from the roof to the inverter closet. If it looks bad, I'll punt.

                      And I do appreciate the advice, just want to make sure the facts are understood since y'all mostly don't do solar. This is an important sale for me and I do want to get it right. It's for a turnkey monitoring box that makes pretty pictures, like this one --

                      Julie in Austin

                      Born to brew, forced to work ...

                      Comment


                        #12
                        I belive tallgirl is reffering to the feed from the solar array. I can't see there being any pulsing at this point of the system before the inverter.

                        Comment


                          #13
                          Originally posted by ELA View Post
                          What do you mean when you say the The DC is switched at a fairly high frequency ? Do you mean when it is input to an inverter to produce the AC output? What is the current level that is switching?

                          If there is any high frequency present in the conductors (whether riding on 60 hz or DC) you can have noise issues from coupling into low amplitude dc signals for transducers.
                          That's why the Cat5. Because it's UTP -- Unshielded Twisted Pair -- any current that's induced into one wire in the pair is also induced into the other and they cancel. The inputs to the device that will be measuring the voltages from the sensors are differential op-amps -- inherently immune to common mode voltage problems.

                          Also, they aren't transducers, like split-cores or 4-20ma CTs. It's a thermistor with a voltage amplifier, and I believe that produces a 0 to 6 or so volt signal that's proportional to the temperature. The solar irradiance sensor I hope to up-sell later is a Davis Instruments irradiance sensor with a voltage amplifier that produces 0 to 2.5 volts corresponding to 0 to 1000 watts / m^2 irradiance.

                          I might need to go wrap some Cat5 around the conductors for a smaller array and see what they do, but every time someone has told me that UTP is susceptible to noise, I've been unable to produce any noise problems -- the stuff really is pretty robust in noisy environments. I used to have space in a lab that used 1.2MW and was liberally coated in Cat5 running all over the place, and in places it had no business being.
                          Julie in Austin

                          Born to brew, forced to work ...

                          Comment


                            #14
                            Originally posted by flatlander View Post
                            I belive tallgirl is reffering to the feed from the solar array. I can't see there being any pulsing at this point of the system before the inverter.
                            There actually is. The charge controller "voltage tracker" operates at around 25KHz. It's like any other switch mode power supply -- there's a lot of high frequency noise. There's some 60Hz "noise", but the 25KHz noise dominates. I think I've measured the AC component at around 4 or 5 amps when there were around 150 amps DC.
                            Julie in Austin

                            Born to brew, forced to work ...

                            Comment


                              #15
                              Originally posted by tallgirl View Post
                              Nope, neither of those class circuits. The one proposed sensor is a back-of-solar-panel temperature sensor, with a possible solar irradiance sensor being added.

                              I'm not expecting there to be any problems with the Cat 5 -- I've pulled more Cat5 than I can shake a stick at in my life and as long as I didn't actually PULL on it, I've yet to break a single conductor. Plus the radii for any bends are going to be huge on account of it's a really big piece of pipe -- 8KW low voltage solar array and all the 156% derating rules that go with solar.
                              The big radius actually makes it more likely to break a small conductor when it is pull in with larger conductors. You have no control of where the cables will be in relation to each other in the conduit. Often the small conductor will end up on the long side of the bend and it will have to slip by the large conductors to make up that difference in length. That will put excessive force on the smaller conductor.

                              We use #14 wires for our motor control circuits and they are pulled in with the motor power conductors unless those conductors are larger than #4. When the power conductors are larger than that, it is just too easy to break the #14s so they get installed in a second conduit.

                              If these are short runs with only a single bend, you may get away with it, but if there is any length and you have two or more bends I would not even think about it.
                              Don, Illinois
                              (All code citations are 2017 unless otherwise noted)

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