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    occupancy switch says neutral not needed

    This Lutron Meastro occupancy-sensing switch boasts that no neutral is required. In addition to the line and load wires, the switch has a bare wire and a green one, and the instructions say to connect both to the box's ground.

    http://pdf.lowes.com/installationgui...36_install.pdf

    This seems most odd to me. I thought the whole point of 404.2C (requiring a neutral in every switch box, with some exceptions), is that when "smart" devices such as this are installed, that no currents are introduced into the EGC system. Is the operating current of the logic in the switch so low that it's been deemed acceptable to have such currents flowing through EGC ?

    Odder still is that
    they don't even suggest to use a neutral if there happens to be one in the switch box.


    #2
    Originally posted by RustyShackleford View Post
    Is the operating current of the logic in the switch so low that it's been deemed acceptable to have such currents flowing through EGC ?
    Certainly been deemed acceptable by the manufacturer; not so sure about the NEC.
    Master Electrician
    Electrical Contractor
    Richmond, VA

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      #3
      Can you tell us if there is continuity between that green and bare pigtail?

      I'd bet the green is actually the "neutral" for the smart device, and it won't work if that lead is not connected to something grounded.

      I have heard there are some devices similar to what you describe but you are supposed to connect the insulated lead to the neutral if present, otherwise EGC is acceptable to the instructions. Can't say I have seen them though.
      I live for today, I'm just a day behind.

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        #4
        You can make a capacitive power source without the need for a connection to neutral, so long as your power needs are just a few milliwatts. That's likely what they are doing here. The load is in series with a cap which doesn't really affect it, but the cap creates a small reactance, that reactance then is exploited as a pseudo voltage drop to power up the micro-electronics. You can't get much out of that without causing more problems, but it's often enough for very low power circuits like timers, sensors, etc.
        __________________________________________________ ____________________________
        Many people are shocked when they discover I am not a good electrician...

        I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

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          #5
          Certainly been deemed acceptable by the manufacturer; not so sure about the NEC.
          Lutron is a pretty major manufacturer. Can't quite feature them suggesting actions that violate NEC. I guess putting this device into older construction, that is not compliant with current code, would not conflict (or would it ?). But not specifying to connect to neutral in newer work (compliant with current code), would seem awfully odd.
          Can you tell us if there is continuity between that green and bare pigtail?
          Will check when I can. I doubt it. Green goes to PCB inside, whereas bare goes to the device yoke.
          I'd bet the green is actually the "neutral" for the smart device, and it won't work if that lead is not connected to something grounded.
          I'd bet you're correct.

          Comment


            #6
            Originally posted by Jraef View Post
            You can make a capacitive power source without the need for a connection to neutral, so long as your power needs are just a few milliwatts. That's likely what they are doing here. The load is in series with a cap which doesn't really affect it, but the cap creates a small reactance, that reactance then is exploited as a pseudo voltage drop to power up the micro-electronics. You can't get much out of that without causing more problems, but it's often enough for very low power circuits like timers, sensors, etc.
            But NEC now thinks we need a neutral conductor to supply a proper return path.
            I live for today, I'm just a day behind.

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              #7
              Originally posted by Jraef View Post
              You can make a capacitive power source without the need for a connection to neutral, so long as your power needs are just a few milliwatts. That's likely what they are doing here. The load is in series with a cap which doesn't really affect it, but the cap creates a small reactance, that reactance then is exploited as a pseudo voltage drop to power up the micro-electronics.
              The cap will eventually get discharged and a new potential cannot be developed without some current flowing. Unless somehow it's poaching current from the hot line-to-load connection somehow.

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                #8
                Originally posted by RustyShackleford View Post
                The cap will eventually get discharged and a new potential cannot be developed without some current flowing. Unless somehow it's poaching current from the hot line-to-load connection somehow.
                In an AC circuit current doesn't stop flowing, it reverses every half cycle. DC circuit the current stops flowing once the capacitor is charged.
                I live for today, I'm just a day behind.

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                  #9
                  Originally posted by kwired View Post
                  But NEC now thinks we need a neutral conductor to supply a proper return path.
                  Yeah, that's the rationale behind 404.2(C) I believe. So if somehow Lutron has developed a smart switch that does not use any current for the logic - which I believe violates the laws of physics - then maybe that NEC update shouldn't have been approved.

                  I believe it's more likely that Lutron developed logic that uses so little current that they convinced UL to approve it.

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                    #10
                    Originally posted by RustyShackleford View Post
                    Yeah, that's the rationale behind 404.2(C) I believe. So if somehow Lutron has developed a smart switch that does not use any current for the logic - which I believe violates the laws of physics - then maybe that NEC update shouldn't have been approved.

                    I believe it's more likely that Lutron developed logic that uses so little current that they convinced UL to approve it.
                    I think (grounded) current in such devices was considered fairly negligible before the code change, but then there started to be more of them in use and they maybe were more concerned about the additive effects.
                    I live for today, I'm just a day behind.

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                      #11
                      Though 404.2(C) says we have to provide a neutral to the switch location, it doesn't say we have to use it.

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                        #12
                        This Lutron Meastro occupancy-sensing switch boasts that no neutral is required. ...
                        That suggests Lutron is making this device for the pre NEC 2011 market. Since 2011 on should have a neutral. I mean who on here would not have followed the code?

                        Or
                        Jraef's response is really good. A cap can definitely give a few volts drop with no appreciable heat loss. The OFF position would have to interrupt any bleed current. There has been sufficient micro-energy-magic developed in the last 7 years to make this feasible.

                        Just curious, does the Lutron have a listing mark?
                        Without data you’re just another person with an opinion – Edwards Deming

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                          #13
                          If the instructions say to connect both wires to ground, you have no choice. 110.3 B
                          Electricians do it until it Hertz!

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                            #14
                            I looked at the instructions and I didn't see any sort of listing mark or logo.
                            Cheers and Stay Safe,

                            Marky the Sparky

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                              #15
                              I posted this at stackexchange and got some interesting answers. I think the longest answer "There is more than one way to skin this cat" nailed it.

                              https://electronics.stackexchange.co...t-need-neutral

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