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Filter/Choke on 120/240 circuits for Enphase PV communications

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  • Filter/Choke on 120/240 circuits for Enphase PV communications

    Interesting problem here, I think.

    I've got an Enphase PV system backfed into a panel on a 20A 240V breaker. All is good, wired just as recommended.

    The Enphase microinverters communicate with their Envoy communications box (which provides internet access to monitor PV system performance) by carrier modulation at around (I understand) 140 kHz, over the building wiring.

    Trouble is, I believe, that the client has a number of things that are interfering with that (including, for example, a plug-in intercom system).

    It is possible, in principle, to isolate the Enphase system using Ferrite-core chokes to eliminate that interference. Or, alternatively, isolate the noise-producing circuits. However, since the intercom has to be able to communicate _across_ circuits (wherever they plug it in), really the Enphase has to be isolated.

    Problem: the microinverters are on a 240V 2-pole breaker, but the Envoy plugs into 120. What this boils down to is: these 240V and 120V circuits need to be isolated from the rest of the panel, but not from each other.

    A solution: run a 240V wire out to a subpanel, loop that through a choke, and then split it to a 240 for the inverters and a 120 for the Envoy. Easy peasy.

    However, electrically that's EXACTLY the same as adding a 120V outlet off one leg of the 240V breaker in the main panel, and running both legs through a choke before they get to the breaker in the main panel. Since everything is packed in a fairly small closet, the outlet could be right next to the panel, and dedicated to the Envoy. I can't figure out if there are code problems with this.

    Thanks for any thoughts.

  • #2
    Re: Filter/Choke on 120/240 circuits for Enphase PV communications

    It sounds like you want to tie small wires under the lugs in the main panel without proper over current protection, and yes that is a problem.

    I would look for proper wiring at the main and to any sub-panels before I would blame the intercom or anything else being used in the house.

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    • #3
      Re: Filter/Choke on 120/240 circuits for Enphase PV communications

      Originally posted by keithwins View Post
      Trouble is, I believe, that the client has a number of things that are interfering with that (including, for example, a plug-in intercom system).
      What makes you believe that? Just a wild guess? You need to do a little better than that.

      Any device designed to operate by communicating over the building AC wiring MUST be able to deal with the normal and expected noisy environment that that entails. If it doesn't, get better equipment.

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      • #4
        Re: Filter/Choke on 120/240 circuits for Enphase PV communications

        Originally posted by dgbldr View Post
        Any device designed to operate by communicating over the building AC wiring MUST be able to deal with the normal and expected noisy environment that that entails. If it doesn't, get better equipment.
        Flux capacitors are the new norm, no? Ever seen the [-]S/N thrown off when the thyristor circuit opens? It's like someone unplugged uncle Ted and forty seven stray cats are scratching up some simulcast fever.

        What has Enphase said/done and did you unplug the intercom sys and get any change in performance? Have you gone through all circuits in attempt to isolate the troublemaker[s]? What is it, exactly, that is happening [or not]?
        “I find the curiosity of our men with respect to this animal is pretty much satisfied.”
        ~ Meriwether Lewis

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        • #5
          Re: Filter/Choke on 120/240 circuits for Enphase PV communications

          Okay, here goes:

          I WASN'T considering avoiding overcurrent protection or any such thing, sorry if my post wasn't clear. Everything was still going through the 2-pole breaker of the PV system. It was just tied together (the 120v outlet and the PV feed), and all of it choked, BEFORE going to the breaker, so it was isolated from the rest of the panel and not itself.

          No, I'm not wildly guessing about communication signal strength. The Enphase has a signal strength indicator. Since it's plugged in right next to the panel, it can't be some wire length issue, so I've assumed interference. It has since started to operate, though I haven't been back to the site to see if it's barely eeking by (low signal strength) or if it finally latched on.

          The site includes an operating police station. I can't just turn off the intercom. And there's no meaning, it seems to me, to the "get better equipment" mantra: for one thing, there are no other options for microinverter communication, but for another thing some environments/equipment are noisy enough that you have to isolate noise/signals from each other. That was the nature of the question. I CAN'T isolate the intercom from the panel if it's intended to be used on more than one circuit, so I was going the other direction.

          Enphase has suggested it's interference, and eventually suggested the isolation/choke idea. My question ultimately comes down to, I suppose: any problems adding a choke inside a main panel? I don't think I've ever seen one... If you think not, can you point me to the relevant code? Or at least justify your answer...

          Thanks.

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          • #6
            Re: Filter/Choke on 120/240 circuits for Enphase PV communications

            Colour me confused. Where is the breaker that feeds the 120V receptacle?

            As far as putting any sort of filter around the wires in the main panel- it is not a direct violation of any NEC code but, it is a very open ended question depending on what exactly the filter is, how it is rated and used.

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            • #7
              Re: Filter/Choke on 120/240 circuits for Enphase PV communications

              So the 120V receptacle is fed off one leg of the 240V PV breaker. Exactly like a subpanel, exactly like a 120V receptacle-equipped 240V disconnect (though they are typically additionally equipped with a GFCI on the receptacle.

              The filter consists of some (epoxy-coated) ferrite rings that the wires are passed around. No connections, nothing, just a simple choke.

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              • #8
                Re: Filter/Choke on 120/240 circuits for Enphase PV communications

                Ferrite beads are used to choke or suppress higher frequencies than 140 hurts.
                Do you know how your intercom works? What carrier and modulation it uses?

                I cannot imagine any intelligent designer employing amplitude modulation to communicate over shared power wires. That would be very susceptible to noize and too easy to accomplish in another manner that would be less susceptible to noise.
                You know what makes an ungodly tremendous amount of electrical noize? Inverters.

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