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    “Common” wire

    I had a question about control wiring. When referring to the common wire, is that always the neutral? What about 120 VAC, 24 VAC, 24 VDC industrial control wiring?



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    #2
    Good question.
    In a 120V system, there is no neutral, but we all call the white wire a neutral (need three wires to have a neutral).
    In my control systems, the 120 was from a transformer, so it was X2 and the 120 was X1 abbreviated as 1 on drawings, X2 was white and that was the identification.
    24VAC I didn't use.
    24 VDC you have positive and common. I wouldn't call it negative. And in 24 VDC systems under UL 508A the common is white/blue.

    Get a copy of UL508A, use the colors and identification there, and document on your drawings
    Moderator-Washington State
    Ancora Imparo

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      #3
      Neutral refers to the center point of a power wiring scheme (240/120V, 208/120V, etc); in the NEC it's usually (always?) the grounded lead (see below).

      When you get to low-voltage controls, usually there's only one supply transformer or rectifier, so there are only two leads; one goes through switches/relays/etc to control things and the other is the return, or common, conductor. There is no neutral, as such, since there's only one supply.

      (Yes, there are 120V-only electric services, but then we properly call one the grounded lead, not the neutral, anyway.)


      From the code definitions:
      Neutral Conductor. The conductor connected to the neu-tral point of a system that is intended to carry current under normal conditions.
      Neutral Point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system.
      Informational Note: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential.

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        #4
        The simple answer is that common is the lead that is common to all devices from the same control voltage source. It has no contacts, switches, etc between it and loads. It is run to one side of all devices. It is often grounded (but you should never assume it is). In most ladder diagrams it is the vertical line on the right hand side.

        But this is a very simplified answer. For example, cars often switch the negative lead and it is often called the common.

        Comment


          #5
          Originally posted by Russs57 View Post
          The simple answer is that common is the lead that is common to all devices from the same control voltage source. It has no contacts, switches, etc between it and loads. It is run to one side of all devices. It is often grounded (but you should never assume it is). In most ladder diagrams it is the vertical line on the right hand side.

          But this is a very simplified answer. For example, cars often switch the negative lead and it is often called the common.
          This is the best answer really. In days gone by we in the industrial business used more of what is called “common control”, meaning your control circuit voltage was common to the line voltage, ie 480V coils on devices. In that case, none of the control wires were “neutral”, but one of them was always used as a “common” in order to follow the conventional wisdom for the next guy looking at it, the way Russs57 just described it. This convention was laid out in a set of standards called “JIC” (Joint Industry Council) during the boom of the automotive industry after WWI. The old JIC standards later became NFPA 79 and still apply to industrial controls. So all control circuits must have a common side and IF the control circuit uses a neutral, that is supposed to be the common side.

          The only caveat is that on NEMA designed motor starters, the OL relay trip contact is always on the common side of the starter coil (neutral or not), for reasons that are now mostly historical.

          The alternative is that you can make a control system work by wiring up in any way that is functional, no common circuits at all, and I have seen a couple of systems done that way. It is an ABSOLUTE NIGHTMARE to troubleshoot and can be dangerous for someone that is expecting it to be done per the standards.
          __________________________________________________ ____________________________
          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.

          Comment


            #6
            Originally posted by Jraef View Post
            The only caveat is that on NEMA designed motor starters, the OL relay trip contact is always on the common side of the starter coil (neutral or not), for reasons that are now mostly historical.
            Maybe to make sure they're not accidentally bypassed, disconnected, or otherwise mis-wired while the control wiring is connected.
            Master Electrician
            Electrical Contractor
            Richmond, VA

            Comment


              #7
              Originally posted by Russs57 View Post
              The simple answer is that common is the lead that is common to all devices from the same control voltage source. It has no contacts, switches, etc between it and loads. It is run to one side of all devices. It is often grounded (but you should never assume it is). In most ladder diagrams it is the vertical line on the right hand side.

              But this is a very simplified answer. For example, cars often switch the negative lead and it is often called the common.
              This is the best most concise answer. Note the mention of not being switched.
              IN 24VAC HVAC systems is it often grounded but not always.
              Microwave Radiation Dangers should be openly discussed

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                #8
                Also worth mentioning: The common wire shouldn't be white or have a white stripe unless it's grounded.
                So if you see white, it should be a grounded secondary.

                One should be familiar with the color codes of the control standards (or other) of the panel your in.
                For instance, if you touch a yellow wire in a panel that you powered down, you could still get electrocuted provided its an interlock energized signal from another source.
                John,

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                  #9
                  Yes, or you open up the cabinet and they are all red.

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