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Install heavy load breaker first in panel to avoid lights dimming

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    Install heavy load breaker first in panel to avoid lights dimming

    My instructor years ago told me to keep house lights from dimming when the AC turns on, to install the AC rwo pole breaker first or next to the main breaker, upstream from the lighting breakers. Is this what you also suggest ? Thank you.

    #2
    I dunno-- I'd "assume" that putting the lighting circuits next to the main breaker would mean the lights would get first crack at the electrons, and let the A/C get the leavings. (:

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      #3
      We need someone to tell us the VD on 2" of busbar, say 35 amps then 3 amps an inch later. For this instance ignore the 100' of overhead #2Al, the 30'of 4 cu, and the 10KVA pot with 2.3% impedance feeding the three adjoining neighborhood homes. Non of that matters.
      Tom
      TBLO

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        #4
        If your installation is so weak that the microvolt drop that might take place in 1" of bus material is significant, then you have MAJOR problems.

        No... it was BS.
        __________________________________________________ ____________________________
        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
          I prefer landing 2p breakers on one side (away from neutral bus if only one), highest to lowest, and 1p breakers on the other (neutral bus) side.

          Click image for larger version

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          I especially try to avoid placing 2p breakers head to head.
          Master Electrician
          Electrical Contractor
          Richmond, VA

          Comment


            #6
            Originally posted by Jraef View Post
            If your installation is so weak that the microvolt drop that might take place in 1" of bus material is significant, then you have MAJOR problems.

            No... it was BS.
            I agree, the bus is typically the same ampacity of the conductors feeding the panel so a few inches when calculated against the entire length of the conductors back to the POCO transformer is nothing.
            Rob

            Moderator

            All responses based on the 2014 NEC unless otherwise noted

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              #7
              Stevenfyeager,
              If you want to test his hypothesis on your own, connect your voltmeter leads to a live panel bus, one at the top and one at the bottom of the same bus (wearing appropriate PPE of course). If you read a voltage, that's the voltage drop across that distance. Make sure it is on the millivolt scale to have any possible chance of reading anything... which I doubt.
              __________________________________________________ ____________________________
              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


                #8
                Yes, this is a mitigating technique with minimal effectiveness. In other words, better is not best. Some have argued it is a placebo effect; I cannot and have never scientifically proven this theory, but I have used it. And I explain it to customers when I do use it (thus the placebo argument).

                Proper sizing of the branch circuit conductors accounting for voltage drop will be a far more effective solution, although costs will rise.

                Take a 29 amp MCA AC unit with a 50 amp MOCP. The LRA is about 100 amps. (These are rounded numbers off a brand name single phase 240V unit)

                You could size the circuit with #10 copper and be code compliant. The voltage drop on start up would be significant compared to other options (and the light flicker would be measurable).
                You could size the circuit for the MOCP and use #8 copper. The voltage drop would be less (and the flicker).
                You could size the circuit for the LRA and use #3 copper. The voltage drop would be closer to non existent, but costs and termination issues would arise.

                OR

                You could understand that Kirchoff has a current law, and that his law is static (fixed in time and must be true at all milliseconds of time). The moment that the AC contactor pulls in, current flows to a new 100 amp load in an instant. Each electron comprising the flow of current does not instantaneously travel from the utility to your condenser exclusively, but takes some time to travel (although we are speaking about milliseconds).

                So in the instant that the contactor makes, electrons are "stolen" from every connected circuit on the bus in the panel, with the system equalizing in very short order. The more circuits between the new load and the source, the more circuits are affected. The systems affected that our human eyes can see are lighting circuits - otherwise known as flicker.

                Because the electrons want to come from the source, if you put your AC breaker closer to the main feeder, fewer electrons will be "stolen" from the other circuits and tend to come from the source.

                Let me be clear - this does not solve the problem, it only slightly mitigates it at best.

                Comment


                  #9
                  Originally posted by Nuber View Post
                  You could size the circuit with #10 copper and be code compliant. The voltage drop on start up would be significant compared to other options (and the light flicker would be measurable).
                  You could size the circuit for the MOCP and use #8 copper. The voltage drop would be less (and the flicker).
                  You could size the circuit for the LRA and use #3 copper. The voltage drop would be closer to non existent, but costs and termination issues would arise.
                  This would only apply to any lights on the same circuit as the AC (which there normally would not be).

                  Actually, increasing the AC conductors' size would cause more starting-current-induced voltage drop on the conductors that the AC and light share; i.e., ahead of the panel.

                  Reduced starting voltage caused by excess voltage drop on the AC circuit would decrease the effect of voltage drop on the shared service/feeder conductors, reducing flicker.
                  Master Electrician
                  Electrical Contractor
                  Richmond, VA

                  Comment


                    #10
                    I don't want to dim my air conditioner while turning lights on. Now I'm totally confused. Where should I put the breakers?

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                      #11
                      Originally posted by MAC702 View Post
                      I don't want to dim my air conditioner while turning lights on. Now I'm totally confused. Where should I put the breakers?
                      As I said in post 5: 2p, highest to lowest, down one side, and 1p, high to low, down the other side.

                      If there's only one neutral bus, put the 1p breakers on that side. Usually suits KOs, too.

                      Terminate all grounds first, neutrals second. That's why my panels almost look empty.
                      Master Electrician
                      Electrical Contractor
                      Richmond, VA

                      Comment


                        #12
                        Originally posted by MAC702 View Post
                        I don't want to dim my air conditioner while turning lights on. Now I'm totally confused. Where should I put the breakers?
                        It doesn't matter even a little bit where they are placed in the panel. In most dwelling units the biggest source of voltage drop when large loads are started is the utility transformer.
                        Don, Illinois
                        (All code citations are 2017 unless otherwise noted)

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                          #13
                          Ok, thank you for your input. Here is another part of my situation. I have an 200 amp outside meter /disconnect with 8 (or 16) spaces in it. Then 4/0 cable to a panel inside the home 24 feet away. Would it be best to power the AC from the outside panel (closest to the meter, source) ? Does it make any difference ? It is just about as easy to feed it from either panel. Thanks again.

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                            #14
                            I would use the outside panel for anything outside. That makes one less hole through the wall.

                            It also means less light flicker when the compressor starts.
                            Master Electrician
                            Electrical Contractor
                            Richmond, VA

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                              #15
                              Yes, outside items are fed from an outside panel, especially when it’s closer.
                              Tom
                              TBLO

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