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Automatic Transfer Switch after the Main Disconnect

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    Automatic Transfer Switch after the Main Disconnect

    The customer has 25 kw optional generator to back up his whole house. The ATS has two 200 amp breakers and the generator has a 125. Like a lot of other states, the meter has a main breaker panel on it. There are no lugs available to hook up this generator on the load side of the meter, just buss bars for two breaker locations. I am resigned to the fact that I have to hook up the ATS after the 150 amp main breaker.

    However, I, and my customer for that matter, would prefer it if every time the house main is flipped off, it would NOT be required to also turn off a breaker in the Automatic Transfer Switch to prevent the generator from kicking in. This would especially be in the case of fireman or some other urgency requiring turning off all the power to the house and unwittingly kicking in the generator.

    Given that this job is schedule for tomorrow (I know, I know, I should be at home watching the game on Saturday) my best guess to this point is to do the best I can by labeling the panels to identify the need to turn off an additional breaker. Is there a reasonable cost effective way (i.e. NOT replacing the service) to one-switch this operation? 'Cuz, still, in the back of my mind I ask, "In the even of an emergency is someone going to pay attention to a little sticker?"

    Possibly a shunt trip?

    I would rather beg for forgiveness then beg for permission.


      091023-1925 EST


      If I understand the logic of your system, then there is an outdoor breaker at the meter, and the ATS is someplace beyond this. If the breaker is manually turned off or tripped some other way, then normally the backup generator would automatically start and the transfer switch would change state.

      Can you connect a voltage detector on the input side of the outside breaker? Obviously needs fuses or its own breaker to the input of the voltage detector. This detector could be a continuous duty relay. Then is it possible to easily connect the voltage detector output to some part of the logic of the generator-ATS system such that if AC power to the meter is not lost but AC power is lost to the ATS system that automatic turn on of the generator-ATS system is inhibited?



        Is it possible to replace the main breaker at the utility with one that has a form "c" contact. Then use that contact to enable/disable the generator from starting ?
        Advise is a dangerous gift, even from the wise to the wise.



          Does the ATS circuit allow remote sensing of loss of power from utility i.e. can the ATS loss of power sensing relay or transducer be wired to upstream (line/utility side) of the disconnect/switch which you require not to initiate transfer.
          Last edited by mjolnir; 10-24-09, 11:00 AM.
          Keith :)


            I don't have my code book at home, so this might not help. But since the generator is a second service to the house, which is allowed because it is a standby source, then the code already requires you to put some label to inform interested parties (such as fire fighters) that there is a second source? They know how to handle such situations. They can turn off the second source, before entering the house.

            If you are concerned about non-emergency situations, such as turning off power in order to do some maintenance or adding more circuits, then all the homeowner has to do is to disable the generator (close a fuel valve or something) to prevent it from automatically starting.
            Charles E. Beck, P.E., Seattle
            Comments based on 2017 NEC unless otherwise noted.


              ATS Query

              Thanks Charlie B,
              Since yours is the only reply that I understood, I think I'll go with it. :grin: Seriously, thanks to all others. I am just a simple residential electrician, and most of the comments sound like they are a world away from my field! Commercial or industrial maybe? Marking the panel and ATS cabinet looks like the simplest and most cost effective. Glad to know the firefighters know what to do.

              Will finish the install on Monday.


                p.s. The home owner is a pretty savvy guy in the HVAC & plumbing business who hooked up his own gas line to the generator, so I feel confident that he will disconnect the generator at the ATS if he needs to shut off the power.


                  Sg 1

                  SG 1,

                  Would I be able to find a form C contact to replace a simple residential Square D 150 amp main breaker?


                    Contact your Square D supplier to see. They may only be available in industrial type breakers. You never know unless you ask. If not available, why not mount an enclosure at the service with a switch to do the same thing ? Not as fool proof, but handy. I am sorry about not providing more detail about how the form "c" contact would actually be used.
                    Advise is a dangerous gift, even from the wise to the wise.