Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Contactor Wiring Question

  1. Jul 13, 2015 #1
    If I have a contactor that I want to use with two separate control sources, on a site with a single phase power source, is it possible to connect two 120V control signals to the contactor coil, potentially live at the same time and connected to the same source, without causing a short, or damaging any equipment? The circuit is being designed to automatically turn on security lights at intervals set on a time clock, but with the addition of a manual override to turn on the lights as needed. I am not an electrician, but the work will be completed by a certified electrician once the planning for the building upgrades is complete. I'm just trying to understand how this would work. My understanding is that since the power is coming from the same breaker panel and the same power source, the two control voltages should always be in phase, eliminating the possibility of constructive and destructive interference. I would think that this setup would be considered a parallel arrangement? Meaning there would never be more than 120V supplied to the coil if both inputs were on, but the current available to the coil would be higher? Or would that supply too much current to the coil and cause problems? I'm just curious, I don't know how the electrical contractor will actually hook everything up when the time comes, but I have to admit I find this concept rather fascinating and I'd like to know a little more about it.
  2. jcsd
  3. Jul 14, 2015 #2


    User Avatar
    Gold Member

    If what you are suggesting is two switches in parallel sourced from the same exact breaker and connected to the same contactor terminal, then there is no issue.

    If it is coming from a different breaker in the control panel, then NO! since it could come from different input leg (I guess you say it is single phase), and NO! you must not short the output of two breakers.
  4. Jul 14, 2015 #3
    Thank you! I definitely learned from that! In the real world, is it likely an electrical contractor would do it this way, or would they likely do something entirely different? I know its hard to say because it depends on the contractor, but is there a simpler way to accomplish the same task?
  5. Jul 14, 2015 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    not only mustn't you short them,

    you mustn't mix their return wires either. What goes out through one breaker must return through THAT SAME breaker's neutral.
  6. Jul 14, 2015 #5


    User Avatar
    Gold Member

    Just tell the contractor to wire them from the same breaker. That is probably what he would do anyway if you told him nothing.
    (assuming you are in a country with an enforced Electrical Code)
  7. Jul 14, 2015 #6
    Why does it have to be from two different sources? The same source can feed the timer AND the bypass switch - this is very common.
  8. Jul 14, 2015 #7


    User Avatar
    Gold Member

    If, indeed, you MUST use two sources from two different breakers, then you also need two contactors. The extra contactor, whose coil is powered by the second breaker through the bypass switch, switches power from the first breaker to the primary contactor. (or vice versa, the timer powers the second contactor)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Contactor Wiring Question
  1. IEC Contactors (Replies: 4)

  2. Helping in contactor (Replies: 9)

  3. Wiring Question (Replies: 16)

  4. Wiring Question (Replies: 1)