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Thread: Induction in switch loop

  1. #1
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    Induction in switch loop

    If you run a switch loop in emt with no other conductors, no neutral. Will the conductor in the switch loop experience inductive heating? Or will it cancel out?

  2. #2
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    The current will be traveling two directions and will cancel out.

    Roger
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    Quote Originally Posted by BassFace View Post
    If you run a switch loop in emt with no other conductors, no neutral. Will the conductor in the switch loop experience inductive heating? Or will it cancel out?
    I don't think this is allowed anymore.
    Bob

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    Quote Originally Posted by petersonra View Post
    I don't think this is allowed anymore.
    Bob, I think you are thinking about 404.2(C), the exceptions actually allow you to omit the neutral to the switch in many installations.

    Roger
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    Quote Originally Posted by roger View Post
    Bob, I think you are thinking about 404.2(C), the exceptions actually allow you to omit the neutral to the switch in many installations.

    Roger
    I don't think we are talking about running to the switch, but rather from the switch to the switched device.

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    Quote Originally Posted by gadfly56 View Post
    I don't think we are talking about running to the switch, but rather from the switch to the switched device.
    I agree, I was just pointing out what I think Bob was referring to.

    It is perfectly fine to just run a feed and a switch leg in a raceway or cable, they will cancel each others EMF effects.

    Roger
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  7. #7
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    Quote Originally Posted by roger View Post
    I agree, I was just pointing out what I think Bob was referring to.

    It is perfectly fine to just run a feed and a switch leg in a raceway or cable, they will cancel each others EMF effects.

    Roger
    Got it.

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    Thanks Roger! I am an apprentice and myself and another apprentice were arguing about the effects of EMF in a switch loop. He said it wouldn't cancel I said it would. Thanks for settling the argument.

  9. #9
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    Quote Originally Posted by BassFace View Post
    Thanks Roger! I am an apprentice and myself and another apprentice were arguing about the effects of EMF in a switch loop. He said it wouldn't cancel I said it would. Thanks for settling the argument.
    Don't worry about it, we've been wiring 2 wire switch loops for over a hundred years without any problems.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  10. #10
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    BassFace:

    Often times experiments may be a useful way to prove to someone a concept.

    But first about your first post. Induction heating is not of the wire, but of the conduit. However, heating the conduit will transfer some heat to the wire. The wire itself is not where the induction heating is occurring. Rather the AC current thru a single wire in the conduit is inducing a current in the conduit, and it is the induced current in the conduit that is heating the conduit.

    On experiments. You can take a 500 or 1000 ft roll of wire and use it to sense some 60 Hz stray magnetic fields. Small diameter wire is desirable from a size and weight point of view, but you can somewhat usefully use #16 or #14. I ran a test with #16. Both ends of the coil must come out of the spool so you can connect to the coil. Besides a mV meter you probably want about 2 ufd of capacitance (paper, Mylar, or Polypropylene) across the coil (across the coil is the same as meter input) to reduce high frequency noise. This way I was around 0.1 mV of background. Other wise without the shunt capacitor of 2 ufd it was about 10 mV.

    With a 250 W lamp load on a normal rip cord and the cord close to the sensing coil the meter reading is not much different than background. About 0.3 to 0.4 mV. In this case we have exactly the same current in each wire of the rip cord, but one current is flowing in the opposite direction of the other, and their magnetic fields at only a short distance from the wire pair mostly cancel each other.

    Next, if I rip the cord apart to create two separate wires, actually I used two separate wires, put one wire close to the sensing coil, and the other wire about 1 foot away, then I saw about 5 mV of induced voltage. Probably not the maximum I can produce. This demonstrates that when you sufficiently separate the two current paths, to and from, that a substantial induced voltage can be produced.

    I have better sensing coils than this, but I wanted to see what I could do with something anyone on this forum might be able to find.

    .

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