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I have a room where I need to toggle the lights by a switch and the outlets in the room to not be affected by the switch.

As they are on the same circuit, am I able to run a 3 wire cable from the switch so that I'm feeding switched and non-switched power throughout? A little ASCII art to explain:

Incoming cable                          Outgoing cable
--------------            --------      --------------   
Hot (from service)        |switch|      Hot (from switch)
Neutral (from service)    --------      Hot (from service)
                                        Neutral

I suspect this isn't an issue because everything is on the same circuit, but thought I'd double check since I would normally pair a neutral with every single hot line.

EDIT: Just so there are no misundersandings, the incoming and outgoing cable also have a ground wire in them.

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In AUS this is illegal and does not conform to AS3000. It would also render any insurance invalid and if any licensed professional came across this in Aus they would be forced to do an inspection and "make safe" all wiring - at your cost. But you are probably not Australian. –  user13766 Jul 1 '13 at 12:04

3 Answers 3

up vote 7 down vote accepted

There is no problem with this. In fact it's very common, especially when wiring ceiling fans. The power (2 wire /w ground) enters the switch box, then a switched hot and unswitched hot are supplied to the fan (3 wire /w ground). This allows the light to be toggled by the wall switch, while the fan is toggled by the switch on the fixture. It's also a common practice to wire duplex receptacles this way, where one half of the device is always powered and the other half is controlled by a switch (in which case the tab separating the two halves should be removed on the ungrounded (hot) side).

The grounded (neutral) conductor will carry the combined current of the two ungrounded (hot) conductors, so any inductive coupling will still be canceled out. Since the circuit is protected by an overcurrent device (circuit breaker/fuse), there is no way to overload the grounded (neutral) conductor. The current on the ungrounded (hot) conductors will be limited by the overcurrent device, so too much current should never flow on the neutral.

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1  
+1 Note that for duplex receptacles to be wired this way (which I have in many rooms) the tab on the hot side joining the two halves needs to be broken off. –  bib Jul 2 '13 at 3:09
    
Thanks @bib, I figured that fact was outside the scope of my answer so I didn't mention it. –  Tester101 Jul 2 '13 at 13:40

This is OK except at some point the 3-wire exit cable (+GND) is going to branch off to go to the outlet box. This most likely will be happening in the box that the switched light mounts to. In that case you will obviously connecting in a neutral / unswitched hot wire pair off to the outlet(s).

If there is more than one switched light box you may extend the three conductor cable (+GND) through to the far light box if the outlets feed off the end of the last light box. If the outlet boxes branch off before the last light box then that last light box(es) will require just a two conductor cable (+GND).

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You will have hot/neutral/ground coming in.

To your light(s) that you want to toggle you need to have hot/neutral/ground.

To the rest of the outlets you need hot/neutral/ground going out to them.

If everything is starting at this light switch then you would have

  • Your "power" line coming in - 3 wires
  • You would have 3 small wires to go from live to switch
  • You would have have your line out to the outlets
  • So to recap - You would have 3 black, 3 (white) neutral, 3 green (ground) all individually capped.
  • You would install the other end of your 3 "short" wires to the light switch.
  • The other "end" of your light switch would feed your lights, and the near end goes into light switch.
  • The other end of your outlet circuit (3 wires) goes out to your outlet run
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"You would have 3 black, 3 neutral, 3 white all individually capped" ... what color is the neutral? –  BMitch Jul 1 '13 at 12:02
    
Ha. Just a typo. Fixed above. –  DMoore Jul 1 '13 at 14:44

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