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Just looking to get someone's opinion on the legality or practicality of this circuit:

Three-way light option

There was another question posted recently where someone was asking about the ability to switch a second light both independently and dependently of a three-way circuit. This was the first idea that came to mind, but I've never seen a circuit like this and could only imagine the headache trying to troubleshoot this if you stumbled upon it in the future. Basically a regular three-way circuit, but then connect a third switch in reverse with another source of power tapped from the common in.

Condition matrix

              | Circuit 2 Pos A | Circuit 2 Pos B
Circuit 1 ON  | L1 *, L2 *      | L1 *, L2 *    
Circuit 1 OFF | L1 o, L2 o      | L1 *, L2 o

I do not believe it is against code directly, but is there something prohibiting this setup? (Neutrals at each switch box, but I ignored that for clarity in the picture.)

  • From a user experience perspective, the third switch could be frustrating. In the case where the two upper lights are on, the third switch would serve no function. – isherwood Feb 22 '18 at 22:25
  • Basically he wanted the upper set of lights to work like a regular 3-way. The lower light should be cut off when the 3-way cuts off. But if the 3-way is off, he wanted a way to turn on that light by itself. – TFK Feb 22 '18 at 22:27
  • Just a regular 3 way, but that bottom light has a bypass power source. – TFK Feb 22 '18 at 22:28
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    It is not a code violation since it is fed from the same branch circuit but I have never seen something like this in a real world example that I can remember. – Ed Beal Feb 22 '18 at 22:50
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    Between your 3-ways you need 4 wires in 1 cable, the classic problem of carrying always-hot and neutral through a 3-way complex. 9 times out of 10 this involves illegally using 2 cables or bootlegging ground. The right answer is conduit between the 3ways or rare /4 cable. – Harper Feb 22 '18 at 22:57
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The problem with "electronics style" schematics is they completely ignore wiring methods. Wiring methods are the heart of electrical. Understanding this craft (or lack of understanding) is the #1 difference between electricians and electrical engineers.

So I've redrawn, first using wire colors for function so I don't lose my mind... and second making a real effort to consider wiring methods and routings.

enter image description here

Now, I used red for both switched-hot circuits. In actual field application I would probably use orange for the second switched-hot, otherwise the next gal might mistake the 2 reds for travelers.

Now, with wiring methods considered, small problem: you need four conductors between the 3-way switches. Again back to wiring methods, you can't just hork this together with a couple of /2s, and you can't slapdash a solitary THHN wire along a /3. You will need the less common /4 cable, or run conduit between and use THHN.

I snubbed blue for neutral because /4 actually has a blue.

Other than that, it's as straightforward as can be. We can plainly see it's a "tree" topology and no tricky loops, so currents will be equal in all cables. It's all good.

  • I wouldn't call a ladder control diagram an electronics schematic, but sorry for my quick drawing. I shouldn't have included the neutrals. .. I see the issue with the 4 wire better now though. – TFK Feb 24 '18 at 5:04
  • Well the root of your idea was superb and you nailed it. But you should include the neutrals because keeping the currents equal is a non-trivial exercise. It helps to draw things as a tree and never encircle any areas on the diagram. – Harper Feb 24 '18 at 5:43
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Here is my two cents: If your connection at light 2 back to the main neutral were to have a break (and you were still connected neutrals from light 1 to light 2) then light 1 could become hot on its neutral line - something that would never happen in the 3-way only scenario.

IMO while not a code violation it wreaks of failing in the spirit of the code - Safety.

EDIT 2-23-2018

Typically a circuit with only the 3-ways you expect Hot from one wire to your load - even if they are paralleled easy enough to recognize. This circuit 'hides' the fact there are two paths for the hot for lamp 2 - which if the neutral becomes free from Lamp 2 to main yet lamp 2 to lamp 1 the neutral remains - of course this will cause someone to investigate why the lights do not work. When working on the circuit the potential is that now you can easily have hot on that neutral at lamp 1 and this is not as apparent in this particular scenario. A truly parallel circuit each load operates via one switching circuit (whether 1-way, 3-Way or otherwise) you expect what you have there could be a current path via any of the loads IF you have power coming out of the (switch circuit) . This scenario you you have 2 power paths that can operate one of the loads (which behaves in certain scenarios as if it is part of that 3-way switching circuit with one path for power) - my point is that it is much less apparent that you can have that neutral line become hot through the load. Again this would never happen in the 3 way only scenario - if the neutral at Light 1 became disconnected (light 1 would at the socket be hot through the load - but that is easily recognized and the wire itself once disconnected from the socket - behaves as a neutral would - not as a hot would).

My point here is that to an unsuspecting person troubleshooting this - they can easily find hot on a neutral - something the 3-way only circuit would never present them with - it might be hot on the socket but the neutral wire if disconnected from lamp socket 1 - could present itself HOT (if perhaps there is a neutral to main disconnect issue at lamp 2.

I hope my edit makes clear what I am trying to get at here... I am not talking about whether the circuit works or not; as it does. Nor code as it is acceptable in code. I am just saying look at it from the wires being hidden (wall and ceiling) and the operation of the circuit (now you or a DIY'er walk's into it to troubleshoot it and neither light is working - what will you touch and disconnect? How many people know that the neutral is not a hot wire and might 'touch' that wire - you know remove the wire nut so you can check things or disconnect from socket etc.. How many of you have simply undone a nice white wire grouped together with a wire nut and touched that bare white wire ???? I am sure a few of you - even well seasoned guys have done so...

  • A load becoming hot on its neutral side (via back feed through another load) can happen with any loads that share part of their neutral path when that shared part breaks. - The safety principle that you imply would require all the lights in the building to be controlled by one switch. – A. I. Breveleri Feb 23 '18 at 12:45
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    Neutrals could be tapped off where the common is tapped off. It doesn't have to run between the last two lights. – TFK Feb 23 '18 at 13:37
  • @A.I.Breveleri - you are correct. I was looking at the circuit given your typical scenario of what you might expect. Typically a circuit with only the 3-ways you expect Hot from one wire to your load - even if they are paralleled easy enough to recognize. This circuit 'hides' the fact there are two paths for the hot for lamp 2. When working on the circuit the potential is that you can easily have hot on that neutral (by disconnect from the main) - much less apparent than normal. – Ken Feb 24 '18 at 2:26
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    I see your edit and you are correct. Seeing the guts of this switchbox could be a nightmare for anyone expecting a typical 3-way switch. This is why I figured remarking of the conductors might be worthwhile. For the neutral though, I think it's a non-issue as it could be connected like Harper's post above rather than my hastily drawn picture. – TFK Feb 24 '18 at 4:49

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