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I have an existing porch light on a (spring-style) timer. I would like to add a switch for more convenience, so that the light could be manually turned on for a few seconds or for several hours.

The function of the 3 way switch would be to turn the light on directly or transfer power to the timer (in series). Would this be an okay (code compliant) way to add a (simple) switch to a timer?

Another part of the reason I ask, is because a digital timer (with simple on/off button) is like $30 in my area, whereas a 3 way switch is like $1.50. And the timer just happens to be in a box with an outlet switch that could easily be eliminated (meaning, I don't have to buy a remodel box or anything else).

Edit- New picture for clarity:

enter image description here

  • Is S2 a 3-way device? If S2 only has two terminals (as opposed to two travelers and one common) then your wiring arrangement will cause the light to be always on. – Shimon Rura Aug 4 '16 at 20:47
  • @ShimonRura S2 has 2 terminals... no it won't always be on... it would only be on if the timer was on or if the light was powered by S1. – Ben Welborn Aug 4 '16 at 20:50
  • Digital countdown timers start at less than $20 in my area (e.g., lowes.com/pd/…), so you may not want to discount that idea entirely. Your usage is exactly what they are for, so it seems a bit needless to hack together a solution involving multiple devices when a single one will work perfectly – mmathis Aug 4 '16 at 20:52
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    I see... so with S1 in position 1 (feeding the red-wire traveler) the light is only on if the timer is wound. In position 2 (feeding black traveler) the light is on regardless of the timer. Interesting scenario, makes sense when these two switches are adjacent but would seem wrong in most normal applications of 3-way switches. – Shimon Rura Aug 4 '16 at 21:14
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    @ThreePhaseEel It's like an egg timer. On for up to 60 minutes. Spring powered timer. – Ben Welborn Aug 4 '16 at 22:46
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I looked at your circuit. With the switch in one position it would be on with the switch in the second "timer position" it would only be on until the timer expired. If listed devices in boxes that do not exceede the box fill for wires and devices this would be legal in the U.S.

  • I suppose this is the answer... for lack of any evidence to the contrary. But I am kind of wondering whether this wiring arrangement could apply to a digital timer. I know it's not necessary (because digital timers already have on/off buttons), but I wonder about applying 120vac to the "wrong" side of a digital timer. – Ben Welborn Aug 5 '16 at 14:03
  • You might want to wire the timer between the mains power and the three-way. You can avoid feeding hot into the timer load terminal. – A. I. Breveleri Aug 5 '16 at 16:59
  • @A.I.Breveleri do you mean have the timer set up like the second picture in your answer? The second picture in your answer is a sensible arrangement for a digital timer. – Ben Welborn Aug 5 '16 at 21:06
  • @Ben Welborn: Yes. All three diagrams - your proposed circuit and both my proposed circuits - will work perfectly safely with a spring timer. - However, my second diagram eliminates parallel wiring and allows you to restore your undergarments to the untwisted position. – A. I. Breveleri Aug 5 '16 at 21:19
  • @A.I.Breveleri your first picture is a fire hazard and it is illegal. But I know that it would work... just like parallel resistors. – Ben Welborn Aug 5 '16 at 21:20
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A three-way switch is not necessary. You can connect a single-pole single-throw in parallel with the spring timer.

SPST with timer

EDIT: Future readers should note that this is electrician-safe only because both switches are in the same gang box. With the switches in separate boxes, someone working on this circuit in future might be surprised to find voltage on what he thought was a load wire that he had just switched off.

Also note that this is equipment-safe only because a simple mechanical spring timer can tolerate hot voltage on its load terminal. (In fact the spring timer probably does not have "LINE" and "LOAD" labels.)

If anyone ever wants to cobble together a similar setup with a timer that (1) doesn't have an "ON" setting, and (2) may not tolerate hot voltage on its LOAD terminal, then he will need a three-way switch. But it can't be wired according to your proposed circuit. Instead, the timer must be wired between the mains power and the three-way.

SPDT with timer

Here the three-way selects between the always-on hot or the timer switched hot. It never feeds power back upstream.

  • If both switches are on, that would create a parallel hot wire. It would not be safe and is against NEC code. Look up parallel conductors in the NEC. – Ben Welborn Aug 5 '16 at 12:15
  • @Ben Welborn: I cannot find the section in the NEC that describes or touches upon this circuit. Looking up "parallel conductors" in the NEC leads to section 310.10, which covers a completely different definition of the term "parallel conductors". - Could you please explain more. – A. I. Breveleri Aug 5 '16 at 15:58
  • Yep, 310.4 Conductors in Parallel: they are allowed as long as they are 1/0 or larger (so like 1/3 inch or larger), AND they have to terminate the same, be exactly the same length, the same insulation, same size and same conductor material. Switches and 12 guage wire have resistance... the voltage will be about the same and it might not pose an issue within an hour, but it really is a fire hazard. – Ben Welborn Aug 5 '16 at 16:51
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    @Ben Welborn: You are misapplying the standard. NEC 310 covers using parallel conductors in place of a single larger conductor. - This circuit is safe because either parallel leg can easily handle the full load. – A. I. Breveleri Aug 5 '16 at 17:02
  • When both switches are on, they are both conducting and they will have a difference in voltage... a switch does not have high resistance or create a huge current draw, but let's consider something that does, like a light bulb. What would happen if you put a jumper across those wires? It would create a short circuit (pow). A parallel circuit is essentially the same thing as a short... the flow will go through the less resistant switch, and the voltage difference in the wires can cause heating. As another example, consider a 30 ft loop of wire and the voltage difference across that short. – Ben Welborn Aug 5 '16 at 20:59

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