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I would like to add a Honeywell RPLS740B to a circuit with two three-way switches to control four outside lights. I realize the RPLS740B is not compatible with a three-way circuit, but my question is if it is possible to wire it such that three-way switching is "disabled" but the circuit still works with the timer? For example:

I have identified the switch on the line side, fed by a two-wire cable from the panel. Can I install the timer in place of this switch and tie the load side of the timer with both travelers leaving the box? It seems to me that the timer would then power both of the travelers and the outside lights would be on with the timer regardless of the position of the second (load-side) three-way switch. I would be OK with that compromise (i.e., loose three-way functionality from second switch).

Is it safe to tie the travelers together? This would obviate the need to cap any wires.

Is an alternative to cap the red traveler, install the timer in place of the first line-side three-way, and use the black wires throughout the circuit? This would leave the second load-side three way as a dummy?

Is there another option?

  • Do these switch locations include grounds and neutrals; are they romex/cable assemblies or spooled wires? – ChiefTwoPencils Jan 17 '15 at 5:43
  • Neutral and ground present, 14/2 or 14/3 Romex cables. – ph0t0n Jan 17 '15 at 11:39
  • Sorry, just want to be sure before I answer you. So, it's 14/2 or 14/3, or 14/2 and 14/3 at the separate locations? Does the line side have 14/2 feeding it and 14/3 going out to the other location? Have you verified that it doesn't terminate in another fixture or box and if it does do you have easy access to it? – ChiefTwoPencils Jan 17 '15 at 18:34
  • 14/2 into the box on line, 14/3 between boxes, 14/2 out to fixture. Standard three way arrangement with three way switches between line and load. – ph0t0n Jan 18 '15 at 4:21
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Can I install the timer in place of this switch and tie the load side of the timer with both travelers leaving the box?

Yes (probably) and no. Unless the timer pulls an amount of current that would put the circuit over 80% of the 15 A supplied by the breaker (12 A) you can install the timer on the original circuit but don't tap the travelers together.

You can't run 14 AWG wire in parallel, which would be the case if you tapped them together. This is done when dealing with larger wires where the cost factor compared to additional ampacity isn't economical.

NEC 310.10(H) Conductors in Parrallel

(1) General

Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sized 1/0 AWG and larger where installed in accordance with 310.10(H)(2) though (H)(6).

Note: You can actually run conductors smaller than that in parallel. Those exceptions don't seem to apply here.

Is an alternative to cap the red traveler, install the timer in place of the first line-side three-way, and use the black wires throughout the circuit? This would leave the second load-side three way as a dummy?

Yes, this is the correct thing to do based on the above information. However, I wouldn't leave a dummy switch there. Just remove the switch and replace its cover with a blank plate.

Is there another option?

Well, there's a couple additional options depending on current and future plans for operation.

  1. If the device is capable and there's a need, you can use the red as a second timed leg. This would give you the ability to use it for some future fixtures. Obviously, additional work would be done.
  2. You could use the black for a constant hot and the red for the switch leg and use the black to feed a single-pole switch as sort of an override. The timer would turn them on whether the switch is on or not, and the switch could turn them on without manually switching the timer circuit any time you need.
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You are correct in that the RPLS740B is not directly compatible with a three-way circuit; however, since this timer uses a neutral wire for its return instead of returning via the load, you can use it to drive a SPDT relay that replaces one of the three-way switches.

Another option, of course, would be to take your existing timer back and install a RPLS540A instead; this timer is three-way compatible, but returns via the load.

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