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I'd like to install a Jasco Z-Wave switch (45709) and its add-on switch (45710) in this newish (2004) house, but the wiring instructions do not correspond to what I have behind the current (mechanical) switches. It currently appears to be wired the same as this diagram:

3-way switch wiring

The colors match, and I've confirmed the black wire on the left hand switch is hot regardless of switch position.

Some of the connections are obvious:

  • the traveler wire just goes from switch to switch
  • the black on the left switch is line (but not needed; see below)
  • on the right switch line can be tapped from the wire nut
  • the black going to common on the right switch is the load

But that still leaves the question of where the neutral connection comes from in each switch. To be clear, the 45709 expects line, neutral, load and traveler, and the 45710 expects neutral and traveler. (Wait, what? It doesn't need line. I guess the traveler wire carries DC power as well as signalling.)

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Unfortunately, that style of wiring -- essentially a "switched loop" -- is not compatible with "smart" switches that require a neutral.

The 2011 NEC finally recognized this and now requires neutral at switches, but your home obviously predates that requirement.

There are a few options, but none are easy/cheap:

New wire

Replace the wire between switch and light with a three-conductor (14/3) wire. You get hot, neutral, and switched hot.

New power feed

Run a new power source (which would include neutral) to either switch, and disconnect the existing source in the light.

The existing 3-wire between switches carries hot, neutral, and your signal/traveler, and the 2-wire between switch and light carries neutral and switched hot.

This requires access to one of the light boxes, as well as another power source.

Use an in-line module

Buy and install a Z-Wave "in-line" dimmer or relay in the light box to control the light.

z-wave inline module

The existing 2-wire between the light and switch would just be hot and neutral, and you'd setup the switch to control the in-line module via z-wave signalling. There is no switched-hot going to the light. The existing 3-wire between switches carries hot, neutral, and your signal/traveler.

Nice thing about this option is it doesn't require changing wiring (and thus no holes/etc). The module will cost ~$60 but that may be the cheapest option compared to the alternatives anyway.

  • Thank you for your comprehensive answer. I like the last option, but just to confirm, would I be keeping the original mechanical switches or using the Jasco switches? The Q&As on Amazon suggest that the original switches are kept, because the module can respond to them being switched, but you seem to be saying that I'd be using the Z-Wave switches to control the module. Can you elaborate on that? (These will be my first HA devices.) – Dan Ellis Sep 20 '15 at 7:28
  • I guess you actually have either option; it depends on your purpose for adding controls in the first place, and if you'd like dimming capability or to be able to control other z-wave devices. – gregmac Sep 20 '15 at 16:28

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