Help installing Nest Smoke/Co alarm to capped ceiling light wires

Question

I am trying to install a Nest Protect (co/smoke alarm) and need to connect a black wire to the power line (which is the the one with the red cap after I used a voltage detector) and the white wire to the neutral line. I need help with the following:

1) how do I know which is the neutral and which is the interconnect - I think the neutral is the single black line pictured with the yellow cap as the white line needs to be connected with the braided line for the other lights to work so I thought that would be the interconnect. Does that sound right?

2) how do I join the black wire from my Nest to the power line when the power line is those three lines with a copper tip in the red cap?

Answer 1

Based on information from Nest's website, the Nest Protect doesn't have wired interconnect, only wireless, so you shouldn't have to worry about the interconnect wire - unless your user manual says something differently?

I'm guessing that the white/braided wires with the white cap is your neutral line. Since there are three hot wires running through your junction, there would likely be three neutrals as well. This can be confirmed by:

1. Disabling power to your circuit (turn off breaker)
2. Disconnecting the wires under the white cap
3. Turn power back on and check to see if any lights don't work
4. If so, that's your neutral

As for joining into the black wire - I would try to remove the copper connector and then tie in your Nest's wire using a new wire nut. If the connector is crimped on too tightly, you'll have to clip the wires back just behind the connector, re-strip them, and then install the new wire nut.

Answer 2

That is not a standard/modern/common box. I'm willing to bet that the conductors there are never grouped into multi-conductor jackets, and instead are run through your walls as individual conductors or knob and tube.

In such an arcane system there is no telling what is hot or ground just by the color of the wires.

A wired nest protect however just needs 120v across its two wires. (They do not have wired interconnects or Earth Grounding.) So multimeter it until you find 120v and use that.

I'd also mention that red cap you removed is probably not the best choice to disconnect or mess with. It looks like a semi-permanent, crimped junction. Try the two ends with twist-off wirenuts.