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I'm remodeling my master bedroom, and in the process I have to fix some shoddy electrical work from previous homeowners.

It seems that they tapped into the existing circuit to add an exterior flood light controlled by a switch in the bedroom. There is a conduit powered from the flood light's junction box, which continues the circuit to an exterior receptacle, and a buried conduit that leads out to a shed in the backyard.

The original wiring job was haphazard at best. There was an unsecured junction box just floating in the attic. One NM cable and an unsheathed wire marked as hot (red in picture) lead to the exterior flood light box. Unfortunately, I neglected to take pictures of precisely how the floating junction box was wired, but I believe it must have been like in the picture (labeled "unknown"). The light switch had 2 NM cable legs connected to it.

So, my question

In the current state of the remodel, I've got the power in for the circuit, and the 12/2 and floating hot wire to the exterior light. I assume I'll need to replace the 12/2 + hot with 12/3. How exactly do I work in this light switch into the existing circuit, sending both a switched hot leg and a powered hot leg to the exterior junction box?

Picture of situation

Should I run 12/3 from the light switch box, and power it light so? light box wiring?

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    Can you re-pull the run from the attic-blob to the floodlight switch box with a 12/3 cable? – ThreePhaseEel Sep 16 '18 at 2:48
  • Oh yeah, the walls are all open. I've got power in, a destination, and a blank slate. I actually ended up wiring it like my second picture, with power down to the switch and 12/3 from the switch to the flood light box. Breaker is back on and everything appears to be in order. – Bill Sep 16 '18 at 20:13
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    post your proposed illustration as an answer and I'll give it a +1 – ThreePhaseEel Sep 16 '18 at 23:11
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Looks like posting it here was all I needed to sort this out in my head.

The wiring is a similar idea to wiring two switches to a ceiling fan with a light, using a 12/3 cable. Only here, I've got one switch for the light, and the other hot leg is tied in so that it's always on, not switched.

I pulled a 12/3 cable from the flood light box (wired the same as first picture) to the light switch, and wired it as shown in the second picture, which I'll post again below:

In case my drawing isn't clear, the yellow boxes are wire nuts, and the terminal at the lower left on the switch is the ground. The red wire (hot) is connected to the light. enter image description here

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