I have taken down a wall that separates the kitchen and living room area, and have to relocate the light switches, thermostats, and base board heaters... After letting all the wiring go from same and relocating the boxes, I've found that the original wiring is not long enough to reach the boxes new location, running from the attic... Do I splice each 12/2 wire, for added length, at the site of the wire. Or do I use separate junction boxes in the attic, then use the appropriate length wiring to reach the new switches?

Thanks in advance for any help offered.

Splices are quite easy. Install a Junction Box at the splice location (in the attic). Splice and join wires with Barrets. Install a cover plate over JB.

Pull up all the cables into the attic, and splice each of the cables that are too short in a junction box, onto a new cable extension that can be re-routed to the device that is needed.

As long as the attic is accessible, junction boxes located in the attic are fine. Be sure to splice each wire properly, and use a blank cover on each box.

Multple cables can use one junction box, but each box has a limit to the amount of wires. Need to allow 2 cubic inch for each #14 wire, and 2.25 CI for each #12.

So, for example, say you had a typical 4x4 JB 1.5 in deep, this has 24 cubic inch of volume, and can only fit a total of 10 #12 wires. This equals 4- 2 wire cables, each black and white is counted, and all the ground wires are counted as one.

Thanks guys for the help... Everything is done as you have stated, aside from one wire that's running a light switch, which is spliced only using caps(no JB), inside the existing wall.. Do you suggest I use a Junction Box for this as well, or should It be fine as is?

No, the splices need to be in a box, with a cover and accessible. If the box will be in a finished wall there is a good selection of blank finish plates.