The first thing to check is whether, with all those existing wires, new wires, and an additional switch, the existing box is large enough to meet code requirements. It looks like you have 6 conductors (plus ground wires) entering the box, and I am guessing that you would have one more cable in the box, plus another switch... correct? That would require 30 cubic inches of volume, for example a 4" X 4" box that was a minimum of 2-1/8" deep. Do you have that volume? And if you have more wires, then you would need a bigger box.

As for what is what in your current situation, it looks most likely that the center cable is power, if I am following those slightly hard-to-differentiate wires correctly.

The wiring option I would choose is to remove disconnect all the wires from the switch on the left and pigtail the power wire to the jumper wire between the switches; this wire nut connection would also include two other pigtail wires. That way you have three pigtail wires, each providing power to one of the three switches. Then you would have one new cable for the new switch, and that would go ground to the grounds, white to the whites, and black to one side of the new, third switch. Essentially you would have all the switches connected the same way, with the power cable having three pigtails, one to each switch, and each of the three black wires from the non-power cables connected to the other side of each of the three switches. Is that clear?

And, whatever you do, turn off the breaker and then use a tester to confirm that the power is off before doing anything. I'm not an electrician, but, that's the best suggestion I can offer based on what I think I see in those photos and my understanding of what you want to do. Let me know if I've misperceived what is going on in that box.

There would still only be two "yokes" in the box even with three switches ;)

(4) Device or Equipment Fill. For each yoke or strap containing one or more devices or equipment, a double volume allowance in accordance with Table 314.16(B) shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap.

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http://freenec.com/T145.html

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Also, be careful trying to identify 14 gauge by the white outside sheathing. There were many years when nm-b was white sheathed for ALL wire guages.

"The power supply will be located in a cabinet just above this box."

Do you mean the transformer for the lighting? ( I see that is what they call it, as opposed to the power supply from the utility company ;)

"Then from the power supply, I'll be running to a compatible dimmer I'll install on the right side of this work box, and then to the lights, etc."

Usually, the order would go, dimmer - transformer - lights, not transformer - dimmer - lights.

In this case, same guage wire as in the switch box to the lighting transformer, then use the appropriately sized wire from the transformer to the lights.

Fred-

My interpretation of the code is that two switches on a yoke counts as two individual single switches, so when I did the box fill calc above, I used three switches. Are we agreeing on that?

Yeah - I obviously don't know the terminology. Apologies for that.

I have wired in my previous two switches (a 15w light fixture and a garbage disposal) to the two-switch "yoke" (?) on the left. That is working fine. I just screwed in the dimmer switch to fill the gap and see how it'll look, though it isn't hooked up to anything yet. Here's what it looks like now:

I didn't realize that work box volume was factor, either. Do they make 2-gang old work boxes can accommodate what I'm going for?

I know that typically you run power to a dimmer first, then the transformer, and then the lights. But the power supply I bought from Armacost is not an AC compatible dimmer. It will only work with 12V DC voltage. Therefore I have to go:

120V Line - Transformer - Dimmer - Lights

I'm wondering: can I hardwire in 14/2 romex from the transformer to the line voltage in that same work box? Then run 16 AWG back to the dimmer, then 16 AWG to my lights? Seems real tedious but I don't see how I can do it differently, given what I'm working with.

Kudzu, do you count this as two separate receptacles?

Bob, you can not put 120 volt and 12 volt in the same switch box unless you put a divider in the box. This will make box fill size 22 cu in or more per side or 44 cu in total. That will be hard to impossible to find.

Getting the 120 volt dimmer, and the correct transformer would be easier IMO.

Okay. Not what I wanted to hear, but thanks so much for letting me know.

Do you think this combination of parts could work? Lutron Lumea dimmer and an LED Wholesalers dimmable transformer?

http://www.lowes.com/pd_11239-539-LGCL-153PLH-WH_1z0yt1r__?productId=3350880&pl=1

http://www.amazon.com/dp/B007KWVFXC/ref=biss_dp_t_asn

Fred-

I count that as one receptacle, but I have thought of two switches on a strap as two devices. I could be wrong...what is your view? How about this...is this one device or more?:

Kudzu, that is ONE "yoke or strap containing one OR MORE devices or equipment,"

And the other was 2 devices (duplex receptacle) on ONE yoke.

NEC DEFINITION

Receptacle. A receptacle is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacle is two or more contact devices on the same yoke.

Even with USB ports, this still only counts as one (1) yoke.

Bob, what is wrong with the recommended dimmer?

http://www.armacostlighting.com/store/shop/24-watt-class-2-electronic-power-supply/

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There are dimmer compatibility charts for most transformers. This is for armacost.

http://www.armacostlighting.com/store/45WCPC-dimmer-info/