I cannot speak for the US system. In Europe there is not one system, It is not a continent wide electrical code. Every country has its own wiring regulations and code. With that said there has been large efforts to harmonize systems from country to country. Also in Europe we do not use the AWG (American Wire Gauge) cables are graded/categorised by there cross sectional area. i.e 2.5mm or 4mm etc. The following is directly from the BS7671 which is the wiring regulations code for the entire of the U.K while other European countries may be slightly different it would be very similar.
http://www.batt.co.uk/upload/files/table4d1abs6004bs6231bs6346_1220253954.pdf
I should also point out it is very hard to have just one table as there are literally 40 pages of tables. Because you have to take into a large amount of factors including. Conductor material, Type of sheath composition. Thermal abilities of the cable and the environment it will be in. The type of containment/fittings. and then there are applied factors such as correction for heat and density of cables and voltage drop. The lists go on and on.
What watt power transformer should I be using?
You can use a power supply like the one pictured, rated for the amount of LED's you will be driving. Read the specs on the lighting. They should tell you how many watts per a given length of the strip. So if it says, for example, that needs about 1 watt per meter, then a 15-foot section would require a 5 watt power supply. Take your total length of 76 feet, and divide by the strip's requirements. It wouldn't surprise me if you ended up somewhere in the ballpark of 20 to 30 watts.
Can I run 14 gauge wire directly from a circuit 15 amp breaker to the transformer?
No. You can't put the power supply inside the wall, and you can't bring the wire out of the wall without going through a junction box. The best way to do this is to put a cord on that power supply, and plug it into an existing outlet. There is no need whatsoever to add a new circuit or connect these directly to a circuit breaker.
Can I plug in all 5 circuits of LED strips on top of each other to the output of the transformer?
Yes, you can wire multiple strings in parallel at the power supply. If the sections are short, you can also wire them end-to-end. Double-check the specs for maximum length of a single run.
Is this the recommended way of doing what I'm trying to accomplish?
The recommended way is to plug into an existing outlet. If your lighting requirements added up to 1000 watts, you'd want a new circuit. At 20, 30 or 50 watts, it is ridiculously small in household circuit requirements.
I would also like to add a on/off switch, do they make a wall switch for 12v?
Any switch can be used for 12v. I would put the switch before the power supply. Put a cord on the power supply like this cord with switch.
Can low volt wires come right out of the wall, or do they need a junction box before being connected to the LED strip lights?
The low voltage wires don't need a junction box, but they do need a junction plate, like those used for phone jacks.
Best Answer
It depends an awful lot what you are using the 14/2 wire for.
For 120VAC, and you want to feed all 3 power supplies
If you plan to use the power supplies that come with the kit, a 120V/15A circuit will supply those easily. They are 5 amps at 12 volt output (60 watts). Allowing for inefficiency, they probably draw 80 watts max on the 120VAC side. Three of them would be 240W, easily within the 1800W (120Vx15A) hard max on a 15A circuit. Like this.
All the DC wiring here can be 18 AWG except for the 120VAC wiring on the left, which must be 14AWG. You see why I don't like those power supplies?
However having worked with LED strips a lot, you are not going to be happy with the final result. You will have 3 separate installations of LED strip that will operate totally independently from each other and respond separately to commands from the remote. Not every controller will see every signal from the remote, so you'll never get the colors or brightness to match each other, assuming all 3 remotes are even compatible with each other (and not serialized). If you are rolling/cycling colors, the three strips will be out of sync. Try it for yourself, but I think it will find it frustrating, amateurish and dreadful, and you'll either rip it out in frustration, or look for a better way to do it.
For 12VDC to the strip lights
There are several ways to make an installation of this size work well. One is to power them all from a single larger DC supply, and have all that go through dimmers/amplifiers. Here on out I assume you have a larger 12VDC supply of your own.
The "standard" in LED strips is generally:
12 volts DC
300 emitters per 5 meters
2 amps per channel per 300 emitters -- in a 3528 single-color strip, that's straight up 2 amps. 5050s have 3 emitters - commonly RGB but sometimes WWW (all three white). They have 3 channels, 2 amps per channel or 6A total.
Of course there are modifications on that. Double density or half density; 24V; coolwhite-warmwhite-yellow, just to name a few.
Your example seems to be a peculiar one. It's 5050, but they seem to be alternating RGB with WWW. They claim that the WWW can be controlled separately from the RGB (you would hope!) but that means they are doing something weird in terms of how the strip is wired. It is not 3 channels, it looks like 4.
But since half the total lights are on the "white" chanel, half the 6A power is on that channel, or 3A per strip. The three RGB channels split what remains, 1A each.
Power all three strips from one supply
Three strips together would be 9A on the white, 3A on each R/G/B color, or 18A for any feeder wire that is handling all the power for all of them. Also, 18A on the return ('neutral') wire.
The 120VAC wire must be 14AWG. The feed between power supply and controller must be 12AWG (14AWG would not suffice - I believe that was your original question.) If any distance is involved, go10 AWG and you'll want stranded wire so it's more flexible. The return back from the strip must also be 12AWG and the "white" wire must be 16 AWG or larger. Color wires can be 18AWG or larger.
Since you're working in DC power, there is no requirement to have the "return" wire run in the same cable as the "supply" wires. So you could use 14/4 for the supply to all 4 colors, then run a single-strand 12AWG wire for the return. Or 12/2 for the white and return, and 18AWG thermostat wire for the rest. Since it's low voltage DC you are not obliged to use white/blue for neutral and green for ground.
Another option is do a separate "homerun" for each 5-metre strip, so the return would be only 6 amps, which would permit you to use 18AWG intercom wire of however many strands you need.
One controller, three power supplies
You could power each strip separately, and control them centrally using amplifiers. To do this, assemble each LED strip in the normal way, but use an amplifier where the controller would go. Then use one controller, and split its R G B W and common wires to each of the amplifiers. This means all three strings will be the same color, do the same patterns, etc. You will end up with 2 spare controllers, but you will have to buy 3 amplifiers.
All the wire can be 18 AWG here except for the 120VAC wiring which must be 14AWG. The wires to the controller draw very low current, but wire thinner than 18AWG isn't any cheaper.
(it's possible to hook up one string normally and use amplifiers only with the other two. I don't recommend that, it'll be hard to troubleshoot, and the strings will not be equal.)