Manufacture stated to run 12v 5 amp per 5m led. Im going to be running 7.5 meter, which is using 1 set plus half another. From research its suggested to go to 24volt power supply. Question 1 is can I connect the 2.5 m strip to end of the 5m without any real issue and push current that far or do I need to wire the 2nd strip back to the power supply? This is going down a 24 foot walk in closest so dont really want to run 2nd set of wires back. From my calculations a 24v 7.5a should supply this 5630 dual row led 120 led p meter. Is there any flaws in the above logic, or can I safely run a higher v power supply. I want to be able to leave these one all time if I chose to.
Without having extremely detailed specifications of the LED strip and what you consider to be acceptable the answer cannot be determined.
Connect it to the end and see if it gives acceptable performance, I would expect that it will. if it does not (light too dim at the far end) you can then add a cable from the junction, (or move the power suppply to the junction) etc.. use a 12V power supply capable of 7.5A pr more, there's no simple way to use 24V with that setup.
ideally you should connect power to the strip at several points.
eg: by running a cable parallel with the strip
You may find that it's possible to drill a series of holes in the ceiling and fish the supply cable through them, and then fix the led strip such that it conceals the holes
Question 1, yes, you can just add on the extra 2.5 Meter section to the end of the 5 Meter section. But even brightness is an issue due to the poor resistance of the FPC the tape is made of. See last paragraph for the solution to this.
Question 2, The highest you should ever run a simple 12v led strip is 14V. The average strip will power the leds with 18 mA at 12V, up to around 22 mA at 14V. Anything more, and you risk 1) burning out the leds much faster, 2) cause the resistors to fail from dropping too much heat, and 3) Cause the Flexible Printed Copper that the led strip is made of to heat up p too much. With its high resistance per foot, you need to worry about burning it out or melting the glue.
24 Volts is guaranteed to burn out the leds though. Assuming 150 ohm resistors and a high 3.5 volt forward voltage drop, that's 13.5 volts on the resistor. Ohms law
I = V / R so 13.5 / 150 = 0.09 Amps. That's 90 milli amps through leds rated for 20 milli amps. And that's
P = V * I 13.5 * 0.09 = 1.2 Watts through resistors meant for 0.125 Watts or less. You'll burn out everything on the strip with 24 Volts.
The solution is simple. You should power the strip from 12 Volts but power correctly. Worst case, power it from the middle of your strip, at the 3.75 Meters mark. Best case, from the middle and both ends for the best brightness. Middle ground, power it at two points. 2.5 Meters and 5 Meter points.