I'm planning out some runs of WS2815 12v 60 pixel per meter lighting around the roofline of my house. A couple quick questions about power injection:

My longest run will be 21 meters and I intend on injecting power at each 5 meters.

Does each power injection point require a dedicated home-run back to the power supply or can a single injection line be used the entire length and then just splice in at each 5 meters?

you can use 1 power line and inject every 5 meters no problem as long as your wire is thick enough. the thicker the more current you can deliver to the lights.
what guage to use depends on your current draw and if you plan on going full white and if you want 100% intensity and distance between power supply and first injection point... so many variables.. it best if you do some tests to make sure

You need to make sure you have at least 8v to inject at the furthest run. Since you are tapping into your feed wire you have a slightly different calculation for each 5m length. If you dont mind over designing a bit you can calculate voltage drop as if the feed wire had to deliver the full load for its entire length. This makes the calculation much simpler. Get total wire length, determine total current for all of your strips, pick a wire gauge that has 4v or less drop over the length you need. There are online guides that give resistance per foot for the various wire gages.

2019 - Going to visit my Daughter in New Zealand (again). I will be dark for the 2nd year in a row. Sigh..

OK. I completely appreciate the input. I just want to see if I'm understanding the power injection calculations correctly. Is this diagram correct on amperage draw per injection point?

Can you put the power supply in the middle? It would simplify things greatly and make the wire size much smaller. 76.6 feet is a long ways from the psu..

Hi uncledan. Actually, I can't. I'm attempting to run the strips around the perimeter of the house and I'm very limited on the location of the power supplies. At the moment, I'm just trying to make sure I understand how to compute the loads which I why i added the diagram. Am I on the right track as far as load per injection point or am I not really understanding this?

OK. I completely appreciate the input. I just want to see if I'm understanding the power injection calculations correctly. Is this diagram correct on amperage draw per injection point?

Based on this, your wire would have 20 amps in the first 16', 16 amps in the next 16' etc. This is why I said it is simpler to over design. Calculate the voltage drop for 20A over the entire length and choose a wire that drops less than 4v using that calculation. This will result in a "measured" voltage drop much less than 4v.

2019 - Going to visit my Daughter in New Zealand (again). I will be dark for the 2nd year in a row. Sigh..

You could also use 24V or 36V PSU and use a buck converter to drop the voltage to 12V at the injection points, that way you don't have to use a heavy a wire.

Based on this, your wire would have 20 amps in the first 16', 16 amps in the next 16' etc. This is why I said it is simpler to over design. Calculate the voltage drop for 20A over the entire length and choose a wire that drops less than 4v using that calculation. This will result in a "measured" voltage drop much less than 4v.

I think I'm having my "AHA" moment. I was indeed misunderstanding how to run the voltage drop calculations, but based on your comment, I now "get it". I did the calculations as you suggest and it appears that I need to run 10 gauge wire to accommodate this. I also re-ran the more complex calculations based on each injection point just to make sure I understood. Thanks again for your info and patience.

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