whats voltage drop on 40 yards of ws2815 leds
You can't run 40 yards of this light type without using a feeder.  

That means you need to run a parallel + and - wire and connect it to the strip at least every 5 metres (5.5 yards or 16 ft). 

So the voltage drop will be decided by the size of feeder wire which you use.   You can compute that in any common voltage drop calculator. 



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