If you use all 4 pairs, the resistance of Cat5 cable is 0.03 Ohms per meter. For 24V DC low-voltage lighting (which is becoming the standard now) and 15W fixtures (equivalent to 120W incandescent lamps) you're looking at 0.6W lost over 50 meters of cabling: (15/24)^2 *(0.03*50). This is reasonable.

If you want to go to 100W to power a fairly large room, you're indeed looking at 25W loss, which is not just wasteful, but unsafe.

> With common 24V fixtures, that's not going to work well over Ethernet cables. You either need to significantly increase the voltage or the wire gauge.

Structured cabling for low-voltage DC is actually a thing these days, especially for 48V systems. People are literally using patch-panels to connect switches and lights together.

But I found another complication recently, I want tunable white lighting (i.e. adjustable color temperature), and right now all manufacturers simply do this by having essentially two LEDs that are dimmed individually. So I'm waiting to see what will happen in this area.

> And at that point, you should just wire with Romex instead (or in parallel to Ethernet)

Yeah, that's the plan.