Too much volt drop may indicate a wiring upgrade would be prudent. You say there "exists a voltage on the DB fuse box". Do you mean that the box is made of metal and the casing is energized, or are you just concerned about the voltage drop between the power supply and the fuse box? What guage wire did you use for each run and how long is it? What is the actual operating current of the 12V LED strip? Don't answer in the comments, just edit your question.

@KH see update.

Is there any particular reason you're using such tiny wire for such a comparatively long run? I don't know what you mean by ripcord cable, but I'd suggest upgrading to 14 or 16 guage FT4 rated cable. If you waste too much power as heat in your wiring you kind of give up the efficiency benefits of using LEDs in the first place. You can look up resistance per meter/foot for whatever wire guage to pre calculate your volt drop and keep it down to a few percent that way as an option too.

Do you mean 0.5 mm diameter or 0.5 mm² cross-sectional area?

@KH please see attached spec sheet for the cable.

@Transistor please see attached spec sheet for the cable

Ok so if you have 12V to start with, 2% of that is 0.24V. If you want to have a 3A load with that much volt drop, you can only have 0.08 ohms in wire resistance for a 2% volt drop. You have an 8m run doubled to 16m due to the return wire. That's 16/1000 km and your wire has resistance of 39 ohm/km, giving you a total resistance of 0.624 Ohms, which is far too much.

@KH I see even with a 1.2A LED strip the maximum resistance is 0.2 ohms

39 Ohms per kilometer is about 0.5mm2 if the material used is copper. This is very thin.

@KH what are the consequences of using the wire? I see my alarm using a far thinner wire to power the PIR sensor that is also located about 8m from my alarm box.

Power waste is the main concern. If you had to add extra voltage converters that's another concern because they cost money and also waste typically 10-20% of the power that goes through them. For long daisy chain LED strips you can also end up with one end visibly dimmer than the other.

PIR sensor uses far less current.

@KH so I will adjust the cabling to accommodate the length needed. Please post your calculation and conclusion as an answer.

@Joey, ” I see even with a 1.2A LED strip the maximum resistance is 0.2 ohms” how so?

@StarCat 0.24V/1.2A load of the LED strip. 0.24V is 12V time 2%.

So is 3.2A your actual load then or is it 1.2A?

@KH so 3.2A is the rated output current of the power supply and 1.2A is the actual load i.e LED strip.

@Joey, I see. For a 3.2A load, 8m of cable and a loss of less than 2% (0.24V) you’d need 2mm2 or thicker wire. For 1.2A you would need about 1mm2 at a minimum, but I would use at least 1.5mm2.

@StarCat how did you convert your calculation to know exactly what size cable to use or is this just experience ????

1mm2 of copper wire is about 17-18 ohms per kilometer. The rest follows from that. I work a lot with 12V systems on my boat so I know how important wire cross-section is with low voltages and high currents and how to calculate cable losses.

Well the proper way is to look up the resistance per length of the size of wire, which can be expressed in a variety of ways and calculate that way, but in practice I usually type "wire resistance calculator" into google, which told me 16m of 16AWG wire(1.3mm^2) is roughly 0.21 Ohms, which is probably acceptable in your case. While I was at the store I'd check the price on 14AWG and if it's cheap enough, grab that instead to knock the resistance down to 0.13 Ohms. You also need to take into account the volt drop from the fuse box to the strips for that matter.

I see I’ve made an error and did not account for the round trip. Still, 1.5mm2 is about right for your situation (1.2 A).

@StarCat I just graduated as an Electronics Engineer and did not take power cables as seriously until now when doing my own installation at my home. Experience is an invaluable asset! Thank you.

Well now that you're doing stuff on your home, remember if you're burying it in a wall, it should be ft4 rated. Every wire type has specific uses. Electronics wire is not for buildings and fixture wire should not be run through walls. On the other hand building wire is sometimes fine in electronics where heavier guages are needed, although stranding may be inadequate.