Cable wires - confirmation of live and ground

Question

I bought a generic DC power extension cable and I'm trying to wire it to a connector with a cable attached. The connectors cable clearly has two wires - a red and black/+ and -. This is what I'm used to, coming from the world of breakout boards and tiny wires. The generic wire, once stripped, however has outer bare wire strands surrounding a sheathed tiny wire:

(I've attached a photo. I've twisted up the outer wire so I could test it with my multimeter). My logical assumption is that the inner sheathed wire is + and the outer is -. Is this correct? I've hooked it up to my solar panel and tested it on my multimeter. It shows 7v when I've got the red wired touching the inner sheathed wire and the black wire touching the outer, and -7v the other way round. I just don't want to connect it up and fry my solar charger board. It's be great if someone could give me a 'yes! you're right!' confirmation please?

Answer 1

Cables don't care which way around you use them. The only difference between a live wire and a ground wire is that one of them is connected to live, and one of them is connected to ground. The electricity doesn't care what colour of wire it's flowing through.

Feel free to use green and yellow wires for 240V. But don't actually do that because the next electrician who sees the cable might electrocute themselves by mistake! For mains electricity, colour codes are there so that people don't electrocute themselves or blow things up by mistake.

Since this is not mains electricity, and you are dealing with only a few volts (nowhere near 120 or 240V) and there is no risk of electrocution, and probably nobody will see it except you, you can use whichever wire you want for whatever purpose you want.

If you are going to have a ground wire, I would suggest using the outer wire for ground. That way, in case the black insulation gets damaged and the wire touches something metal through a hole in the insulation, it has less chance of damaging your circuit. Otherwise, it doesn't matter.

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Edit: I missed that your cable already has a connector on one end. In that case, the connector is wired a certain way around. When your multimeter shows a positive voltage, the red lead is touching the positive wire, so in this case, the inner one is positive.