wiring conventions for DC ?

In past lives I used a wire marker that had a number on it. Depending if the wire was for AC power or control I needed something to identify each end. since they were all the same color which was usually black.

Wire markers can be adhered using a stick on tap that wraps around the conductor or a plastic clip on type. Both types can be found at any electrical stores and probably through the web.

Cool, didn't know about those. So I could get by with a 100ft spool each of red and black.

But I think my local City Electric will cut to length for a comparable price to 100ft spools, so no reason I couldn't get 4 different colors. Maybe they have red and black, but with a white strip, that'd be perfect.

White means neutral and green means ground. Any other color should be OK.

Wire markers are a great idea. Mike's suggestions are always perfect.

The hard part is figuring out a marking or color scheme that means something to you or the next owner, ten years later. Perhaps put a diagram or color chart next to an indoor junction box, where it's easy to spot.

I'd be curious when extra copper will pay itself back for you. In other words, if you replace AWG 12 with AWG 10, you'll pay $x more now and harvest $y more electricity every year. Longer runs have longer payback time, so there isn't a one-size-fits-all answer other than at a minimum, "must meet code".

Yeah, no white wire. I'm thinking black and red, the polarity of which should be obvious to anyone, and a second pair with any kind of marker. Red & black with white stripes would be awesome, but if wireandcableyourway-dot-com doesn't have it, it probably doesn't exist. Could cheap out with a 250ft spool of black and three types of markers, but that's a bad idea I think.
Well, 100ft rolls of 10awg and 12awg THWN are $29 and $25, respectively, at Lowe's, so it'd cost me $8 to upgrade. Voltage drops would be 0.9v and 1.4v, respectively. Let's say I'm running at 2/3 the OC voltage on average, about 8*38.5*2/3, so 12awg (1.4v drop) loses me 0.7% and 10awg loses me 0.4%. I'll looking to generate something like $400/year, so the 10awg increases my $production by about $1.20/year. Or maybe there's a squared effect in the loss, not sure if panel acts more as a voltage source, or ...

I would look at pricing of colored cable in the AWG size you want. Don't be surprised if something other than red and black (like blue or orange) may cost more per foot.

bob-n stated any white color is normally considered a neutral so don't use that. It may be much easier to just label the same color #1 & #2.

Yeah, I think using two peculiar colors (in addition to the black and red), isn't a great idea (unless it's black and red with white stripes). Or nothing and something; or is that poor form ?

By the way, something I've wondered about about ... I imagine the DC output terminals of solar panels are not referenced to any other voltage. Is there some reason one couldn't do it with three wires, so they share the '-' ? Obviously that shared '-' carries twice the current. Similarly, is there some reason you can't tie the '-' (shared or not) to ground, same as you tie "grounded conductor" (aka "neutral") to "equipment grounding conductor" (aka "ground") in regular house wiring ? Only at the inverter or junction box end, not both - same as with a subpanel.

You're right that the DC output wires from the panel are isolated. You could have a common ground as you describe and replace two thin wires with one fatter wire. But that limits your flexibility. With independent wires, you can put them in series or parallel or completely different systems. Also, microinverters are set up to accept two wires from each panel and string inverters are set up to accept series connections. The world is designed around floating panels. You could engineer a new world, but as you said, you'd need fatter wire for the shared terminal, so it wouldn't save you much.

As to connecting one side of the panel to the mains or to ground, think about lightning survival. With the whole panel floating, a weak lightning strike will not cause current through the conductors in the panel. It will instead go through the frame and rails to earth. With one side of the cells grounded or connected to mains, a strike may cause current through the cells to earth, damaging the cells. Of course, a direct hit will burn down the house, but the weaker stuff, like a nearby strike or a weak discharge is more likely and survivable if you do it right.