[Delphinus]

I used 18g on my DIY fluorescents on my 50g. It was a chore to find solid core instead of stranded in that small a size.

Anyways, what I used, turned out to be inappropriate for the fluorescent sockets. I don't know if it was the heat produced by carrying the amount of current that they were, or if it was the heat from the lamps itself. What happened was the insulation became brittle, and eroded away in places leaving exposed wire. One day, I thought I heard an odd noise so I started looking at all my wiring, and in an obscure spot, I found a spot where there was arcing across two wires (plus little puffs of smoke).

Anyways, so I guess my point is, whatever guage you use, check the rating for the insulation. I think that the wires used in typical 40W ballasts is 18g, so 18g ought to be OK. The problem going to a larger wire size is that you need a fairly slim wire to slip into those typical fluorescent sockets, but obviously going to too thin a wire causes worse problems.