For me, I just match or exceed the cross sectional area.

For example, if wire gage "A" has 10 sq inches but I want to use multi strands of wire gage "B" which has a cross sestion of 1 sq inch.

..then I simply use 10 strands of "B" to match or exceed the equivalent cross section of "A".

Rule of thumb is that going two-in-hand is going thicker by three wire gauges.  (Don't know if that's exact or just quite close.)  That matches going three in hand (halfway between two and four times the arad) resulting in going down to the crack between dropping four and five gauges.

Going N-in-hand, rather than a single wire of the same total cross-section, has a couple advantages:
 - It's easier to wind.
 - There's a slight drop in eddy current losses.  (Not a big deal, unless you're coming down from copper bar or pipe to thin wire, because most of the copper loss is resistive drop from the output current.)

ULR;
Thanks for the math check.
The thinner wire also seems to be easier to find for free to near free.
I'll also make note of the resistance that comes along with too.

Z-
Yup, Theory is nice to get a starting point, but nothing like doing the do and see.

All;

We can't put up here in the city, anything near what could be deemed a true HAWT of even good sized VAWT.

I have gotten back to a small idea from a few years ago of building lots of smaller sized VAWTs bolted along the fence line, and linking them together for battery charging. Certainly not the best place and will more than likely be nothing more than battery charging will see.  

I'm currently not out to put AmerenUE out of biz or anything pie-eyed like that. Grid tied is for someone with deeper pockets than me...

However, after the last power loss, where only a few neighbors and I were the ones with lights and coffee, more people are open to the idea of what I'm attempting.

City inspector ( whom I now know by sight and first name) is understanding and interested in the small stuff too.

The NiCd battery packs seemed to have gelled into good sized 20Ahr units and bringing them up to 14.4Vdc seems to make the MSW inverters work just fine, even powered a small 600 watt Microwave without kicking the LVD or melting anything.
AND with G-s dump controllers, the NiCds are happy little charged campers:> BUT I can't keep relying only on the 1/2 dozen HF PVs.

Thanks ALL for the input.
Next stop coil winder :)

Cheers;