When I chose the wire for my hops I went with the assumption that one hop plant might weigh up to 20 lbs, so in your case thats 12 plants per cable, x 20 lbs = 240 lbs per cable. Not entirely sure how that translates into tensile strength, but I just went by the estimated max weight of my hops.
fwiw, I use 7/16" nylon with a rating of 350 pounds for suspension line, with the longest section supporting 11 plants, this their fifth year.
But it's really only supporting the upper half of my plants: they climb ~15 feet to the top railing of my 2nd story deck on drops that are attached to the deck structure, then they go the rest of the way up to ~23 feet above ground on the drops hanging from the suspension line.
If I had to guess I'd say the deck carries around 40% of the weight and the suspension lines carry the rest (and pretty much all of the cones).
If using nylon I'd upgrade to 1/2" for its ~500 lb rating if it's going to hold 10-12 plants on its own...
Don't know why i can't find any of his posts, but theres a process engineer from a hop farm in Michigan that posts here all the time.
He suggests that a mature plant is more like 35 lbs per plant, and in high winds that load on the main line can double if not more. In the end he suggests estimating closer to 100 lbs per plant just to be totally safe.
I just started this year on my father in laws farm, so I have quite a bit of space and some opportunity to expand, so I went with 5/16" 7x19 aircraft cable, which has a rating of like 9800 lbs breaking. I have 60 plants, 3.5 foot spacing, with 10 plants between poles. I'm using the same for the guy wire.
I'm also in an open field with considerable winds, so if your pretty close to a house, wind may not be as much of a factor
For a 60' run, you should seriously consider galvanized "aircraft" cable. As others have suggested, you need to think about more than just the weight of the plants. A long row will act like a sail in the wind - applying hundreds, if not thousands of pounds of force.
For 40-60' runs I have used 1/4. It has a breaking strength of 7000lb. Since this size is often used for ziplines, you can also pick it up cheap on amazon or ebay. 2 weeks ago, I had 250' delivered to my door for $60.
I have 8 plants on 550 paracord this year. So far no problems with it. It does however have a lot of stretch to it, so it has to be mechanically tightened. I have a cheap $4 ratchet strap at the end to solve that problem.
I personally wouldn't really go 60' only reason is sagging is going to be bad unless you have the ability to tension everything. But with only 20 plants you could get away with 3/16" guy strand (1x7) or aircraft (7x19) cable. Both are pretty cheap. We run 1/4" guy for our row cable @400' of length on our short rows. Our long rows will have over 1000' runs. Same 1/4" cable. Plenty of smaller fields use 3/16" And some even just a #9 steel wire. But you'll need some supports to prevent too much sag
I'm the guy that says 30 to 35 lb/plant. Think of your worst case scenario. At full maturity, some can hit 30 lbs but it will be a rainy night so add another 5 lbs for water. So we are at 12 * 35 = 420 lb
Now the wind starts blowing/gusting. If it is a constant wind, those hops will turn into a sail and you can roughly double the weight. (I'm assuming you aren't in some really high wind area.)
420 * 2 = 840 lb
Or the wind is gusting and your hops are rocking back and forth. To calculate the force on the top wire as the hops hit bottom center of the swing is too much for this forum but let's just say 4 times the normal load.
420 * 4 = 1680 lb
True, that's not all at a point source so you can ease off of that number a bit...but trust me on this...you do not want to have to pull 60 feet of hops off the ground and back into the air, so be conservative.
1/8" to 3/16" aircraft cable should have enough strength. You should be able to get it cut to length at a home improvement or farm store. Don't forget to make sure the anchoring system can also hold up to the weight.
Oh, also don't forget to add in the weight of kid/dog/fat neighbor running into them at full speed. That crap happens.