^you can get away with a bit more than 80% load, but i would absolutely not exceed 90%. for the risks it creates, you may as well just install another breaker+wire run, instead of risking a failure in your wiring.
Yes, I agree. Better safe than sorry! I only wanted to share my experience with running at 100% myself on PVC (Polyvinyl Chloride) insulated wiring. This is the type of wiring generally used inside a home. It's the type of wiring I purchased for my setup. Mine is not inside of walls or conduit.
PVC wire heat resistance [On a scale from 1 to 5] is 2. Which means the insulation of the wire would be considered a little below average [or fair] in its ability to withstand heat. Especially, over long periods of time inside of a non-ventilated space like walls and/or conduit. Hence, the reason why I've said I would not push this wire over 80% if it's inside of walls or conduit. The normal temperature rating for PVC wiring can range from 60C, 75C, 90C. Usually, the temperature rating is noted on the packaging but not always. My 10 AWG (10/2) wiring is rated at 90C (194F). Even 60C is 140 degrees Fahrenheit.
When I ran my four (4) 30A circuits on 10/2 PVC wiring part of the summer at 95 to 100 percent, the wire and outlets worked well. They did not get too hot to the touch at all. I could keep my hand on them without having to take my hand off. They felt warm but not hot. Remind you, my wiring for my rigs is not within walls. I DO NOT RECOMMEND doing what I did if your wiring is within walls or conduit. I have since added another 30A circuit to allow me to run my circuits at 80 percent instead of 95 to 100 percent.
The only reason I've mentioned this is to say what the wire is capable of handling if not inside walls and/or conduit. Am I telling people to do this? No! I'm only informing people of what can actually be done with the wire. I personally believe it would be beneficial to stay on the side of caution by adding an additional circuit(s) to keep them all at 80 percent or lower. Especially, if your circuits are inside of walls and/or conduit. I personally, will run my 30A circuits at 85% when I get all of my S7's. Remind you, my circuits are NOT inside walls and/or conduit.
So, don't think I'm advocating for everyone to run their circuits at 100 percent. I would say you could if your circuits were not inside of walls and/or conduit but I would only do it for a short time as I did myself. I put off adding another circuit for approximately 40 to 45 days. My wiring and outlets performed well and did not get too hot at 95 to 100 percent load until I added another circuit. That's all I'm saying...
EDIT: The main reason I would NOT recommend running at 95% or higher on this wiring for a long period of time is because air is a poor insulator. Without shielding the strands or even solid wire, an electrical potential can over-stress these air voids between the strands or between multiple conductor(s) and the outer shield of those conductors. This air breaks down or ionizes, going into what's called, corona (partial discharges). This will form ozone and can chemically deteriorate cable insulation over time. If you bought high dollar wiring with semi-conductive strand shielding, It can eliminate this potential by “shorting out” the air.
I mention this a lot in my training when the subject comes up about using alligator meter clips with a bed of nails that penetrates the insulation to have contact with the conductors for testing purposes. I explain when the insulation is damaged, an ionization process can occur from exposure to more air [Especially damp air] to emphasize the importance of cutting off the portion of wire they damaged with their meter clips and re-splicing to avoid having potential problems with that circuit in the near future. Those meter clips with a bed of nails are responsible for many of the opens they have in the local telephone loop. Most opens begin at a low resistance value and they increase in resistance as the copper wire gets smaller in diameter over time due to this ionization and electrolysis.
http://www.amazon.com/Alligator-Clip-Nail-Angled-Medium/dp/B00M1V7E5A/ref=sr_1_1?ie=UTF8&qid=1447014797&sr=8-1&keywords=bed+of+nails+clip
So, again, I'm NOT advocating individuals load their circuits at 85% or higher OVER LONG PERIODS OF TIME! I see I need to be more specific when I elaborate on a subject because it can be taken the wrong way at times. If you are in your initial setup and or install and your wiring is NOT inside walls and/or conduit, one could load their circuits greater than 80 percent without much fear if it's for a short period of time. The longer your circuits have a load over 80%, the life of the insulation for the circuits can deteriorate over time. This deterioration can result in the insulation being more susceptible to higher temperatures and thereby reduce its ability to withstand heat over time.
So, if you would like to mine with more rigs but don't have all of your circuits installed yet, you could do so for a short period of time [under certain conditions] until you have the other circuits installed for those rigs to get everything within a 75% to 80% limit. That's all I'm implying. I've done this myself while putting my setup together. Remind you: MY WIRING IS NOT WITHIN WALLS AND/OR CONDUIT! My setup is completed now and enough circuits are installed for the amount of rigs I have at present to keep me at 85% or less. I feel 85% is plenty safe for my install because my install does not have wiring within walls and/or conduit.
When I add more power to my setup for more rigs in the next month or so, I will add enough circuits to keep them around 80% or less. I'm presently running my circuits at 67%. Each 15A breaker on my 30A metered PDU's say they are running at 10 amps [Approximately 66/67 percent]. When I get more S7's (batch 6) at 1042 watts each, I will run them at 5 x S7's per 30A PDU. This will be 72 percent of its maximum. I would probably be fine with 6 x S7's (batch 6) at 86.6 percent of it's maximum for a short period when it gets close to time for the move to a new location for my setup.
When I do move my setup to another location, I will have every circuit at 80% or less.
How much working hours that cost, have you count it? (Personal question)
Money is solution on lot of problems 
I work on weeks, not getting job for weekends 
there is a dealer in europe, but the price is 2 times more expensive.