A qualified, licensed electrician has been with me from the start of this project and has wired everything I have in my attic. He also made me some custom cords that I’m using for the temporary garage setup. He’s seen everything I’m doing in my home (as have two more electricians that work under him). I get my information from them.
Average temp in the attic very much depends on the weather outside. In the beginning, before I had my radiant barrier up and when the weather was hotter, it would get toasty up there. Uncomfortable to be up there for any amount of time. Now it’s much better, pleasant in fact. I’d estimate 70-75 degrees.

I am aware of the “80% load” rule of thumb, but in reality this is very conservative. A brand new breaker switch can probably sustain 100% of its rated current (or more) especially if there is some airflow in the breaker box. I leave my door open with a fan on it. The other day I noticed my attic (100 amp service) was pulling 99.x amps and probably had been doing so all day. That’s a bit close to the line for me so I moved one machine to the house circuit, but now I run in the low to mid 90s up there and I’m perfectly comfortable with that.

Incidentally, the D3 is not nearly as overclockable. At the very best I can get 20 MH/s, which is only 15% over stock. And most machines can’t reach 20.

It was definitely a lack of internet connection that caused the problem. It was the red “fault” light that made me think it was something electrical, however if you test this at home, you’ll see this does indeed light up if you lack an internet connection. I would have figured this out but when I got home from work the wife and kids were waiting in the car, ready to leave town, so I had essentially no time to troubleshoot. So instead I churned with worry all weekend long, imagining the worst. I did begin to think it was the router towards the end of the weekend (it was the only common denominator) so it was such a relief to get home and see that router off.

With regards to overclocking, there is absolutely no doubt you can safely overclock 5-10%, or be like me and push the envelope a bit and go for 20-25%. I’ve been doing this successfully for months now with no ill effects, in a hotish attic no less. When you have lots of machines you will notice that some are more overclockable than others, due of course to the silicon lottery. Some machines will barely take a frequency of 425M, and others can approach 500M. Here are some numbers I’ve gathered. Take this with a grain of salt as these numbers came from just one machine.

384M - 504 MH - 810w
400M - 520 MH - 845w
425M - 555 MH - 905w
437M - 572 MH - 935w
450M - 590 MH - 965w
475M - 620 MH - 1020w
481M - 630 MH - 1065w
487M - 635 MH - 1070w

When you start approaching the limits of a particular machine, you will notice hardware errors accumulate very fast and your real time hashrate will be higher than your average. For me, when the difference is more than 20 MH/s, that’s the limit. I have some machines that can hash at 650+ MH/s.

YES. Such a relief.

Unbelievable! It WAS the Ethernet. What happened was I had a few heavy duty splitters on some of my 120v extension cords, and one of them tripped that happened to contain both that fan and my 24 port Ethernet switch. I didn't notice the switch was off in my panic right before I had to leave town. Up in the attic, a similar but unrelated thing happened where a 120v line went down that contained the switch up there. What a relief!!!! I'm back up and running as I type this. I shuffled some things around so this shouldn't happen again.

I'm going home this afternoon, will report back.

You know, the Internet thing makes a bit of sense because the Ethernet switch for the garage is on the same outlet as that fan that stopped working. But would no internet connection really make the miners act this way? Not even boot up and show a red fault light? That doesn't seem right. However, it definitely is the one thing in common with all the garage miners. Even their power supply wasn't the same: some had 230v or 120v from the attic, some had 230v from the dryer circuit, and some had 120v from the rest of the house.

I'm using BITMAIN's PSU, the single one (A3) for each miner.

It wasn't hot in the garage at all. The garage door was open just enough for the exhaust fans of the miners to stick out. I had a side door open with multiple fans arranged such that cool air would be pulled from outside and blown across the miners. Miner temps were in the low to mid 70s. I overclocked them the day I got them because I'd had such success overclocking my attic miners (where it's much hotter BTW). For weeks I was running even more aggressive overclocks with no problems at all. I can't imagine overheating as a result of overclocking is the problem. They wouldn't all fail simultaneously like that, plus I know for a fact the miners will shut off before they let themselves overheat. The one fan that went out wouldn't have had much of an effect on airflow.

Our car charging setup is nothing like that. It's a Volvo XC90 (plug-in hybrid) and we don't even have a 230v charging station. We use the 120v cord that comes with the car and plugs into a regular outlet. It was using a 20 amp circuit that had no miners on it. But again, even if the car charging pushed things past what the house can handle, I don't see why that would damage the miners. It should cause the main breaker to trip and that's it, right? Am I missing something?

I wonder if the electric company is throttling my service or something? That would explain why all of them were affected (and the fan too). Maybe some machines are more sensitive to decreased wattage/voltage than others, which explains why now some work and some don't? And that would explain why the lighted ends of the extension cords still work, yet they won't power a miner. It's clear whatever happened was a result of some house electrical problem (again, the fan) but the question is, did it do lasting damage to the miners.
Also, an electrical surge doesn't seem likely since miners in both the attic (behind a whole-house surge protector at the sub-panel) and miners in the garage (no surge protection) were affected. Other than a surge, what sort of electrical derangement can permanently damage a miner?

Thanks for the answers. The miners in the garage were a temporary proposition, for two weeks at the most. I'm aware of what you said about the way electrical circuits work. Here are some details of my setup:
The attic does not have a separate meter (I'm told this is not legal where I live). Thus the 100 amps going up there are a part of the overall 200 amp service. My permanent attic setup consists of six 230v circuits on which I have the capacity to run four machines. Currently I have only 15 up there. There are a few 120v 20 amp circuits up there for fans and whatnot. I also routed the circuit to my sup pump (which I don't have) to the attic, which is also a 20 amp line. To power the temporary miners in the garage, I ran four heavy duty 230v extension cords to the garage, plus 120v extension cords rated at 20 amp from some of the attic 120v circuits, plus the former sump pump circuit. I never power more than two miners (about 2000 watts total) on a 20 amp 120v circuit, and never more than four on the 230v 20 amp lines. Then I used a number of unused (or lightly used) 20 amp circuits from the rest of the house to power a number of the garage miners. Finally I had my electrician make a custom cord that plugged into my dryer outlet, which is a 30 amp 230v line. His cord split into three NEMA 5-20 230v outlets, which I further split to power two miners each, 6 miners total. All the extension cords and splitters I used were heavy duty and were rated to handle the amps/watts being asked of them.
Even if the wife and her EV pushed the house past the brink, it should have simply tripped the main 200 amp breaker. Why would that have done damage to the miners? That's the point of a breaker.
With regards to heat, I doubt this was the problem. I had all the miners lined up next to each other (which they were made to do, since they have tracks on the side that fit into each other) and they were exhausting directly out the garage door, which was open just a crack. There were also a couple fans blowing across the miners themselves. In the past, before my attic setup was complete, some of those miners/PSUs would get extremely hot. When this happened, either the miner or the PSU would shut down, as it is designed to do. This happened many times. I don't understand why this wouldn't have happened again if heat was the issue.
And by the way, I did an experiment where I took the power cord from a working machine and plugged it into a non working machine. No luck. Then I took the PSU from the working machine and hooked it to the non working machine. No luck still. So whatever the problem, it does seem to be the miner itself. Unfortunately right after this happened I had to leave town for the weekend, so I haven't had a chance to play around any further than that. One thing that gives me hope: when I came home, I unplugged everything and then immediately plugged them back in, and several machines came back to life. I'm hoping when I come home Sunday afternoon, even more will will work again.
I'll keep you all updated. Again, thanks for the input.

I had a horrible thing happen today and I need guidance from someone who understands the technical aspects of an Antminer L3+. I have 9 of these running in my attic and recently I added 25 in the garage. Most of the miners are running a moderate overclock to ~600 MH/sec using about 1000W. Today I came home to find every single miner displaying a flashing red "fault" light and a solid red light on the front of the control board. I have absolutely no idea what happened. It wasn't a particularly hot day. There were no thunderstorms. My wife was home most of the day and reported nothing out of the ordinary and said around noon all the miners in the garage "had a green light." None of the circuit breakers in my house were tripped. I have the miners hooked to various 230v and 120v circuits throughout the house via heavy duty extension cords. I've done the math and none of the circuits should be overloaded. I have a 100 amp sub panel in the attic that usually draws around 85-90 amps. I also have a "whole house" surge protector for the attic service. In fact since I had just brought all the garage miners online the night before, I had my electrician go by this afternoon and measure my whole house and attic-only load: it was about 186 amps whole house (I have 200 amp service) and 92 amps in the attic. When I got home all the other electrical components in the house were working, with one exception: a fan I was using in the garage had stopped working, even though it was plugged into a seemingly working outlet. I switched outlets and it started working again. All the lighted ends of my extension cords looked normal. When I got home today I quickly unplugged all the miners and tripped then reset every breaker in the house. Now only a handful in my row of 25 miners in the garage power up. The ones that don't work have a weak flicker of lights on the back when initially plugged in, then the fault and control board lights come on. The rear fan spins (slowly) but not the exhaust fan. Interestingly, the miners in the attic fared better. Only 4 of them don't power up.

Can someone shed light on just what the FUCK has happened here??? I don't understand it. Is it from the overclocking? But why did everything fail at once? My wife was charging her electric car earlier in the day, did that screw something up? And what was up with that fan in the garage? Finally, most importantly, what do I do now??? I just took delivery of the garage batch a few days ago, but I gather the warranty is void because of the overclocking... Is it the control board that has gone bad? If so BITMAIN sells them for $60 each, which wouldn't be too painful of a fix.

I really need some help with this. Thanks in advance if you have any information that might help.

They are sold, thank you.

I got lucky and was able to secure a quite a few L3+ units in last night's sale. I'm not sure how many I'll keep, but I certainly can't keep them all. If you are interested in purchasing more than one L3+ in the Oct 21st batch, PM me for more details. Although my account here is new, I'm a reputable person and I'm willing to take extraordinary measures to make you comfortable that I'm not scamming you. Again, PM me for more details.
Thanks.