Lightning struck my home last year and blew everything connected to bus bar A on the circuit panel; computer, furnace, one socket, and routers. Recently my electric range and hot water heater stopped working. After frustrated troubleshooting, I replaced every circuit breaker, including the main 100 amp (SIEMENS MBK100A 100-Amp, identical to old one), which did not resolve the issue. I removed every breaker except the main, and bus bar A was showing 20V, B 120V (120V going in on both cables). I switched the hot cables, and bus bar A was showing 120V, B 20V. Any advice on what the issue could be?
The moral of the story is "Measure stuff".
If you had measured the 3 voltages between hots and neutral on the utility side of that panel's main breaker, just like this guy does here... you could have effortlessly proved out the entire panel, and would not have had to replace anything.
Once you found the voltages were faulty there, the next step would be to pop the covers off that outdoor disconnect switch/breaker, the one you can't de-energize, and check them there.
You certainly know the troubleshooting tree, if voltages are bad on that disconnect's output yet good on its input, then that disconnect is faulty. If they're bad on both input and output then problem lies between here and the utility transformer, the only things left are:
disconnect to meter pan, meter pan proper, and meter pan to weatherhead, these things being your responsibility, but unlikely to be the culprit.
the meter itself, crimps to your service entrance wires, service drop to the pole, and all pole top gear. All the power company's responsibility. 98% of the time, the problem is the service drop, since it's been whipping in the wind for 30 years. They fix this for free.
I realize the lighting increases the chances of it being something else, but it doesn't raise it anywhere near 100%.
So the other moral may be "don't assume it's the same problem". I had that this year, was so wrapped up in a bearing problem on my car, I forgot to check the tire.
Re-torque everything properly.
The problem with pulling apart feeders and large breakers is now you have to re-torque those connections. The original installer used a torque wrench.
That is particularly important on the "main breaker", and you need to look at the panel labeling, figure the torque required there and for the main breaker-to-lugs attachment, and get a torque wrench of appropriate size and use it. Very important!
The need to properly torque large lugs has been known for a long time. What was more recently discovered is this matters on small connections too. That is why Code now requires you set proper torques on everything with a torque spec, meaning most devices and certainly everything inside a breaker panel. So you'll also need a torque screwdriver, or one of those "mini" 1-120 inch-pound torque wrenches with a bit holder and appropriate bits, to set all the small screws.