.k. I'm staying at this place with friends while the haus is being built. A few things beforehand. There are two fridges and one chest freezer, two pools, a hot tub, various lighting and TV sets, power outlet demand excluding the former, a garage with power tools that see intermittent use, a pole barn with 20 amp max draw, two microwaves, a washer and dryer, AC units, and water heating and softening equipment, all squeezed onto a 200 amp circuit. There are sub panels for the hot tub room and the AC. Whenever something kicks on, the lights flicker, and the power even (...) completely off for about five seconds. I'm not sure if the transformer is overloaded or what, but would a 400 amp panel upgrade be a viable solution?

You're building your own house?
The most I can provide right now (doesn't mean I can't tell you more later) is to upgrade any FPE panels to something better, or you have a super sized fire hazard.
What matters is wire resistance.
We have the same problem, but to fix it, you need to upgrade the main power line going from the electricity meter at the utility poles, which is a hard thing to do.

Yes, we're having the house built. I'm talking about the place were staying in now has these issues. I saw that replacing the wire from the trannys to the meter (50 or so feet) is about 3500 bux. I'll suggest this to the owners. Yeah I heard federal pacific panels are fire hazards. I'll check what the panel is.

The dimming/flickering lights are the wire resistance, so you would need to mainly replace the feed cable from the transformer to your panel (or from the distribution bus; I do not know, which topology you have there - if you have your "own" transformer or that is common for the whole street and so on). What is most important are the longest sections, include the one upstream the meter. But that is the hardest to replace (mainly to organize that), as most likely it does belong to the power company and it is not yours.

Just w new panel won't help at all. But if the present one is known for being problematic, of course it is worth considering to replace it as well...

Owning appliances does not mean switching them on all at once. Check where you really are standing in regards to loading/overloading your panel :

Do you happen to trip the main breaker occasionally by overload ? (assuming intact breaker that works, not FPE)

At times of presumably high load, is the front of the main breaker hot ?

At times of presumably high load, write down the appliances in use and their current draw. Calculate to see what is being drawn

(clamp meter might be used, but there is big safety issue with using that on the main 200A feed, from the possibility of moving some old wire with bad isolation and making a short circuit while trying to put the clamp around it)

Check what Phase are your loads connected to : Plug an extension cable to one receptacle, and measure V between the Phase on it and Phase in other receptacles. Youll see ~0V for same Phase and ~240V for different Phases. Map to which Phase belongs everything and check that the significant single Phase loads (that are in use at the same time) are more or less distributed between the 2 Phases and not all on one

Overall, 200A is a lot. You can manage with it, at most with some sense when switching on stuff. As for loading and overloading :

 - Normally electrical systems should not be loaded above 80% of their rating

 - For the time you are there and using more power, its not forever, and it is ok to load the system up to 100%

 - Short overloads are acceptable. For example, here with our small ratings of 25A main, it is ok to overload it by few A for a minute to run the microwave - It is made to handle it



The few seconds darkness is not ok, but that may be related to a failure of something and allways not to having to upgrade the panel

Is the darkness related to starting of some specific load ? Check especially motors, as their starting current is huge and then it goes down, and high resistance in the wiring may slow down the starting of a motor from the normal split second down to the few seconds range. Heaters would cause a "step" change in lights brightness when switched on or off, but not a "Starting transient"

If such load found, what is it ? Is it 120V or 240V ? What is the power of it ?

Does the darkness affect everything or some part of the house ? Everything on one Phase by any chance ?

Does something else get brighter at the same time ?

Let that load work and measure the voltage drop across everything on the way to it, that is upstream from the part of the circuit affected by the power outs : For example input and output terminals of breaker etc, or Phase to Neutral voltages before and after a length of the wiring. If somewhere the voltage drop is higher than normal (if it is 240V then just compare the voltage drop of 2 Phases), then there is bad connection with the cable or commutation component where the voltage drop is found

And just sniff around, maybe you would be able to catch by smell the connection or component that causes the few seconds of darkness, if it is one connection somewhere, shortly after such darkness event and before the smell dissipates away

It was warm in the breaker room, only because of the water heater and furnace. Some breakers were a bit warm to the touch, nothing concerning. I did see something interesting. A 100 amp sub panel connected into the main panel to a 70 amp breaker.
Breakers arranged by ampacity on the small panel.
Left:
15 (off), 20, 20, 20, 20, 15, 50, 30 (all on)
Right:
50, 50 (on), 20 (off), 20, 50 (on)
More images to come.

There is no problem with limiting to 70A breaker a panel capable of 100A

The order of the breakers is not important, it does not give hint in finding where the voltage drop happens. First, try to point out what load is causing the drop when switched on, and what part of the house is affected by the drop