Is 1970s wiring "safe"?

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The 3 flat my dad owns built in 1915 had gas and electric lighting when it was first built. All the conduit is black iron just like the gas piping, and it all came down to the basement to one electrical panel (which is now a junction box with another conduit leading to the current breaker panels). The previous owner rewired everything in the building except for the stairwell lighting and the third floor apartment. That apartment’s original setup was just 2 circuits staggered across the apartment. Found this out when some tenants tried to run a window AC in the kitchen and another one on the other end of the apartment in the front bedroom. Several more circuits have been added over the years to be able to run window AC’s in each room.
Down in the basement stairwell there is an original porcelain lamp socket and a twist-knob style switch, and it works.

At my grandmas house we rewired it in 2008 after major flooding and I found a similar wiring layout despite this house being built in 1950 and not 1915. All three bedrooms shared one 15 amp circuit, as well as the bathroom and part of the living room. The kitchen had two circuits, one of them shared with one outlet in one of the bedrooms and the other with two outlets plus half the living room, garage, and sunroom. The rest of it was a hodgepodge mess from changes made to the house over the years, like when the basement was finished they just wired the basement outlets or lights into whatever circuit was nearest. One of the furnaces shared with the washer, and the dishwasher in the kitchen above it. One of those kitchen circuits was wired in 14 gauge wire which was only good for 15 amps, someone had installed a 30 amp breaker on it many years ago. Now each room has at least one circuit and the 60 amp service is now 100 amps.
 
Knob and Tube...

I think knob and tube has got a bad rap.

 

Some years ago I read an interesting book (The Well Built House, I think) that discussed knob and tube wiring. The gist of it was that back in the day, knob and tube wiring as originally installed was typically done by licensed professionals who were meticulous and the result as installed was quite safe.

 

The problems with it came from various non-code alterations:  Homeowners extending it by splicing into the original wiring; aforementioned replacement of lower amp glass fuses with higher amp fuses; overlaying wires in attic spaces with flammable insulation that does not allow it to dissipate heat properly. There were also some cases of substandard wire insulation - tar based that embrittled over time, hardened and cracked, and failed.
 
SudsMaster-
Yes, Yes, and Yes!

That's exactly what we were told when we had our house inspected. The electrician we had out tidied up a few things, but advised us to keep our K&T intact for those very reasons, and to be careful if we added any insulation in wall cavities with wiring (ie: don't do it).
 
John, you say rewiring an underwired home will never reduce power consumption.
Suppose this is because a too small wire gauge also means there is a resistor in series with the current consumer.
So you don`t have to pay for the extra heat generated by insufficient wiring because the current consumer would also run "cheaper" at a lower voltage?
I always thought it`s most efficient to run everything at proper voltage and keeping conductivity loss as small as possible.
 

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