Thanks for the "electrical input" and the nice compliments. everybody.
I have been studying a lot shop photos, but it's hard to distinguish between all the lines. So let's start with the basics. My questions might sound ridiculous to some of you, but I really have no clue.

I need fresh and waste water for the first and second floor. I guess those would be bigger pipes like the blue one in my sketch below, right? How many? On both walls?
The electric conduits: I guess one fat cable comes out of the ground to a box and what then?
Air compressor: I guess I want a stationary, electrically powered. How would that set up look like? Where do the pipes go? What is the compressed air used for? How do the hook ups look like?
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Waste water pipes, here in UK if its soil pipe coming down from upstairs toilet it would be 4" diam pipe,
used to be cast iron with spigots, but is now plastic with smaller spigots or shoulders, Usually such would be routed out through the wall & down the outside, but I have seen them on the inside of industrial buildings
if its just waste water from a hand or kitchen sink then the pipe would be about 1.25" diam. with an S bend just below the sink bowl.
fresh water in, you might have 3/4" or 1/2"pipe coming up from the floor to the tap, could be 3/4 copper, or earlier it may have been galv steel pipes about 7/8" outside diam. ( I know plumbers use inside diam dimensions, but as a modeller I find it easier to think what the OUTSIDE diam is when visuallising material choice).
Earlier again it could have been lead pipe, but due to health concerns, & scrap values very few places have any of that left unless it is a protected hertitage building. More modern could be alkathene water pipe, which as it came off a reel could be quite characteristicly wavy instead of dead straight.
Off course if its 2 floors & similar to a domestic arrangement it might even have a rising main up to the top floor or attic header tank, with the feed pipes coming DOWN from the header tank to the taps & appliances.
EDIT, of course as your are into 1/87 with this, probably pipe dimensions are not going to be that noticable.
And although I am modelling in 1/12th, I am following these garage threads with great interest, even if I dont comment much, great inspiration
