I see that in some cases where a 400A switch is fused at 400A and 600A switch is fused at 600A. For a 30 apartments service switch, can a 800A switch fused at 800A or does it require to be fused at lower amperage? Any code restriction shown in the code book? Please help.

Perform a load calculation per the NEC and size the Service accordingly.

Obviously, 800A fuses will be accepted by a 800A switch -- presuming they are the right type.

Your query gives the appearance of someone new to electrical engineering -- as against field wiring of apartments.

As a general rule Service equipment is sized per the Code and fused to the maximum of the frame size. Further, the typical Poco will only run a limited selection of wire sizes for their drops or laterals. You'll want to know what those values are, too.

After the MAIN, typically a fused bolt-switch, each dwelling will have a disconnecting means. These days each dwelling will be normally individually metered. If so, the Poco will have very definite preferences as to locations and meter heights and working clearances.

I do not recommend attempting any design work of this nature unless it is overseen by an experienced engineer or electrical contractor active with your Poco. In this market, a newbie is going to get into trouble.

I would say you have to select the fuse that protects the wire you are using.

You can also have a situation where 400a switchgear is wired with an ampacity of 250a (or some lower number) and as long as the appropriate fuse is used, that is legal. As far as I am concerned this may be a problem because, if that fuse blows, you know the "handyman" will put a 300 in it or whatever fits. I questioned this in a proposal (to the 99 I think, maybe 2002) just to get their take on it and they said, basically, we only inspect to what is installed at the time we inspect and you can't do anything about unqualified people coming along later and overfusing. I suppose that was not the same CMP that came up with the type S requirement

Please not that "800-amps" is something of a 'magic number' as far as the code is concerned. For example, fuse sizes change at this point, and you can no longer 'round up.'

In short, you can't put a 799-amp fuse in 800-amp gear - at least, not without some sort of adapter.

I don't encounter this threshold very often - so jump in if I'm confused!

Reno, 240.4(B) says "Devices Rated 800 Amperes or Less." so the break is at 801 amps. (similar to language referring to 601 volts). I agree the difference is largely semantic since plus/minus 1 volt or 1 amp is well within the tolerance of the devices but they have to draw the line somewhere.
I do agree with Tesla that you need engineering guidance when you get up into this big stuff.

Thanks, Greg, for the correction! I had a feeling I was not quite correct.

(Look it up? I should have, but my access to my materials is severelt hampered these day, as I move about in the job search. With luck, that quest will be ending soon).