Selection of Upstream Breakers

It depends on local regulations, especially on how diversity may be used in load calculations.

NFPA 70e

Total average load plus 20 - 25% rounding off the nearest common since breaker that fits in that range.

You don't really have any choice, all electrical installations are governed by the electrical codes....consult your local codes....

https://www.fs.fed.us/database/acad/elec/greenbook/3_basicdesigns.pdf

Is this a homework question? 18.04 amps sounds a little precise (truly, 18 amps + 40 milliamps? In a power distribution circuit?) given the ordinary realities of varying mains voltage and so forth, and so sounds a bit on the academic side to me. Just curious.

If it is a homework question, what do your lesson's study materials tell you?

It only gets worse.

Electrical Panels Rating

Be careful.

One of my faults is telling the truth when a poster demonstrates a complete lack of competence in a subject, especially when discussing something with a high potential for lethality, like electricity.

I've been accused of being a meanie for giving someone truthful advice that they are in way over their heads.

I wouldn't give a blind man a loaded gun and invite him to shoot it.

Yes, I know, but I think at this stage the greatest risk the OP currently faces is a paper cut. I have this hunch he's a student who's not studying his lessons thoroughly enough to pass his sparky exam.

His dog ate his code book.

I weep for our future.

Incompetence seems to be in vogue these days.

Most students I've encountered (and I've encountered quite a few, trust me) are pretty incompetent. Probably why they're students. At least we can give them credit for attempting to remedy the situation by attending school. Unfortunately for some, merely attending classes doesn't seem to have much effect. They also have to study, a foreign concept to some. Our OP sounds like he may fall into this latter category. My advice to him is log off this forum and get his nose back into the books. At the very least this will remove him from live circuits.

If he'd said 18 amps, I would be concerned, but he said 18.04 amps. He's either designing a space probe's power distribution bus where every milliamp counts (highly unlikely methinks) - or he's in school and this a homework problem.

It depends not upon the loads connected, rather upon the cables connected, about which the forum has been told exactly nothing. After all, the circuit protective device is only there to protect the cables.

British Standard 7671 gives full protocols for this sort of assessment and to post further on the topic here is abstruse. If in doubt, consult a qualified Electrician.

I feel if sum of all currents is 18.04Amps, then to have close overload protection main incomer must be 20Amps. However as I can see, that the selection of outgoing MCBs is on much higher side which needs to be rectified to have close protection. Today MCBs are also available in 0.5A, 1A, 2A, 3A, 4A, 6A. It is recommended to use those depending on max load on each outgoing. Thanks.