basically its often totally impractical to design on the assumption that everything will be on at once. therefore you must decide just how much capacity your installation needs.
there are some conservative guidelines in the regs but theese tend to give fairly gross overestimates (ie if we belived them we would all be neededing 200A supplies)
so allowing for diversity is a combination of looking at guidelines, making educated guesses and ultimately experiance as to whats workable.
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