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I have 3 500 THHN .7073 sq. in area and I choose a 2" EMT AS THE RACEWAW WITH 2.013 Sq. in. area, and I use chapter 9 note 4 that allows me to fill the conduit to 60%, am I correct? because this looks like it's too small, but I'm doing practice quizzes on line and the person is saying 2" EMT IS THE CORRECT SELCTION?

Air supply piping from and air compressor. I see that some air compressors are connected with stainless steel braided hose. Does this hose have to be listed for fire protection use? I can’t seem to locate any relevant information in NFPA 13. 
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If I have a hypothetical 480/277V, 3 Phase, 4 wire feed to a distribution panel. (Spare panel for future use, we do not know the load) Do you de-rate because it is 4 wire? Following 310.15(B)(5)(c) If the majority is non-linear (meaning greater than 50%?) we would de-rate because of 4 wire. (Correct?) Or does this apply only to a branch circuit,
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At a recent NFPA event, we were asked:   A question we struggle with often is when is the requirement for hydraulic calc’s necessary based on amount of added sprinkler heads?   Tell us, is this a question you struggle with as well?   VOTE and read below for a response from NFPA's Principal Fire Protection Engineer, Matt