We are required to install an underground, man accessible 480 VAC single phase concrete electrical vault which will contain a 480 VAC single phase feeder & disconnect, 480/120/220 VAC transformer and 120/220 distribution panel. What are the NEC code requirement for this vault?

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Are you the electrician? GC? Client?

This is not really a question for a Home & Garden DIY message board.

Your topic is well beyond the scope of this forum. Your situation is unique, and requires several detailed questions asked, and a fair amount of Code research.

Due to the occupancy by people, there are some critical issues that need the be considered in the design.

You may view the 2008 and 2012 NFPA 70 National Electric Code at NFPA (http://www.nfpa.org) and review ARTICLE 450 - Transformers and Transformer Vaults.

You need to do a quick register, and look for list of standards near the bottom left of the home page.

I strongly recommend that you seek professional advice with this project.