I've struggled with sizing my power sources and associated PDU's in my server racks. Somehow I feel like I'm way oversizing things based on nameplate requirements on power supplies.
My simplistic approach is to look at the nameplate draw on my power supplies, add them up and size my feed for that. Doing that with a rack that has 10 servers, 1 storage array, and 4 Cisco Switches leaves me with 6 - 20 amp circuits? How do I arrive at this figure? Well, I look at absolute worst case which in my mind is half of my power supplies have failed and I'm starting up all my equipment at the same time on 3 circuits connected to the half of the power supplies remaining. Because I have to assume all load will be concentrated on half of my outlets that effectively doubles my requirements.
Does anyone have a better methodology than this? My customers freak a little bit when I tell them I need 80 or 90 amps for 8 or 9 servers and some switches. It does sound a little over the top. I know one option might be to configure the high draw equipment (like servers and arrays) to not power on when power is restored. I don't know if I have an option to delay power on either. That would be nice. Do switched PDU's have a feature like this where it will stage the outlet power ons. That seems like it would be effective.
I know I can use the calculators to determine steady state power draws which are much lower than the startup power draw. My ultimate concern is startup power draw for all components at once. From there I effectively have to double the amount of capacity I deliver to the cabinet to account for starting up on only half of the available capacity because half of my power supplies are failed.
Based on some research it appears switched outlets that can stage themselves might be the answer. Others agree or have better solutions?