EDIT: I misunderstood you were talking about power feeds, the normal case is the run "48% as if it's 100%" (because of power spikes, but also most types of transformers run more efficiently under specific levels of load (40-60).
Normally this is factored into the Rack you buy from a hardware provider, they will tell you that you have 10A or 16A on each feed, if you exceed that: it will work, but you are overloading their feed and they might complain about it.
The poster was speaking more of the power delivery going to the power supplies, not the server's power supplies themselves. So say each PSU 1 is wired to circuit A, each PSU 2 is wired to circuit B. Circuit A experiences a failure. All servers instantly switch over all their load to their PSU 2's on circuit B. Suddenly circuit B's load is roughly double what it was just moments ago. If proper planning wasn't created or followed, this might overload circuit B, meaning all PSU 2's go dark regardless of the server being able to do the change over or not.
But, point taken: yes your power feed should be running at <50%. But that just means you treat 50% as 100% just like any resource.
Mostly this is outsourced to the datacenter provider; they'll give you a per side rating. (usually 10A or 16A) which also matches the cooling profile of the cabinet.
