Why hot swap server PSUs are derated for lower line voltages

[japlytic]

I compared two 1200W units (both with active PFC) around the same manufacturing time, an ATX unit and a compact hot swap server unit: the ATX unit had 780uF input DC filtering capacitance compared to 390uF for the server unit, and the server unit had smaller PFC components (and heatsinks) for its size.
Also, the ATX unit was rated at 1200W output under full line voltage range, but the server unit (full rating at 200-240V line input) was derated to 900W at 110-120V input and 800W at 100-110V input.

Typically (in my belief) in a 100-120V region, server rooms have 220-240V mains power, and I have seen server PSUs which require a three phase AC input, one of which was an IBM unit built by Fuji Electric.

[Curious.George]

Re: Why hot swap server PSUs are derated for lower line voltages

Typically (in my belief) in a 100-120V region, server rooms have 220-240V mains power, and I have seen server PSUs which require a three phase AC input, one of which was an IBM unit built by Fuji Electric.

Most servers are 110VAC machines. Typically with dual redundant power supplies (but you could still use a single 120V branch circuit to power them; a pair of branch circuits if you want to protect against a branch circuit fault). You're pressed for space when you're trying to cram that stuff into a 2U form factor which means power supplies have to be smaller/sleeker.

Larger blade servers move up to 220V mains -- but that's usually because the power required starts to creep upward beyond what you can comfortably find on a 20A branch circuit; mine (14 blades in 7U) has a pair of 2300W power supplies in it (I have to plug it in where my electric clothes dryer resides when I want to use it).

Disk farms tend to be bigger pigs. But, not because any single shelf needs gobs of power; rather, because its common to put a dozen or more shelfs in a single rack! (figure ~700W/shelf)