[BBLISA] PDU Power Ratings?

[Rich Braun]

John Rouillard asked for clarification
> Ummm, how does that work exactly. Max continual load on any pdu is 80%
> of the circuit. ... max continuous load of 16 amps.
>
> Now if devices D1...DN have dual plugs and one goes to pdu P1 and the
> other goes to pdu P2, each pdu will see 1/2 the amperage load

You are correct for the two-phase 120V case.  If you go to three phases, then
you get P1, P2, P3.  Each has 208V @ 20A.  For simplicity, let's say I have
the following devices that each draw 6A @ 208V.

Device A: connects to P1 & P2
Device B: P2 & P3
Device C: P1 & P3
Device D: P1 & P2
Device E: P2 & P3
Device F: P1 & P3

Six devices, 12 power cords, each cord drawing 3 amps.  There are 3 PDUs so
each has 4 devices connected.  Without any faults, therefore each PDU is
supplying 12A.

Q: What happens when P2's breaker trips?
A: Device A and Device D draw an extra 6 amps from P1; device B and device E
draw an extra 6 amps from P3.

That third phase, wired this way, gives you a lot more headroom in the event
of a fault on a single phase.  (My example rounds up a bit, to 90% load during
a failure; a conservative data center admin will respect the 80% max cited by
John.)

-rich