As you know, some servers use redundant power supplies; they advise putting the two power supplies on independent circuits so a breaker trip won't down the server.

Common North American 120V receptacles have 2 sockets on them (called "duplex receptacles")... and they can be split and fed from 2 separate circuits (each with their own neutral; not an MWBC, though that too).

I have heard a rather interesting claim over on our DIY forum that HP once (10+ year ago) had installation instructions that said to "split" a duplex receptacle and use each half for an independent circuit.

I have my doubts about that, since a) why would they care?... and b) it's a NEC violation.

It's technically legal to feed 2 circuits to one recep, but the circuit breakers must be handle-tied to protect a future maintainer. Handle-ties tend to cause common trip, which defeats the purpose!

Does anyone have any recollection of such an instruction existing in HP or other server's installation manuals? This might have been perhaps 10-15 years ago.

  • Without evidence, I can't prove it (I can't find the document) - but I do recall seeing something about that in a "practices" guide for installing Poweredge 1950/2950 servers, which about matches your timeframe. However, I seem to recall that the idea was to multi-gang the sockets, rather than splitting a receptacle across duplex. I still wouldn't want to do even that, personally. It should be very clear that they are using differing circuits. – Spooler Sep 4 '20 at 18:51
  • Wouldn't a more relevant question be if this is good/acceptable practice rather than if some server manufacturer(s) mentioned/suggested such an approach in their documentation 10-15 years ago? – Håkan Lindqvist Sep 4 '20 at 19:02
  • What I heard from the past, which no longer seem right or talked is that splitting both PDU to different input can lead to problem as the if the circuit A got a short not blocked by the UPS, you could trigger a problem on your input B electrical circuit back to the breaker. It's not something heard from HP on the other side – yagmoth555 Sep 4 '20 at 19:32

I've worked with HP gear for 25 years and I seriously doubt that. It would be gross negligence.

The following is consistent with other HP rack documentation I have read in the past:


Power considerations

Power is best managed within the rack by the use of a power distribution unit (PDU). Depending on the configuration, it may be necessary or desirable to use multiple PDUs to connect all devices inside the rack.

Each PDU should be connected to a dedicated (unshared) branch circuit that is suitably rated for the continuous load of all the equipment connected to it. The total power load for a PDU should not exceed 80 percent of the branch circuit rating.

If a PDU is not used, each piece of equipment within the rack should be connected to a dedicated branch circuit.

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