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I have a 20 AMP/120V circuit. The only device plugged into this circuit is a 1000 Watt Dell UPS. Right now several devices plug directly into the 1000 Watt UPS.

I wanted to order a PDU to help untangle my cabling a bit.


If I order a 16 amp PDU, it seems like I could accidentally overload the UPS (just as I could by plugging items into it directly).

I was wondering if one of the two things existed:

  1. A PDU suitable for use in a Dell full-height rack rated @ 120V/8 AMPS OR
  2. A PDU with a configurable shut off point. (One I could configure to shut down at 8 amps even though the PDU is rated much higher.)

I had a conference call with a Dell and APC rep, who suggested that no such thing exists and implied that it's unnecessary for my situation.

Why is this unnecessary for my situation? To me relying solely on whatever overload protection mechanism the UPS has seems a bit scary. Am I being overcautious and/or misunderstanding the situation? Also, does a PDU like the one described exist (even if not available through Dell)?

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1 Answer 1

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You are thinking about things the wrong way around. The PDU just distributes the power that is being delivered to it. For the PDU 16 Amps is the maximum current it can distribute. Your UPS has a maximum delivery of 8 Amps. The delivery of the UPS is inside the safe limit of the PDU.

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I suspect I must be (thinking about things the wrong way around) because most people I ask about this have a similar response. Here's my thinking though: The UPS has a maximum deliver of 8 Amps. This means if I load it with devices pulling more than 8 Amps, I'm relying on the UPS to shut itself off (or whatever it does). If I have an 8 Amp PDU connected to the 8 Amp UPS I have two lines of defense: 8 Amp PDU will flip first. If that fails, UPS will shut itself down. – user426724 Sep 7 '11 at 15:25
Your PDU is still only getting the power from the UPS whichever way you look at it. – Iain Sep 7 '11 at 16:54
Yep, the PDU is only getting power from the UPS, but that's exactly my concern. Right now I can easily overload my UPS by plugging too many devices into the PDU. If the PDU has a breaker that flips below the max rating of the UPS then the PDU would trip before I overload my UPS. Essentially I'm thinking of the PDU as a secondary mechanism to prevent overloading UPS (secondary to whatever internal mechanisms UPS has). I have a feeling I'm missing something here... – user426724 Sep 7 '11 at 17:57
The UPS should have overload protection. You should carefully work out how much load you are placing on the UPS and ensure that you don't overload it. – Iain Sep 7 '11 at 18:02
Yep, I've done this. I was hoping to use the PDU as a second line of defense (second to the UPS' overload protection). For example, if I set the PDU to trip at a little below where the UPS trips, the PDU would trip first. If I set it to trip at the same amperage as the UPS, it would trip if the UPS overload malfunction failed. It sounds like this just isn't done though. I'll get a 16 amp PDU and just be very careful with what I plug into it. – user426724 Sep 7 '11 at 19:54

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