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We're building an office for a small business, and the electrician is starting rough-in next week. I'm doing all of the low voltage wiring myself. We have electrician for 120V wiring.I have a dedicated space in utility room for 42U server rack.

At a minimum, the rack will include:

  • basic server running ubuntu server 12.04
  • dsl modem (12v wall wart)
  • router (12v wall wart)
  • 48 port switch
  • multi-function laser printer
  • 3 satellite boxes
  • 7 powered Cat6 to HDMI converter plates (12v wall wart)
  • 2 stereo receivers
  • multi-room home audio receiver
  • 2 bluRay DVD players
  • 2 IR remote repeaters (each has 12v wall wart)
  • 1 gateway device for garage doors (12v wall wart)

Totals:

  • 12x 12v wall warts
  • 11x 120v devices
  • 23 120v outlets required.

I will likely get 2 rackmount power strips, each having 12 120v receptacles. Can these 2 rackmount pwr strips be plugged into a regular 2-recepticle wall outlet, if the outlet is on it's own dedicated breaker?

Michael Hampton
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dan
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    With that load, probably. Go 20-amp (think NEMA 5-20 receptacles) – ewwhite Jan 17 '13 at 15:34
  • 20 Amps?? I have a Watguard Firewall, PaloAlto Firewall, 2 HP Proliant DL320 with 4 HDD each, 2 Cisco Catalyst, 2 UPS...at peak my two metered PDU shows 1 Amp of usage. Am I missing something in the calculation here? – Alex Jan 17 '13 at 15:39
  • I would go with a 4-Plex rather than a 2 recepticle and bump to 30 amp. If you are like me, the power requirements will likely go up in the future and I like extra wingle room. Most rack mount power strips I've worked with are the regular 3 prong adapter so you should be fine. – Harold Wong Jan 17 '13 at 15:43
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    @Alex There is **no** way you're just pulling 1 or 2 amps for that lot. Not a chance. – Dan Jan 17 '13 at 15:43
  • I think you're missing a fire suppression system, pulling the sort of load that lot must be *really* generating through a normal power supply, and possibly a working power meter. – Rob Moir Jan 17 '13 at 15:44
  • @Alex How are your PDUs metered? You know you actually have to have the meter in-line with the PDU for it to work, not just velcro it to the side of the PDU :) – MDMarra Jan 17 '13 at 15:46
  • My [PDUs are the meter](http://www.tripplite.com/en/products/product-series.cfm?txtSeriesID=939), they are both inline, they are both pluged into regular outlet on 15 Amps Breakers. – Alex Jan 17 '13 at 15:58
  • Where is your air conditioning? – Michael Hampton Jan 17 '13 at 19:02
  • @Alex Can you log in to the iLO or the HP Insight on your DL320's and see how many watts they are using? – longneck Jan 17 '13 at 19:03
  • @alex It doesn't matter if your meters are in the PDU or in a rolls royce parked outside your home after attending a grand parade in their honour as power meter of the year. There's no way the systems you describe are pulling the load you describe, unless they're powered off. – Rob Moir Jan 17 '13 at 19:15
  • @RobM Right, I'd expect a 1A pull from vampire power draw with that equipment - def much higher for running gear. – MDMarra Jan 17 '13 at 19:21
  • I'm not using the iLO, but lets just for fun that the 460W power supply was using the whole thing non stop, on 120V that's still a little less then 4Amps for each server. Switches and Firewall has 60Watts if I remember correctly, so again, with the switch used full blown that would be .5 Amps. So the very maximum I can reach is around 10-11Amps. – Alex Jan 17 '13 at 19:30
  • Wow, I was actually wrong, both firewalls and the switches have 23 to 27 Watts power supply. So the 4 of them together at maximum capacity would consume about 1 Amp. – Alex Jan 17 '13 at 19:38
  • I wanted you to look up the consumed watts because those HP server should be drawing about ~230 watts at idle. That's about 2 amps. Something tells me your amp meter is b0rked. – longneck Jan 17 '13 at 19:39
  • @longneck, power drawn is subject to a great amount of variables. CPU quantity and clock speed, amount of RAM, disk speed, server load, RAID controller present or not, heat. Also, I would seriously doubt both PDUs would have defective meters. Not to mention that if the consumption was as high as claimed here, I would have noticed on my electricity bill. – Alex Jan 17 '13 at 19:54
  • @Alex wow, i completely misread the spec sheet on the dl320g7. you're right, idle draw should be ~35 watts for single power supply, and ~50 for dual. – longneck Jan 17 '13 at 19:59

1 Answers1

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First off, you're missing some key electrical equipment (a UPS!).

To determine UPS (and power circuit) requirements, calculate peak potential load:

  • Determine the amperage of each load source.
    This is usually printed somewhere on the device or in its specifications.
    If it's in Watts instead of Amps remember that power (watts) is voltage times amperage, and do the math.
  • Sum up the peak loads
  • Add 5% to 10% as a safety margin.

This tells you how much power you need for your equipment. Work with your electrician to select a UPS strategy (carrier-grade UPS system, or individual rack-mount units protecting individual circuits), and to determine how many circuits you need to install in order to safely supply that load (typically circuits are 20-amp, occasionally you see larger or smaller).

Now that you know how much power you'll be using, balance it. Some tips:

  • Inductive loads (air conditioners, anything with a motor) get their own circuit(s).
    (Don't connect these to a UPS, you'll regret it!)

  • High-Current loads (laser printers) get their own circuit(s).
    (Again, don't connect these to a UPS, you'll regret it!)

  • Don't load any circuit to 100% capacity -- leave yourself some headroom.
    (This becomes especially important when you wind up power-cycling the whole rack at once)

  • Unless it's key to the infrastructure, forget the home theater equipment.
    Plan and power this stuff separately unless it's getting connected to stuff in your rack and is a key part of your infrastructure.
    I don't know your application, but generally you don't see many home theaters in datacenters.

voretaq7
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