How do you select a PDU?

[Pandasaurus]

I wasn't sure if this should go here or in Networking. If Networking is more appropriate, mods feel free to move it.

Obviously, I'm not an electrician. I actually have almost no idea what I'm doing when it comes to this, and my Google-fu is either even worse than I thought, or I found the answer I need but didn't realize it. So, can anyone help explain the thought process to the poor new guy?

That said, I'm trying to figure out how one selects a PDU (or more than one, if necessary) for a rack. Do you go by amps, or watts? The average PDU available with 5-15R/P sockets/input plug (standard US household plugs) has a 15 amp capacity, if I'm understanding correctly. If I am, I'm going to be using a rough total of ~16 amps just with 3 switches and 3 routers, no other hardware. (technically, if Cisco's numbers are right, it could be in the area of around 200-300 amps on power-up. They say a single 2970 switch draws 50 amps on power-up)

On the other hand, some of the sources I've found say you calculate your input amps * volts to find your capacity in watts. If this is the case, I would have ~490 watts excess capacity with the same hardware, assuming everything is at maximum consumption (which it likely won't be). I'm thinking this is the correct way, but... Like I said, I'm new to all this, and clearly am not understanding something. Anyone able to help me out?

 

[Red Squirrel]

The plug it has, and it's capacity, and the amount of outlets is probably the biggest factors. Idealy you want something that will easily be able to plug into your UPS(es).

I recently bought two Tripp Lite PDUs that have a 20 amp twist lock but also have an adapter to go to a regular 20 amp plug. one PDU will be plugged into the surge protector and the other into the UPS.

Some more expensive PDUs also have a management interface and amperage metering. The management interface can be neat as you can turn on/off outlets remotely and such.

 

[Pandasaurus]

No UPSes at the moment. This is for a home lab environment, so nothing is particularly mission critical (and may or may not be running 24/7, I haven't decided yet. I may only power it up when it's in use). I would like to get some, but for my planned hardware, I'm looking at spending well over twice as much on the necessary UPSes than I have on the actual hardware. (Translation: If I remember my price-shopping from a couple nights ago correctly, I'd need at least $2k in UPSes to power everything for <5 minutes) That's just not in the budget until I get a real job, sadly.

As far as the PDU(s), I know which plug I need (5-15R, with a 5-15P input cord), and how many outlets (roughly). I'm looking for a basic (possibly metered, but that's not a requirement) model. Something in the sub-$150 range, if possible. There's a couple Tripp Lite (and an APC or two) models I can get in that range (around $90 shipped) that are also metered. The problem is, as I said in the OP, I'm not sure how to calculate the needed capacity, if I need more than one or... What.

 

[dawza]

The 2970 spec sheet lists 190W max power draw (from the wall, which in the US would be 120V, so <2 amps @120V). The amperage you are listing/concerned about is DC amps, which is @12V in this case.

The chances of you ever seeing that full power draw for any sustained period of time are quite low. My entire network rack draws <80W most of the time, and that includes my home server.

You're likely viewing DC amperage (equipment needs) and AC amperage (from the wall/power strip) as one and the same voltage-wise (they are not). Just get a PDU you like that is within your price range, and a UPS as well; my 1U 700VA UPS keeps my stack running for just shy of an hour. You can get a pretty nice PDU for $150-- super basic models start at $40 or so.

Your 490W (AC wall draw) estimate (almost certainly a worst/highest-use-case scenario) is more in line with reality.

 

[Red Squirrel]

For UPS I'm a big fan of inverter-chargers. They cost way less per watt and per minute of run time.

http://www.tigerdirect.ca/applicatio...507&amp;CatId=1451

Though if I was to do it over again, I would have gotten this one:

http://www.tigerdirect.ca/applications/SearchTools/item-details.asp?EdpNo=5017615&CatId=237

Might still get it in the future and setup a separate battery bank.


I have 2 100ah batteries and my whole rack stays up for 4-5 hours. I'll be adding 2 more batteries once I'm done the DC cabling for the new rack. I'm waiting for some 100a switches to come in the mail so I can complete this. I'll be able to isolate the batteries for maintenance/replacement without taking anything offline.

 

[lakedude]

The 2970 spec sheet lists 190W max power draw (from the wall, which in the US would be 120V, so <2 amps @120V)...

This

The whole thing will draw less than 12 amps @ 120VAC, meaning you can use pretty much any 15 amp surge protected powerstrip and not have anything to worry about. If you are really worried you can turn your switches on one at a time to avoid a peak in inrush current.

I wouldn't spend over $25 of my own money to power those things unless of course you need a UPS.

 

[lakedude]

That 50 amp number must be DC inrush current. If dawza's 12VDC number is correct this translates into a 5 amp AC inrush @ 120VAC. Steady state draw is going to be under 2amps @ 120VAC.

E=IR and P=IE baby!