Hello all,

I'm a software engineer and am in the process of setting up a server rack in my home to serve as a lab for my work. The rack will hold 6 server grade computers, 4 regular PC's, and some other network related equipment.

I have 100 amp service into my home now, which I'm upgrading to 200 amp service tomorrow. I'm having a professional electrician do the work. During the bidding process, I told each electrician why I needed to upgrade to 200 amp service. I even showed them the servers which will be consuming so much power. I pulled out one of the power supplies to one server, and it reads like this:

AC INPUT
100-127V~,50/60Hz,8.3A
200-240V~,50/60Hz,4.0A

DC INPUT
140-224V,5.5A

DC OUTPUT
12.2 V 48 A
5 Vco 0.2 A

TOTAL 585W MAX

Five servers each have one of these power supplies. One server has three of these power supplies.

One electrician looked at a power supply and simply added up the 8.3 amps as described in the specs above, and came up with approximately 64 amps (~8A x 8 power supplies). He said because these servers will be on constantly, we'll need to dedicate the additional 100 amp service just to the rack of servers.

This made some sense to me, but of course I'm not an electrician. But, in the grand scheme of things, this no longer is making much sense to me. Because I've had several of these servers running on a single 20 amp circuit, along with 2 large monitors, an aquarium (pump, water heater, large bulb), 2 laptops, and several other network devices, and the circuit never failed. So, I'm confused about why I need a whole whopping 100 amp sub-panel installed next to my rack of servers.

Besides, ultimately I will be installing uninterruptable power supplies (UPS) in front of the servers. A UPS only has a single power cord, and between 4-8 outputs to run 4-8 servers. How do we calculate the amp's required on a single outlet that's plugged into this UPS? I don't get it.

Can someone clarify this whole capacity planning thing? I mean, from what I gather from this electrician, I'll need to run a separate circuit for each server. That's 8 separate circuits for just the 5 servers, and then a few more for the other network equipment?

Not making sense... :confused:

I know this may sound like a silly suggestion, but have you considered virtualisation of your infrastructure ?

This would not only save space, but also a shed load of power and associated cost.

6 VM'd servers will easily run on 1 physical machine.
You could then use another for the desktops, reducing your load from 10 physical boxes to 2..

Hi CurlyBen,

I am virtualizing these servers. I require quite a few machines in my lab to do my research. I'll end up with 40+ virtualized servers in this environment.

Additional information:

This is the type of PDU that is common in these server racks.
Rack PDU, Basic, Zero U, 20A, 120V, (24)5-20

And this is a UPS that is common in these server racks.
http://www.apc.com/products/resource...total_watts=50

Thanks.

The electrician did 8 * 8.3 to arrive at 64 amps.

Don't know what he was thinking, as all this load, while may all at 120 volts, he must have forgotten (Hmmm or did he), that he has two 120 volt legs to use, assuming you have a typical USA service at 120/240 volts, and he is to balance this single phase 120 volt load on that service, in which case, would apply 32 amps on each of the two 120 volt legs.

Hopefully a service load calculation was done, to determine that a 200 amp service is adequate to handle the existing load in the home, plus this 32 amps per 120 volt leg.

A 200 amp service cannot be loaded more than 160 amps continuous, or 80%.

Does this help? Or help confuse you more?

You say that "several" of these units plus other stuff runs fine on a 20 amp line. Take a clamp on ammeter and determine what the actual load is. The computer assembler bought these power supplies from an outside vendor. Though they may be capable of supplying all that power, about 1,000 watts each, it may be that the actual load may be a fraction of that. If these eight units were actually supplying that sort of power you could heat a small house from the waste heat. A full sized electric oven is around 7,000 watts. Once you know the actual load I think a better decision can be made.
Googling tells me that laptops consume max 65 watts, lcd displays about 40 watts and the average desktop computer 160 watts. Not big loads. Use that ammeter.
PCs and power (the ranting continues) ( discussion of this issue)
If you don't own a meter these will do the job.

http://www.sears.com/shc/s/p_10153_1...=clamp+ammeter
http://www.sears.com/shc/s/p_10153_1...5x000001&aff=Y

Thanks for the input, everyone. So, I did a little more research and was able to find a post with some excellent power consumption information for the very servers that I will be running. Here is a snippet:

IBM eServer x336

Specs: Dual 3.0Ghz Xeon, 4GB RAM, Dual 575W Power Supplies, Dual 146GB SCSI drives

•Connecting Power Peak: 1.06A
•Stdby Steady: 0.79A
•Power On Peak: 2.5A
•Powered low load: 2.12A
•Loaded (7.0+ with disk): 3.25A
•Disk activity only: 2.40A
•Reasonable Load + Disk Activity: 2.85A peak
•heavy cpu/disk load – multiple instances of cpuburn and cat'ing /dev/urandom to a file – 3.2A

This is very good news, in that these servers will actually never use the rated 8.3amps on the power supply. In fact, the rarely peak to half the rated amp's on the power supply.

Given the numbers above, and multiply that by 5 (peak 3.2amps x 6 = 19.2amps), I could essentially put all 5 of these servers on the same 20amp circuit, right? Or is there such a thing as a 30 amp circuit? That should cover these with plenty of additional amp's in case all servers were to draw peak amperage at the same time. Right?

Then, all I need to worry about is the big boy server, which has 3 of these similar power supplies. I'd probably be safe putting that on it's own 15-20amp circuit, I would think.

Then one additional 20amp circuit for the remaining gear (switches and other peripherals).

What do you think?

Thanks for your time. Much appreciated!

Sorry, I had my calculation wrong up there.

peak 3.2amps x 5 = 19.2amps

By the way, here is the link referenced for those power readings above.

Network Jack Blog Archive Power Consumption – Actual amp readings

OMG. Wrong again.

peak 3.2amps x 5 = 16amp peak

Got it right this time for sure!

For you the right circuit is twenty amps. Two 20 amp circuits for the servers, each carrying a max of 14 amps, should be just right. Plus one for everything else. Loading a circuit up is a no-no. Don't believe everything you read on the back of computers.

That's exactly what I'm looking for; what type of circuits do I need and how many. This whole journey with setting up this server rack has been very enlightening. Throughout my 10 years in technology and working with servers, it's amazing I've survived this long without really understanding power consumption and capacity.

I agree with you, Johnmprimce. Three 20 amp circuits in total should give me more than enough room.

Thanks everyone!

Now, if I could only figure out how to mark those who help as "helpful answers". Usually forums have some sort of rating system. I guess this forum simply rates on number of posts and responses. I can only rate my own answer? Hmm... well, my gratitude goes out to you. How's that? :)

If you care to invstigate power consumption yourself, you could buy a "Watt Minder" and check the loads. By using ohms law you can easily convert watts to amps on each of the units. A voltmeter would be helpful as well, but it's likely your power supply won't be much off from the standard 120 volts supplied nationwide.