Data Center Branch Circuit Design

[tskidmore]

I am in the final stages of specifying the branch circuits for a data center. In order to have maximum flexibility, I would like to use 3-phase 30A, 208V branch circuits as a standard. There are numerous rack PDU vendors out there that can supply various receptacle configurations from this input so this a a greatly simplified way to manage my rack power distribution (at the cost of pole space).

When I discussed this config with my electrican, he raised the issue of how much he would be able to oversubscribe the ampacity of the PDUs. I have 150kVA, 480v PDUs fed by a 250A subfeed breaker. There are 3 42-pole panelboards on each PDU, so I have pole space for 42 of these 30A feeds, which is obviously more than my PDUs capacity.

How many of these 30A feeds would be legal? Is there a standard diversity factor that would apply? The 30A breakers and feeds are more for a convenience- we would (of course) need to carefully manage our current capacity back at the PDU...

Thanks

Tom S

 

[rr]

What's the rating of your transformer? You have 480V/3 phase PDU's, but 208V/3 phase branch circuits.

 

[tskidmore]

It is a Liebert PPA150C315. Not sure exactly what transformer rating you are after- I do not have those specifics in my project proposal.

Per the Liebert guide spec online at:
http://www.liebert.com/products/english/products/pwrcond/ppc/60Hz/guidesp/word/sl_20070.rtf

The transformer will be rated "as described in Section 1.3" which would be back to the 150kVA, three phase 480V.

Unit transformer shall have a K20 rating in accordance with UL 1561 to allow full load operation with highly nonlinear loads. Transformer neutral shall be sized for at least 200% of full load. The transformer shall be designed to operate with 100% single-phase, switch-mode power supplies and associated harmonic phase and neutral currents without derating.

What other info would you need?

Tom S

 

[rr]

Assuming 80% load on a 30A branch circuit:

If I understand correctly, you're only going to able to completely load about 16 branch circuits maximum. This would be (1) 42-pole panel. Anything more would overload the 150kVA transformer.

In my opinion, you should contact a Consulting Engineer in your area to have them perform a load study on your particular application.

 

[wirenut1980]

Connected vs Demand

Connected vs Demand

Tom, Total connected load of the 208 V PDU's might add up to more than the capacity of your 150 kVA unit, but as long as your demand (expected actual load) does not exceed your capacity, you can fill up with as many 30 Amp 208 V feeds as the panel will hold in accordance with its listing...if I am understanding your question.

 

[suemarkp]

Data centers are a problem because the nameplates rarely match the actual load, and the fact that most computers are left on 24/7. I don't think applying a demand factor is appropriate for computing loads (at least server types that are always left on).

So I would measure each connected computer to see what its load really is. I'd then carefully balance the loads and hope to stay under 100% of the KVA rating. If the items in your data center change often, this could be a tedious process.

Alternatively, if you choose a PDU that has built in amp metering, use that to check your phase loading. Or, add three ammeters to your design to continuously monitor your loads.

 

[steve066]

I hope you realize you can't put 20A receptacles on 30A circuits.

Steve

 

[ron]

Tom,
You have the same problem that every datacenter manager has.
The answer is that it depends.
You can be very conservative on initial circuit design and pretend that every circuit will be loaded to its max ampacity, which will limit the number of circuits you can connect per PDU.
The more practical method is to monitor the load as it grows in detail. Assume that circuits will not be fully loaded, and keep on top of the load measurements each time equipment is added, and you can dynamically manage the loading of the PDU's. The second method is the most efficient way to get the most capacity from each PDU.
But .... you must monitor the loads often.
I specify branch circuit monitors for each branch circuit out of the PDU (or RPP) so that the loads can be tracked and managed on the fly.

 

[ramsy]

I would like to use 3-phase 30A, 208V branch circuits..

20A branch ckts, at continuous use, may not overload PDU, permit #12cu rather than #8, and using Wiremold 400 series surface/wireways, with the largest size devices, still has fill area for 30 CCC (15 single phase circuits) without derating.

 

[tskidmore]

OK, so

OK, so

FYI I do have CTs on the PDU input and on each panel board input. I was also planning on using metered rack PDUs as mentioned by Ron, and I do plan on monitoring the load constantly, certainly before any new equipment is installed the capacity will have to be checked at the PDU, the panelboard and the rack PDU phase.

So the potential issue I heard from my electrician was that the AHJ may not approve so many 30A circuits on the PDU because it would be oversubscribed by 3x on the max rated load of all the branch circuits. Again, I know I need to manage my overall load at the panelboards and the PDU, but I want to have max flexibility in my "standard" rack config to be able to deploy equipment in racks based on the business drivers, and not to constantly have to call an electrican to change the branch circuit capacity and termination- I anticipate this will be a very dynamic environment. The "oddball" circuits I can do specific circuits, but I want to cover a large majority of potential configs with my standard.

So it sounds like no one is aware of a code issue with installing perhaps 35-40 30A 3-phase 208 circuits off this 150kVA PDU???

Thanks again