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  #1   IP: 207.114.255.2
Old December 17th, 2007, 11:03 AM
mfarver mfarver is offline
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Default Amps per leg in 3 phase

So this falls into the category of "it should be obvious" but its actually got us completely confused

Our datacenter offers us "208 3 phase 60 amp" service. We will hook that to a trio of 208 single phase power distribution units, each attached to one pair of phase leads. The PDUs have only ammeters to tell us how much current they are drawing. How many amps can each PDU draw before the feed is overloaded?

I've heard answers ranging from 15-60 amps.

One method I was given was based on watts. 208 x 1.73 x 60 = 21590w. Divided by 3 legs = 7196watts per PDU. Divide that by 208V per PDU = 34.6 amps.

Ideas?
Thanks
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  #2   IP: 207.35.6.2
Old December 17th, 2007, 11:19 AM
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joed joed is offline
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The simplest answer is usually the correct one. I would say 60 amps per unit.
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  #3   IP: 207.114.255.2
Old December 17th, 2007, 01:06 PM
mfarver mfarver is offline
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I confirmed that the 3 phase breaker for the supply is in fact rated 60 amps, if that affects the answer.

Mark
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  #4   IP: 24.150.120.191
Old December 17th, 2007, 05:12 PM
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A 60A 3-phase branch circuit has 60A available on each leg. So you have 60A available on each leg for your loads. Here is a breakdown of your situation of the delta connected single phase loads.

So in summary, when each single phase 208V load is drawing 34.64A, the 3-phase line current will be 60A.

However, since these units should be considered to be a continuous load (3 hours or longer), then a 60A branch circuit should only be loaded to 80% of 60A. This takes you down even further, to 0.8x 34.68 = 28A.

To complete this discussion consider the following scenario as well (not what you have). If you had three Wye connected single phase loads, then they could each draw 60A while the 3-phase line current remained at 60A. However, the voltage across your Wye connected loads would only be 120V. The power would be the same (root3 x 60 x 208 ) or (3 x 60 x 120) for this 60A branch circuit.

Homer
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Last edited by Homer : December 17th, 2007 at 06:06 PM.
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  #5   IP: 67.171.2.84
Old December 17th, 2007, 09:19 PM
suemarkp suemarkp is offline
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Homer's got the right answer. Because of the confusion that can happen in 3-phase, especially if you mix 120V with 208V loads, it is best to work in watts. The continuous load makes things worse. Your only saving grace will be the power draw of the servers. They rarely draw thier nameplate rating (and dual power supplies don't double the power draw either).

The ammeters will help you know your actual draw, so limit each PDU to 28 amps.
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  #6   IP: 207.114.255.2
Old December 19th, 2007, 06:50 PM
mfarver mfarver is offline
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Thanks for the clear and excellent description! (It turns out, my math was right... sometimes even I get lucky)

Mark
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